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Aspartame and Risk of Cancer: A Meta-analytic Review a

Sreekanth Mallikarjun & Rebecca McNeill Sieburth a

b

Department of Technology and Society, Stony Brook University, Stony Brook, New York, USA

b

School of Medicine, Temple University, Philadelphia, Pennsylvania, USA Accepted author version posted online: 01 Aug 2013.

Click for updates To cite this article: Sreekanth Mallikarjun & Rebecca McNeill Sieburth (2015) Aspartame and Risk of Cancer: A Meta-analytic Review, Archives of Environmental & Occupational Health, 70:3, 133-141, DOI: 10.1080/19338244.2013.828674 To link to this article: http://dx.doi.org/10.1080/19338244.2013.828674

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Archives of Environmental & Occupational Health (2015) 70, 133–141 C Taylor & Francis Group, LLC Copyright  ISSN: 1933-8244 print / 2154-4700 online DOI: 10.1080/19338244.2013.828674

Aspartame and Risk of Cancer: A Meta-analytic Review SREEKANTH MALLIKARJUN1 and REBECCA MCNEILL SIEBURTH2 1 2

Department of Technology and Society, Stony Brook University, Stony Brook, New York, USA School of Medicine, Temple University, Philadelphia, Pennsylvania, USA

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Received 25 January 2013, Accepted 16 July 2013

Aspartame (APM) is the most commonly used artificial sweetener and flavor enhancer in the world. There is a rise in concern that APM is carcinogenic due to a variation in the findings of the previous APM carcinogenic bioassays. This article conducts a meta-analytic review of all previous APM carcinogenic bioassays on rodents that were conducted before 31 December 2012. The search yielded 10 original APM carcinogenic bioassays on rodents. The aggregate effect sizes suggest that APM consumption has no significant carcinogenic effect in rodents. Keywords: aspartame, cancer, meta-analysis

Aspartame (APM) was approved by the US Food and Drug Administration (FDA) in 1981 and European Food Safety Authority in 1994.1,2 APM is the most commonly used artificial sweetener in the world. APM accounts for 62% of the artificial sweetener market.3 APM is used in more than 6,000 products (including food, beverages, drugs, and hygiene products) and about 500 pharmaceutical products.4 Over 200 million people around the world regularly consume APM through these products.5 There is a concern that consumption of APM increases the potential risk of cancer.6 Currently, APM carcinogenic studies on humans are not available.7 However, some carcinogenic bioassays were performed on rodents before and after the inception of APM. The International Agency for Research on Cancer of the World Health Organization (WHO) claims that carcinogenic bioassay findings on rodents are valid predictors of carcinogenic risks in humans.8 However, the findings of APM bioassays on rodents provide inconsistent results and are criticized. Some studies9 conclude that APM does not cause cancer, whereas other studies7 show that APM causes cancer. This leads to a research question—What is the unbiased aggregate empirical finding of all the existing APM carcinogenic bioassays on rodents? This article addresses this research question by using an unbiased meta-analytic procedure to empirically review all the APM carcinogenic bioassays conducted previously. Heretofore, some meta-analysis related to APM products and its various effects were carried out. For example, Hunty et al10 used meta-analysis to demonstrate that the foods and

drinks sweetened with APM result in reduction in energy intake and body weight. Vartanian et al11 performed a metaanalysis to examine the association between sweetened soft drink consumption, nutrition, and health outcomes. Mattes et al12 carried out a meta-analysis of sweetened beverage consumption and body weight. Malik et al13 conducted metaanalysis to compare the risk of sugar sweetened and APM sweetened beverages in causing type 2 diabetes. Nevertheless, none of these meta-analyses investigated the carcinogenicity of APM. Thus, we know of no review that specifically conducted a meta-analysis of the findings from all the APM carcinogenic bioassays. This meta-analytic review fills this gap. This review follows the PRISMA guidelines14 for reporting meta-analyses.

Methods Meta-analytic reviews are quantitative summaries of research domains that describe the magnitude, direction, and significance of the cumulative findings.15 Most importantly, metaanalysis has a potential to bring order to a set of inconsistent results from various studies,16 as is the case here. The wide variation in the results of previous APM carcinogenic studies motivated this meta-analytic review. The rest of this section presents details of the meta-analytic procedure executed for this review. Eligibility Criteria

Address correspondence to Sreekanth Mallikarjun, Department of Technology and Society, Stony Brook University, Stony Brook, NY 11794–3760, USA. E-mail: Sreekanth.Mallikarjun@ StonyBrook.edu

We searched for articles with no date restrictions (until 31 December 2012) and no particular language limits, as we intended to analyze all the APM carcinogenic bioassays conducted to date. Studies must be randomized controlled experiments. The

134 trials must contain at least 1 treatment group and have a control group to provide consistency across studies. The subjects in the bioassays needed to be either rats or mice. We only considered bioassays in which APM was administered as a standalone compound along with a regular ad libitum diet given to subjects orally. The outcome measure reported had to be the number of subjects who develop cancer (growth of malignant tumor[s] in any part of the subject’s body) during or at the end of the study duration.

Mallikarjun and Sieburth subjects (outcome variable) in each group are recorded. We considered data for the outcome variable—the number of rodents with malignant tumors. Malignant tumors are invasive cancers and can damage other tissues or organs around the tumor.17 Hence, to be consistent and obtain meaningful results, we considered the presence of malignant tumors as the outcome variable in this review. We record the data for the number of rodents that developed at least 1 malignant tumor(s) at the end of a particular bioassay for all studies.

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Information Sources and Search The following electronic databases were systematically searched with no time or language constraints: MEDLINE/PubMed, EMBase, EBSCOHost (all databases), Web of Science, Library of Congress, Google Scholar, and SciVerse Scoupus. The last search was executed on 2 January 2013. In addition, we hand searched FDA files for original APM drug approval studies. Finally, we scanned the dissertation database ProQuest Dissertations & Theses (PQDT) in an attempt to locate any APM- and cancer-related doctoral theses. We used the search term aspartame in conjunction with the following keywords within the electronic databases to search in the main fields of the studies: cancer, carcinogenic, carcinogenicity, neuro carcinogenic, neuro carcinogen, malignant, tumors, neoplasm, or neuro oncogenic. Study Selection APM is used as an artificial sweetener in several food products across the world. Thus, there were numerous articles related to APM in the health and food study literature sources. Most of the articles were either not original empirical findings or contained studies in which APM was administered in combination with other dry or liquid food products (eg, diet carbonated drinks, diet chewing gum, tooth paste, etc). There were some studies that evaluated APM’s effect on outcome variables such as body weight, body mass index, diabetes, and energy levels in humans. All such articles were not included in this review. All such studies were disregarded, as this review solely focused on original empirical findings from APM (administered alone in pure form) carcinogenic studies on rodents. For the same purpose, comments and opinion letters concerning published studies and reports were discarded. The retrieved studies were carefully read to ensure that they satisfy the above-mentioned eligibility criteria. The studies that met the eligibility criteria were checked for the operationalization of the outcome measure before inclusion in the meta-analysis. In case of uncertainty, a second reviewer (the second author) analyzed the studies and discrepancies were resolved by reaching consensus through tallying individual study coding forms and consulting a meta-analysis expert. The references of the selected articles were scanned to find additional relevant studies. Data Basic bioassay characteristics, including the type of subjects, bioassay duration, number of treatment groups, doses of APM given, sample size for each group, and number of affected

Summary Measures As the outcome variables in all the studies were naturally dichotomous (affected by malignant tumors or not), we used odds ratio (OR) as the effect size measure for our metaanalysis. First, effect sizes were computed for each individual study that was included in this meta-analysis. Second, the individual effect sizes were weighted by the inverse of their respective variance and averaged to obtain an aggregate effect size for the overall effect. We did this independently for 3 dose levels (regular, high, and very high) in each subgroup (males and females). We estimated 6 independent aggregate effect sizes, 1 for each of the 3 dose level categories within each of the 2 subgroups (males and females). An OR equal to 1 indicates no relationship. An OR greater than 1 specifies a positive relationship, indicating that rodents fed with APM were more likely to develop malignant tumors. An OR less than 1 specifies a negative relationship, indicating that rodents fed with APM were less likely to develop malignant tumors. An OR of 0.5 is equivalent to an OR of 2.0 (the inverse of 0.5) in the opposite direction.18 As a rule of thumb, ORs of 1.50, 2.50, and 4.30 approximately signify small, medium, and large effect sizes, respectively.19 All the bioassays were randomized controlled trials. We used random-effects models for estimation of aggregate effect sizes.20,21 Sampling error in random-effects models represents random variability at the study level and the subject level, which is essential in our case.18 In addition, the significance testing in random-effects models is based on the total number of studies included in the analysis and has increased generalizability of the results.15

Synthesis of Results In a meta-analysis, testing for the heterogeneity of effect sizes is vital.22 Thus, heterogeneity tests for all the aggregate effect sizes were carried out. In general, a heterogeneity test investigates whether the variability within the effect sizes are due to the subject level sampling error only, which is the assumption of a fixed-effects model. The corresponding test statistic is Cochran’s Q, which is distributed as a chi-square with (k − 1) degrees of freedom (df ), where k is the number of effect sizes.23 We also performed a secondary heterogeneity test— measure of inconsistency—and report the corresponding test statistic: I-squared [I 2 = 100% × (Q − df )/Q].24 I 2 does not depend on the number of studies that are included in the meta-analysis. I 2 represents the percentage of the disparity in

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Archives of Environmental & Occupational Health estimated effect sizes across the studies that is purely due to heterogeneity. However, there is some discrepancy in the interpretation of I 2 values. The PRISMA statement for reporting systematic reviews14 remarks that I 2 < 25% reflects low heterogeneity, whereas the Cochrane Handbook of Systematic Reviews25 states that I 2 < 40% means that there might not be critical heterogeneity. Nevertheless, any test for heterogeneity is not precise and meta-analysts are encouraged to conduct analyses of moderator variables even if effect sizes are not heterogeneous.16 Thus, moderator analyses were conducted using analogue to analysis of variance (ANOVA) for the 3 categorical moderator variables. We used a mixed-effects model for all the moderator analyses.26 A mixed-effects model uses a random-effects model to estimate effects within categories and a fixed-effects model to estimate effects between the categories.20 We used Comprehensive Meta Analysis (CMA) version 2.2 for Windows to estimate the effect sizes and conduct all the analyses.27 In general, CMA is versatile and produces results identical to STATA’s Metan, Metabias, and Metatrim.28

Results Study Selection After careful organization and review of the retrieved articles, 34 potentially eligible articles were preliminarily shortlisted. These did not include duplicates (not original findings), patient case studies, expert opinion papers, qualitative papers, and health magazine articles. After reading the abstracts of the shortlisted 34 articles, 5 articles were found that contained original empirical research findings. One of the articles consisted of 3 independent empirical studies. A manual search was executed based on the bibliography of the 5 selected articles and informal sources such as news and magazine articles. The search obtained 3 G.D. Searle & Company Laboratories’ premarket safety evaluation studies from the FDA register.1 These studies were considered gray literature because they were never published or are generally inaccessible. These studies were conducted during the 1970s by G.D. Searle & Company and did not comply with current standards for conducting carcinogenic bioassays.7 Nonetheless, they were included in this review, as they were authoritative and the FDA used them for the approval of APM.29 We were able to retrieve results and limited details of these studies through hand searching FDA files. In the process, we also found other APM bioassays on hamsters, dogs, and monkeys that were conducted by G.D. Searle & Company. We do not include them in this review, as they are either not carcinogenic studies or the subjects are not rodents. Finally, the dissertation database provided 216 results related to APM and cancer. Although a few studies contained original findings of APM’s effect on memory, anxiety, and neurological disorders in rats, no carcinogenic bioassays on rodents were found. The overall search process resulted in a total of 10 (N = 10) bioassays with 39 effects sizes for each sex. We only included bioassays on rodents in which APM was administered

135 as a standalone compound along with a regular ad libitum diet given to rodents orally. For transparency, we now briefly describe all the 10 bioassays that are included in this metaanalysis. G.D. Searle & Company discovered APM and they conducted 3 APM carcinogenic bioassays on rats prior to the approval of APM by the FDA.1 The 3 studies are E-33/34, E-70, and E-75. Study E-33/3430,31 was conducted on Sprague-Dawley rats from 4 to 104 weeks of age. The study consisted of 1 control group and 4 treatment groups with doses of 1,000, 2,000, 4,000, and 8,000 ppmi of APM feed per day. The control group consisted of 60 males and 60 females. The 4 treatment groups consisted of 40 males and 40 females in each group. The results were considered negative regarding the carcinogenicity of APM.1 Study E-7032 was conducted on Sprague-Dawley rats from prenatal life to 104 weeks of age. The study consisted of 1 control group and 2 treatment groups with doses of 2,000 and 4,000 ppm of APM feed per day. The control group consisted of 60 males and 60 females. The 2 treatment groups consisted of 40 males and 40 females in each group. The results were considered negative to the carcinogenicity of APM.1 Study E-7533 was conducted on CD1 mice (original group of Swiss mice) until 110 weeks of age. The study consisted of 1 control group and 3 treatment groups with doses of 1,000, 2,000, and 4,000 ppm of APM feed per day. The control group consisted of 72 males and 72 females. The 3 treatment groups consisted of 36 males and 36 females in each group. All results reported no indication of cancer with respect to APM administration.34 Ishii35 of Central Research Laboratories in Japan conducted an APM carcinogenic bioassay on Wister rats from 6 to 104 weeks of age. The study consisted of 1 control group and 3 treatment groupsii with doses of 1,000, 2,000, and 4,000 ppm of APM feed per day. The control group and 3 treatment groups consisted of 86 males and 86 females in each group. Later, Ishii et al9 provided more details regarding the bioassay.35 The results showed that APM does not cause cancer.9 The US National Toxicology Program (NTP) conducted 3 independent APM carcinogenic bioassays on 3 different types of mice from 6 to 40 weeks of age.36 We labeled these 3 bioassays as NTP-1, NTP-2, and NTP-3. Each of the 3 studies consisted of a control group and 5 treatment groups with doses of 3,125, 6,250, 12,500, 25,000, and 50,000 ppm of APM feed per day. Each of the groups in the 3 studies consisted of 15 males and 15 females. The 3 bioassays concluded that APM does not cause cancer.36 In contrast to the previous studies, the 3 APM carcinogenic bioassays conducted by the European Ramazzini Foundation of Oncology and Environmental Sciences in Italy showed that APM is carcinogenic. The first study7 was conducted on Sprague-Dawley rats from 8 weeks of age to natural death (the bioassay ended at 151 weeks). The study consisted of a control group and 5 treatment groups with doses of 80, 400, 2,000, 10,000, 50,000, and 100,000 ppm of APM feed per day. The control groups and each of the treatment groups (with doses 80, 400, and 2,000 ppm of APM feed per day) consisted of 150 males and

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136 150 females; the other 3 treatment groups consisted of 100 males and 100 females. The study showed that APM is a multipotential carcinogenic compound.7 The second study37 was conducted on Sprague-Dawley rats from the 12th day of fetal life to natural death (the bioassay ended at 147 weeks). The study consisted of a control group and 2 treatment groups with doses of 400 and 2,000 ppm of APM feed per day. The control group consisted of 95 males and 95 females. The 2 treatment groups consisted of 70 males and 70 females. The study showed that APM is carcinogenic and when consumed during fetal life, its carcinogenic effects increase.37 The third study38 was conducted on Swiss mice from prenatal life (12th day of gestation) to natural death (the bioassay ended at 130 weeks, less than 10% of mice were sacrificed). The study consisted of a control group and 4 treatment groups with doses of 2,000, 8,000, 16,000, and 32,000 ppm of APM feed per day. The control group consisted of 117 males and 102 females, whereas the treatment groups consisted of 62 to 122 males and females in each group. The results confirmed that APM is a carcinogenic agent in male mice, whereas no carcinogenic effects were observed in female mice.38

Study Characteristics All the selected 10 bioassays provide data for the outcome variable—the number of rodents with malignant tumors. Figure 1 shows the extracted data and essential study characteristics for all the APM carcinogenic studies. Subgroups and Moderators All studies reported results for males and females separately, as will be the mode in this review. Thus, we conducted separate meta-analyses for each of 2 subgroups—males and females. Additionally, we conducted 3 independent meta-analyses for 3 different dose level categories within each subgroup. Dose is an important treatment variable. Different dose levels can impact the effect of the treatment. Bioassays were conducted for a wide range of dose levels (80–100,000 ppm). Thus, we categorized dose levels into 3 categories—regular, with doses ranging from 2,000 to 3,125 ppm; high, with doses ranging from 4,000 to 10,000 ppm; and very high, with doses ranging from 32,000 to 50,000 ppm. To maintain independence of aggregated effect sizes, only 1 effect size per study should be selected in a meta-analysis.39 In our case, every study had more than 1 effect size, that is, more than 1 treatment group. All treatment groups had a common control group. To preserve independence, we selected not more than 1 effect size per study for each of the 3 separate meta-analyses. As shown earlier in Figure 1, there was no common dose level across all the studies. Hence, we chose an appropriate treatment group (or effect size) from each study within the 3 categories (regular, high, and very high). Table 1 shows the specific treatment group considered from the studies for each of 3 dose level categories. A dash implies that no effect size is available from that study for the respective dose level category.

Mallikarjun and Sieburth Table 1. Studies and Specific Treatment Groups Coding for the 3 Dose Level Categories Dose level categories Study E-33/34, 1974 E-70, 1974 E-75, 1974 Ishii, 1981 NTP-1, 2005 NTP-2, 2005 NTP-3, 2005 Soffritti et al, 2006 Soffritti et al, 2007 Soffritti et al, 2010

Regular

High

Very high

2,000 ppm 2,000 ppm 2,000 ppm 2,000 ppm 3,125 ppm 3,125 ppm 3,125 ppm 2,000 ppm 2,000 ppm 2,000 ppm

4,000 ppm 4,000 ppm 4,000 ppm 4,000 ppm 6,250 ppm 6,250 ppm 6,250 ppm 10,000 ppm — 8,000 ppm

— — — — 50,000 ppm 50,000 ppm 50,000 ppm 50,000 ppm — 32,000 ppm

Results of Individual Studies Figures 2 and 3 show confidence interval [CI] limits, relative weights, and forest plots for both individual effect sizes (ORs) and aggregate effect sizes (ORs) within the 3 dose levels (regular, high, and very high) for both females and males, respectively. Note that ORs of some studies such as E-75 (all doses and sexes),33 Ishii (2,000 ppm for males),35 and NTP-2 (3,125 ppm for males)36 are not defined, as their control and treatment group frequencies equal to zero. However, we could add 0.5 (or lower) to the frequencies to eliminate zeros and calculate ORs, but that would lead to a downward bias of estimates.40 Hence, we keep the undefined individual effect sizes off the analysis. The individual effect sizes ranged from 0.115 to 5.091 in 95% CI and relatively heavy weights were placed on the Soffritti et al studies.7,37,38 We saw no explicit outliers and hence no sensitivity analyses were carried out.

Synthesis of Results Aggregate effect sizes are reported for both random-effects and fixed-effects models. Overall, aggregate effect sizes ranged from 1.005 to 1.473 (random-effects estimates) in 95% CI. All aggregate effect sizes under random-effects models were not statistically significant (95% CI, p > .05); thus, we failed to reject the null hypothesis for both the sexes, which states that APM consumption does not increase the likelihood of malignant tumors in the rodents. Besides insignificance, the aggregate effect sizes were less than 1.50 (for random-effects models), which is the small-effect-size threshold suggested by Cohen,19 meaning that APM intake has a negligible effect on malignant tumor incidence in the subjects. Tables 2a and 2b show heterogeneity statistics for all aggregate effect sizes. There was a significant level of heterogeneity across the very high dose levels for females (Q(4) = 7.860, p < .10, I 2 = 49.11). There seemed to be a very mild heterogeneity across the high dose levels for males (Q(7) = 9.123, p > .10, I 2 = 23.37); however, there was no statistical significance.

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Fig. 1. Data for the 10 APM carcinogenic bioassays conducted on rodents. M = male and F = female. Numbers in bold represent the number of rodents at start of the bioassay in the particular group. Except for the last study (Soffritti et al, 2010), the rodents at the start of all bioassays in the particular group are same for both males and female. Numbers (not in bold) represent number of rodents with at least 1 malignant tumor(s) at the end of bioassay duration.

Moderator Analyses To investigate heterogeneity for the very high dose level for females, we conducted exploratory moderator analyses. We coded 3 study-level moderators—bioassay duration, bioassay sample size, and subject species type, within each of the 3 dose level categories (regular, high, and very high).

Bioassay duration. The duration of a bioassay can impact the effect of the treatment. Some bioassays are conducted for less than 40 weeks, whereas others last for more than 130 weeks. Truncating a carcinogenic bioassay after a short duration might fail to reveal a possible carcinogenic response to the drug.38 Thus, we considered bioassay duration as a moderator to see the effect of APM on carcinogenicity for different

Table 2. Results of Aggregate Effect Sizes With Heterogeneity Test Statistics for Female Subjects (a) and for Male Subjects (b) Regular Effect size Aggregate (random-effects) Aggregate (fixed-effects) Q(df ), p

I2

Very high

OR

p value

OR

p value

OR

p value

1.329 1.329

0.061 0.061

1.005 1.005

0.977 0.977

1.279 1.520

0.487 0.033

I2 Aggregate (random-effects) Aggregate (fixed-effects) Q(df ), p

High

5.747(8), p = .676

2.254(7), p = .944

7.860(4), p = .097

0

0

49.11

1.239 1.239

0.164 0.164

1.473 1.345

0.185 0.134

1.348 1.348

0.125 0.125

4.062(6), p = .668

9.123(7), p = .244

3.656(4), p = .455

0

23.274

0

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138

Mallikarjun and Sieburth

Fig. 2. Effect sizes, CI limits, relative weights, and forest plots for female subjects.

lengths of observations. We coded the bioassay duration moderator variable into 3 categories—short was 40 weeks, intermediate ranged from 104 to 110 weeks, and long ranged from 130 weeks to 151 weeks. Longer-duration bioassays (treatment duration) are likely to increase the possibility of finding malignant tumors in the rodents. Bioassay sample size. The sample size of a study represents the power of that study to reveal significant findings if they exist. In the 10 bioassays, the sample size ranges from 15 to 150. The current standard sample size for carcinogenic bioassays is at least 50 animals per each group and sex.38 Some bioassays meet this standard whereas others do not. Thus, we considered bioassay sample size as a second categorical moderator. We coded the bioassay sample size moderator variable into 2 categories (dichotomous)—small, with less than 50 subjects in the treatment group, and large, with 50 subjects or more in the treatment group. Subject species type.. This review considers bioassays on rodents only. The bioassays are conducted on 2 types of rodent species—rats and mice. We coded species type as a third categorical moderator variable (dichotomous) to see the effect of APM on carcinogenicity in the 2 rodent species.

As there were only 10 studies, each moderator was analyzed one at a time due to limited degrees of freedom. Table 3 shows coding of the 3 moderators for the 10 studies. Figure 4 shows the results of all 3 moderator analyses (mixed-effects estimates) within each of the 3 dose levels for both females and males along with the total Q values.41 Subject species type in very high dose levels for females was a Table 3. Coding for the 3 Categorical Moderator Variables Moderators Study E-33/34, 1974 E-70, 1974 E-75, 1974 Ishii, 1981 NTP-1, 2005 NTP-2, 2005 NTP-3, 2005 Soffritti et al, 2006 Soffritti et al, 2007 Soffritti et al, 2010

Bioassay duration Intermediate Intermediate Intermediate Intermediate Short Short Short Long Long Long

Bioassay sample size

Subject species type

Small Small Small Large Small Small Small Large Large Large

Rats Rats Mice Rats Mice Mice Mice Rats Rats Mice

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Fig. 3. Effect sizes, CI limits, forest plots, and relative weights for male subjects.

significant moderator (Q(1) = 6.496, p < .01). Rats showed significant increase in malignant tumor incidence (OR = 2.385, p < .05) when compared with mice. Thus, the species moderator seemed to explain significant heterogeneity across the effect sizes for very-high-dose females. Other moderators were not significant; the interaction of the moderator variables might

be a more appropriate way to conduct the analysis. But we cannot proceed further, as the number of studies was small (particularly for the very high dose level) and the degrees of freedom were limited. However, the moderator results were based on a small number of studies; therefore, the results may have some degree of validity.

Fig. 4. Results of moderator analyses and effect sizes for different levels of categorical moderators (mixed-effects model). A dash implies that no study is available for the respective moderator and dose level category.

140 Comment

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Limitations Similar to other typical meta-analyses, this meta-analysis also has certain limitations. Although we considered all the existing and relevant APM carcinogenic bioassays (published or unpublished) in this meta-analysis, the estimation of aggregate effect sizes is based on a small number of studies (N = 10). This suggests that more APM carcinogenic bioassays should be conducted and included in the future reviews. Another limitation is that our analysis considers only 1 outcome variable (the presence of malignant tumors). Other outcome variables such as the development of leukemia and lymphoma are not included in the analysis, as they were not reported by all the studies.30–33,35 Hence, the content validity of this analysis is limited, as cancer can occur in various forms and this review considers only 1, albeit very critical, form of cancer—malignant tumors.17 Meta-analytic procedure requires the analyst to make certain decisions on the coding of the studies and design of the analysis.18 As mentioned earlier, there was no common (or standard) dose level that was followed in all the bioassays (see Figure 1). Hence, we coded the dose levels into 3 categories. Thus, we were able to consider the maximum number of effect sizes from all the bioassays in our analysis without violating the assumption of independence. Despite these efforts, there is some heterogeneity across certain studies due to different species type and duration. We attempted to resolve it through moderator analyses, but there were limited degrees of freedom to conduct further exploratory analyses. Conclusions This article aimed to serve 2 main purposes: (1) conduct a survey and organize the findings of all the existing APM carcinogenic bioassays on rodents and (2) conduct an unbiased meta-analysis and document the aggregate results of all the existing APM carcinogenic bioassays on rodents, as the individual findings of the bioassays are inconsistent. This metaanalytic review estimated the unbiased aggregate effect sizes of all the existing APM carcinogenic bioassays. Based on the current body of scientific evidence, aggregate effect sizes revealed that APM consumed at any dose level has no significant relationship with cancer (occurrence of malignant tumors). However, these findings are based on a small number of studies and data. Thus, this review calls for more reliable APM carcinogenic control studies to be conducted under standardized trial settings (such as rodent types, duration, sample size, etc), experiment design, and treatment levels (dose). This will potentially develop better clarity concerning the relationship between APM and cancer and minimizes the heterogeneity in future reviews or synthesis.

Acknowledgments The authors are grateful to Dr Anne Moyer, Associate Professor, Department of Psychology at Stony Brook University,

Mallikarjun and Sieburth for her valuable suggestions on this analysis, by which this meta-analysis has been substantially improved. They are also grateful to the reviewers for their valuable advice and corrections in the paper. Studies preceded by an asterisk are included in the metaanalysis.

References 1. Food and Drug Administration. Aspartame: commissioner’s final decision. Docket no. 75F-0355. Fed Regist. 1981;46:38285–38308. 2. European Food Safety Authority. Opinion of the Scientific Panel AFC related to a new long-term carcinogenicity study on aspartame. Available at: http://www.efsa.europa.eu/en/science/afc/ afc opinions/1471.html EFSA EU, afc opinions, 1471 en. Published 2006. Accessed October 17, 2012. 3. Fry J. The world market for intense sweeteners. World Rev Nutr Diet. 1999;85:201–211. 4. Aspartame Information Center. Aspartame Information Center homepage. Available at: http://www.aspartame.org. Published 2005. Accessed November 27, 2012. 5. Mead MN. Sour finding on popular sweetener: increases cancer incidence associated with low-dose aspartame intake. Environ Health Perspect. 2006;114:A176. 6. Huff J., Ladou L. Aspartame bioassay findings portend human cancer hazards. Int J Occup Environ Health. 2007;4:446–448. 7. ∗ Soffiriti M, Belpoggi F, Esposti DD, Lambertini L, Tibaldi E, Lauriola M. First carcinogenic effects of aspartame administered in the feed to Spague-Dawley rats. Environ Health Perspect. 2006;114:379–385. 8. Huff J. Long-term chemical carcinogenesis bioassays predict human cancer hazards issues, controversies, and uncertainties. Ann N Y Acad Sci. 1999;895:56–79. 9. ∗ Ishii H, Koshimizu T, Usami S, Fujimoto T. Toxicity of aspartame and its diketopiperazine for Wistar rats by dietary administration for 104 weeks. Toxicology. 1981;21:91–94. 10. Hunty ADL, Gibson S, Ashwell M. A review of the effectiveness of aspartame in helping with weight control. Nutr Bull. 2006;31:115–128. 11. Vartanian LR, Schwartz MB, Brownell KD. Effects of soft drink consumption on nutrition and health: a systematic review and metaanalysis. Am J Public Health. 2007;97:667–675. 12. Mattes RD, Shikany JM, Kaiser KA, Allison DB. Nutritively sweetened beverage consumption and body weight: a systematic review and meta-analysis of randomized experiments. Obes Rev. 2011;12:346–365. 13. Malik VS, Popkin BM, Bray GA, Despres JP, Willett WC, Hu FB. Sugar-sweetened beverages and risk of metabolic syndrome and type 2 diabetes: a meta-analysis. Diabetes Care. 2010;33:2477– 2483. 14. Liberati A, Altman DG, Tetzlaff J, et al. The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions: explanation and elaboration. Ann Intern Med. 2009;151(4):W65–94. 15. Rosenthal R. Meta-analytic Procedures for Social Research. Newbury Park, CA: Sage; 1991. 16. Hunter JE, Schmidt FL. Methods of Meta-analysis: Correcting Error and Bias in Research Findings. Newbury Park, CA: Sage Publication; 1990. 17. Sobin LH, Fleming ID, eds. TNM Classification of Malignant Tumors. Chicago, IL: Union International Control of Cancer and the American Joint Committee on Cancer. 1997;80:1803–1804. 18. Lipsey MW, Wilson DB. Practical Meta-analysis. Thousand Oaks, CA: Sage Publications; 2001. 19. Cohen J. Statistical Power Analysis for the Behavioral Sciences. New York: Lawrence Erlbaum Associates; 1988.

Downloaded by [University of Aberdeen] at 23:51 21 May 2015

Archives of Environmental & Occupational Health 20. Borenstein M, Hedges LV, Higgins JPT, Rothstein H. Fixed-effect versus random-effects models. In: Introduction to Meta-analysis. Chichester, United Kingdom: John Wiley & Sons, Ltd.; 2009:77–85. 21. Schmidt FL, Oh IS, Hayes TL. Fixed- versus random-effects models in meta-analysis: model properties and an empirical comparison of differences in results. Br J Math Stat Psychol. 2009;62:97–128. 22. Hedges LV. A random effects model for effect sizes. Psychol Bull. 1983;93:388–395. 23. Hedges LV, Olkin I, Statistiker M. Statistical Methods for Metaanalysis. New York: Academic Press; 1985. 24. Higgins JPT, Thompson SG, Altman DG. Measuring inconsistency in meta-analyses. BMJ. 2003;327(7414):557–560. 25. Higgins JPT, Green S. Cochrane Handbook for Systematic Reviews of Interventions. Vol. 5. Chichester, UK: Wiley-Blackwell, 2008: 278. 26. Viechtbauer W. Bias and efficiency of meta-analytic variance estimators in the random-effects model. J Educ Behav Stat. 2005;30:261–293. 27. Borenstein M, Hedges L, Higgins J, Rothstein H. Comprehensive Meta-analysis, version 2.0 [computer software]. Englewood, NJ: Biostat; 2005 28. Bax L, Yu LM, Ikeda N, Moons K. A systematic comparison of software dedicated to meta-analysis of causal studies. BMC Med Res Methods. 2007;7:40. 29. Council of Scientific Affairs. Aspartame: a review of safety issues. JAMA. 1985;254:400–402. 30. ∗ File E-33. Appendix: Two Year Toxicity Study in the Rat. PT 838H71, submitted by G.D. Searle & Company to the FDA. Administrative Record, Aspartame. Docket No. 75F-0355, FDA, Rockville, Maryland, 1973. 31. ∗ File E-34. Two Year Toxicity Study in the Rat. PT 838H71, submitted by G.D. Searle & Company to the FDA. Administrative Record, Aspartame. Docket No. 75F-0355, FDA, Rockville, Maryland, 1973. 32. ∗ File E-70. Lifetime Toxicity Study in the Rat. PT 838H72, Final Report, submitted by G.D. Searle & Company to the FDA. Admin-

141

33.

34.

35. 36.

37.

38.

39. 40.

41.

istrative Record, Aspartame. Docket No. 75F-0355, FDA, Rockville, Maryland, 1974. ∗ File E-75. 104-Week Toxicity Study in the Mouse. PT 984H73, submitted by G.D. Searle & Company to the FDA. Administrative Record, Aspartame. Docket No. 75F-0355, FDA, Rockville, Maryland, 1974. Molinary SV. Preclinical studies of aspartame in nonprimate animals. In: Stegink LD, Filer LJ, eds. Aspartame Physiology and Biochemistry. New York: Dekker; 1984:289–306. ∗ Ishii H. Incidence of brain tumors in rats fed aspartame. Toxicol Lett. 1981;7:433–437. ∗ National Toxicology Program (NTP). Toxicology Studies of Aspartame (CAS No. 22839–47–0) in Genetically Modified (FVB Tg.AC Hemizygous) and B6.129 Cdkn2atm1Rdp (N2) Deficient Mice and Carcinogenicity Studies of Aspartame in Genetically Modified [B6.129-Trp53tm1Brd (N5) Haploinsufficient] Mice (Feed Studies). Research Triangle Park, NC: US National Toxicology Program. NTP GMM1, NIH Publication No. 06–4459. Available at: http://ntp.niehs.nih.gov/files/GMM1-Web.pdf. Published 2005. Accessed October 11, 2012. ∗ Soffiriti M, Belpoggi F, Tibaldi E, Esposti DD, Lauriola M. Lifespan exposure to low doses of aspartame beginning during prenatal life increases cancer effects in rats. Environ Health Perspect. 2007;115:1293–1297. ∗ Soffritti M, Belpoggi F, Manservigi M, et al. Aspartame Administered in Feed, Beginning prenatally through life span, induces cancers of the liver and lung in male Swiss mice. Am J Ind Med. 2010;52:1197–1206. Rosenthal R, Rubin DB. Comparing effect sizes of independent studies. Psychol Bull. 1982;92:500–504. Fleiss JL. Measures of effect sizes for categorical data. In: Cooper H, Hedges LV, eds. The Handbook of Research Synthesis. New York: Russell Sage Foundation; 1994:245–260. Overton RC. A comparison of fixed-effects and mixed (randomeffects) models for meta-analyses tests of moderator variable effects. Psychol Methods. 1998;3:354–379.

Aspartame and Risk of Cancer: A Meta-analytic Review.

Aspartame (APM) is the most commonly used artificial sweetener and flavor enhancer in the world. There is a rise in concern that APM is carcinogenic d...
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