164
HOW MUCH DIETARY FAT IN
THERAPEUTIC NUTRITION? VLADO SIMKO, M.D. Associate Professor of Medicine SUNY Health Sciences Center Chief, Section of Gastroenterology Brooklyn Veteran's Administration Medical Center Brooklyn, New York
O UR INTEREST IN the tolerance of dietary fat originated in clinical experience with a middle aged man1 at risk of death by starvation because of insufficient intestinal absorption. After several small bowel resections for Crohn's disease, with less than 120 cm of intestine remaining, his mouth to stoma transit time of 30 minutes was too short to provide adequate contact of food with the absorbing surface. This marasmic but fully oriented patient refused total parenteral nutrition. In this frustrating situation we tested a wide variety of predigested diets available on the market. Laboratory tests of absorption of water, nitrogen, and fat showed some unexpected findings. Diets providing 80-108 g of fat per day were better tolerated than low-fat or fat-free diets traditionally recommended for the short gut syndrome. This patient retained more than 60% of the fat load usually ingested by normal individuals. How much fat can then be absorbed by a full length of a healthy small intestine? This unused reserve for absorption should be used more effectively in malnourished subjects who require intensive realimentation but who have all digestive functions
preserved. Good tolerance of dietary fat was later confirmed in several other patients with a postsurgical short gut.2-4 Simple calculation based on these clinical reports shows that if a bowel reduced to Y6 of its length absorbs 65 g fat per day, the intestinal capacity of the full length of the small bowel may exceed 400 g per day. However, concern about high fat intake remains. Usual recommendations for dietary treatment of a patient with the short bowel syndrome include a low fat diet.5 Of all nutrients, fat is considered by some authors6 to be the most difficult to digest. Clinicians usually judge the absorptive function of their Address for reprint requests: Section of Gastroenterology, Brooklyn V.A. Center, 800 Poly Place, Brooklyn, N.Y. 11209
Bull. N.Y. Acad. Med.
DIETARY FAT165 FAT
DIETARY
165
patients by the amount of unabsorbed fat in the stool rather than by how much fat the starving organism absorbs without side effects.7 This diagnostic rule dates back several decades, when it was concluded that "any degree of steatorrhea is indicative of organic disease affecting the normal function of the small intestine."8 This concept recommends reduction in dietary fat and an increase in carbohydrates in patients who excrete more than 7 g of fecal fat in 24 hours.5 Consequently, industry is hesitant to increase fat content of their dietary formulas. As long as a patient with steatorrhea has no side effects from increased fat propagated into his large bowel, attempts to bring the fecal fat to "normal" by reducing dietary fat are not in his best interest. Table I shows that of marketed diets, even those considered "high" in fat would not exceed 125 grams of fat daily, when the patient ingests 2,000 Kcal per day. Diets low in fat and high in carbohydrate inevitably result in a larger volume load and osmolality. Side effects, mostly diarrhea, not infrequently limit the intensity with which enteral alimentation can be initiated and maintained. In such situations high carbohydrate content of the diet may interfere with absorption of other nutrients.9 Table I also includes a high-fat diet designed and clinically tested in our experiments. 10-12 Table II indicates the composition of this high-fat diet which is liquid and nutritionally complete. The source of fat is either corn oil or soy oil. Clinical experience with this diet is described in a separate section. LIMITS OF INTRALUMINAL DIGESTION OF DIETARY FAT Conditions that may limit fat intake are related to intraluminal hydrolysis of triglycerides, 13-15 intestinal mucosal absorption, 16,17 mucosal reesterification of triglycerides, their systemic transport, 18-20 peripheral deposition of lipids and their utilization.21 Clinical manifestations of fat overload may consist of malabsorption, diarrhea, abnormal mucosal accumulation of lipids, hyperlipoproteinemia, platelet dysfunction, ketosis, electrolyte abnormalities, and dehydration. Lipolytic processes catalyzed by enzymes and assisted by bile acids convert dietary fat into more polar products which can favorably interact with intraluminal water. Micellar solubilization of fat increases diffusion through the unstirred water layer of the gut surface by at least one hundredfold when compared to diffusion of a simple fat molecule. 17 The reserve of bile and lipase seems adequate to allow much higher fat intake than usual. 13,22 Ten percent of intraduodenal lipase is adequate for hydrolysis of the habitual fat load.23 In addition to pancreatic lipase, other potent factors in fat hydrolysis are lingual and gastric lipases. 15,25 In dogs Vol. 66, No. 2, March-April 1990
166
V. SIMKO V.XIK
TABLE I. DIETS AVAILABLE ON THE MARKET WITH HIGHEST FAT CONTENT. HICAL IS A DIET USED IN OUR STUDIES Fat Carbohydrate % Cal giliter % Cal giliter Kcal/ml Diet 32 37 55 162 1.1 Ensure* 30 39 55 145 1.1 Osmolite* 90 40 43 217 2.0 TwoCal HN* 36 80 50 250 2.0 Magnacal** 40 91 45 224 2.0 Isocal HNt 55 92 28 105 1.5 Pulmocare* 67 223 25 189 3.0 HiCal DIET * Ross Laboratories, Columbus, OH ** Chesebrough Ponds, Greenwich, CT t Mead Johnson, Evansville, IN
TABLE II. COMPOSITION OF NUTRIENTS IN A HIGH-FAT, 3KCAL/ML, NUTRITIONALLY COMPLETE (HICAL) DIET
Calories Protein, g Fat, g Carbohydrate, g Vitamin A, IU Vitamin D, IU Vitamin E, IU Vitamin K, jig Thiamine, mg Riboflavin, mg Niacin, mg Folic acid, pLg Vitamin B6, mg Vitamin B12, Rg Ascorbic acid, mg Pantothenic acid, mg Biotin, pLg Calcium, mg Phosphorus, mg Iron, mg Magnesium, mg Zinc, mg Manganese, mg Potassium, mg Sodium, mg
3,000 63 223 189 2,772 227 42 140 2.2 2.5 28 227 2.8 9 170 14 168 560 560 10 227 17 2.8
1,175 672
with the pancreatic duct surgically excluded from entering the intestine,24 corn oil absorption remained 70% of normal. Enzymatic breakdown of dietary fat is further protected by colipase, 14 which acts as a lipase-stimulating factor and prevents bile acid-induced inhibition of lipase. Bull. N.Y. Acad. Med.
DIETARY FAT FAT DIETARY
167 167
Bile is not indispensable. More than 50% of dietary corn oil is absorbed in dogs with bile ducts diverted from the gut.24 Significant amounts of fat are also absorbed in patients with total biliary tract obstruction.26 There seems to be an enormous reserve for unicellar solubilization of dietary fat. THE PROCESS OF FAT ABSORPTION
How the fat molecule is transported across the intestinal luminal surface remains unclear. The concept of particular absorption of an unhydrolyzed molecule of fat,27 as well as the assumption of pinocytosis or microcytosis (membrane engulfment)28 are now of historical value, since they have never been confirmed by electron microscopy. It is assumed that fat absorption occurs by diffusion29 across an unstirred water layer, then through the mucus coating and finally across the microvillous membrane of the intestinal absorptive cell. A fatty acid binding protein has an important role in the transport of products of lipolysis from the cell membrane to the cytosol. 18 LIMITS OF INTESTINAL ABSORPTION OF FAT
We plotted a hypothetical relation between fat load and fat absorption (Figure 1) based on our clinical observations and on the reports of others. Using all these data, we anticipate that the rate of fat absorption in man remains at 90 to 95%, up to a fat intake of 1,000 g per day. However, an important limitation to fat intake may be the response of the large bowel to unabsorbed fat. The vast reservoir of absorptive capacity for fat is in the distal gut (Table III). This reserve compensates for sudden changes such as an increased dietary load or a shortened transit time.30 Borgstroem et al.31 confirmed that absorption of a complex meal is completed in the upper jejunum. Absorption of fat occurs more proximally than protein or carbohydrate. The proximal 25 cm of intestine absorbed 55% of the 30 g of emulsified fat, but only 16% of protein and 28% of carbohydrate.32 These observations have been confirmed by others.33,34 If the same rate of absorption occurs in the distal gut, Borgstroem's experiment applies to the full length of the human intestine, indicating an absorption potential of up to 400 g of fat from a single meal. Wollaeger35 reported that over a wide range of fat intake, the quantity of fecal fat is related to the amount ingested. There was a linear relation between intake and absorption for 20 to 360 g fat intake per day. Others made similar observations. 36,37 Vol. 66, No. 2, March-April 1990
V. SIMKO
168
V.
800 M
0
700K
(I) o 0
I 't
L
600
H
400 .° 0 ci 300o g
FFG~~~~~0 0
0
200
HOW MUCH DIETARY FAT IN
THERAPEUTIC NUTRITION? VLADO SIMKO, M.D. Associate Professor of Medicine SUNY Health Sciences Center Chief, Section of Gastroenterology Brooklyn Veteran's Administration Medical Center Brooklyn, New York
O UR INTEREST IN the tolerance of dietary fat originated in clinical experience with a middle aged man1 at risk of death by starvation because of insufficient intestinal absorption. After several small bowel resections for Crohn's disease, with less than 120 cm of intestine remaining, his mouth to stoma transit time of 30 minutes was too short to provide adequate contact of food with the absorbing surface. This marasmic but fully oriented patient refused total parenteral nutrition. In this frustrating situation we tested a wide variety of predigested diets available on the market. Laboratory tests of absorption of water, nitrogen, and fat showed some unexpected findings. Diets providing 80-108 g of fat per day were better tolerated than low-fat or fat-free diets traditionally recommended for the short gut syndrome. This patient retained more than 60% of the fat load usually ingested by normal individuals. How much fat can then be absorbed by a full length of a healthy small intestine? This unused reserve for absorption should be used more effectively in malnourished subjects who require intensive realimentation but who have all digestive functions
preserved. Good tolerance of dietary fat was later confirmed in several other patients with a postsurgical short gut.2-4 Simple calculation based on these clinical reports shows that if a bowel reduced to Y6 of its length absorbs 65 g fat per day, the intestinal capacity of the full length of the small bowel may exceed 400 g per day. However, concern about high fat intake remains. Usual recommendations for dietary treatment of a patient with the short bowel syndrome include a low fat diet.5 Of all nutrients, fat is considered by some authors6 to be the most difficult to digest. Clinicians usually judge the absorptive function of their Address for reprint requests: Section of Gastroenterology, Brooklyn V.A. Center, 800 Poly Place, Brooklyn, N.Y. 11209
Bull. N.Y. Acad. Med.
DIETARY FAT165 FAT
DIETARY
165
patients by the amount of unabsorbed fat in the stool rather than by how much fat the starving organism absorbs without side effects.7 This diagnostic rule dates back several decades, when it was concluded that "any degree of steatorrhea is indicative of organic disease affecting the normal function of the small intestine."8 This concept recommends reduction in dietary fat and an increase in carbohydrates in patients who excrete more than 7 g of fecal fat in 24 hours.5 Consequently, industry is hesitant to increase fat content of their dietary formulas. As long as a patient with steatorrhea has no side effects from increased fat propagated into his large bowel, attempts to bring the fecal fat to "normal" by reducing dietary fat are not in his best interest. Table I shows that of marketed diets, even those considered "high" in fat would not exceed 125 grams of fat daily, when the patient ingests 2,000 Kcal per day. Diets low in fat and high in carbohydrate inevitably result in a larger volume load and osmolality. Side effects, mostly diarrhea, not infrequently limit the intensity with which enteral alimentation can be initiated and maintained. In such situations high carbohydrate content of the diet may interfere with absorption of other nutrients.9 Table I also includes a high-fat diet designed and clinically tested in our experiments. 10-12 Table II indicates the composition of this high-fat diet which is liquid and nutritionally complete. The source of fat is either corn oil or soy oil. Clinical experience with this diet is described in a separate section. LIMITS OF INTRALUMINAL DIGESTION OF DIETARY FAT Conditions that may limit fat intake are related to intraluminal hydrolysis of triglycerides, 13-15 intestinal mucosal absorption, 16,17 mucosal reesterification of triglycerides, their systemic transport, 18-20 peripheral deposition of lipids and their utilization.21 Clinical manifestations of fat overload may consist of malabsorption, diarrhea, abnormal mucosal accumulation of lipids, hyperlipoproteinemia, platelet dysfunction, ketosis, electrolyte abnormalities, and dehydration. Lipolytic processes catalyzed by enzymes and assisted by bile acids convert dietary fat into more polar products which can favorably interact with intraluminal water. Micellar solubilization of fat increases diffusion through the unstirred water layer of the gut surface by at least one hundredfold when compared to diffusion of a simple fat molecule. 17 The reserve of bile and lipase seems adequate to allow much higher fat intake than usual. 13,22 Ten percent of intraduodenal lipase is adequate for hydrolysis of the habitual fat load.23 In addition to pancreatic lipase, other potent factors in fat hydrolysis are lingual and gastric lipases. 15,25 In dogs Vol. 66, No. 2, March-April 1990
166
V. SIMKO V.XIK
TABLE I. DIETS AVAILABLE ON THE MARKET WITH HIGHEST FAT CONTENT. HICAL IS A DIET USED IN OUR STUDIES Fat Carbohydrate % Cal giliter % Cal giliter Kcal/ml Diet 32 37 55 162 1.1 Ensure* 30 39 55 145 1.1 Osmolite* 90 40 43 217 2.0 TwoCal HN* 36 80 50 250 2.0 Magnacal** 40 91 45 224 2.0 Isocal HNt 55 92 28 105 1.5 Pulmocare* 67 223 25 189 3.0 HiCal DIET * Ross Laboratories, Columbus, OH ** Chesebrough Ponds, Greenwich, CT t Mead Johnson, Evansville, IN
TABLE II. COMPOSITION OF NUTRIENTS IN A HIGH-FAT, 3KCAL/ML, NUTRITIONALLY COMPLETE (HICAL) DIET
Calories Protein, g Fat, g Carbohydrate, g Vitamin A, IU Vitamin D, IU Vitamin E, IU Vitamin K, jig Thiamine, mg Riboflavin, mg Niacin, mg Folic acid, pLg Vitamin B6, mg Vitamin B12, Rg Ascorbic acid, mg Pantothenic acid, mg Biotin, pLg Calcium, mg Phosphorus, mg Iron, mg Magnesium, mg Zinc, mg Manganese, mg Potassium, mg Sodium, mg
3,000 63 223 189 2,772 227 42 140 2.2 2.5 28 227 2.8 9 170 14 168 560 560 10 227 17 2.8
1,175 672
with the pancreatic duct surgically excluded from entering the intestine,24 corn oil absorption remained 70% of normal. Enzymatic breakdown of dietary fat is further protected by colipase, 14 which acts as a lipase-stimulating factor and prevents bile acid-induced inhibition of lipase. Bull. N.Y. Acad. Med.
DIETARY FAT FAT DIETARY
167 167
Bile is not indispensable. More than 50% of dietary corn oil is absorbed in dogs with bile ducts diverted from the gut.24 Significant amounts of fat are also absorbed in patients with total biliary tract obstruction.26 There seems to be an enormous reserve for unicellar solubilization of dietary fat. THE PROCESS OF FAT ABSORPTION
How the fat molecule is transported across the intestinal luminal surface remains unclear. The concept of particular absorption of an unhydrolyzed molecule of fat,27 as well as the assumption of pinocytosis or microcytosis (membrane engulfment)28 are now of historical value, since they have never been confirmed by electron microscopy. It is assumed that fat absorption occurs by diffusion29 across an unstirred water layer, then through the mucus coating and finally across the microvillous membrane of the intestinal absorptive cell. A fatty acid binding protein has an important role in the transport of products of lipolysis from the cell membrane to the cytosol. 18 LIMITS OF INTESTINAL ABSORPTION OF FAT
We plotted a hypothetical relation between fat load and fat absorption (Figure 1) based on our clinical observations and on the reports of others. Using all these data, we anticipate that the rate of fat absorption in man remains at 90 to 95%, up to a fat intake of 1,000 g per day. However, an important limitation to fat intake may be the response of the large bowel to unabsorbed fat. The vast reservoir of absorptive capacity for fat is in the distal gut (Table III). This reserve compensates for sudden changes such as an increased dietary load or a shortened transit time.30 Borgstroem et al.31 confirmed that absorption of a complex meal is completed in the upper jejunum. Absorption of fat occurs more proximally than protein or carbohydrate. The proximal 25 cm of intestine absorbed 55% of the 30 g of emulsified fat, but only 16% of protein and 28% of carbohydrate.32 These observations have been confirmed by others.33,34 If the same rate of absorption occurs in the distal gut, Borgstroem's experiment applies to the full length of the human intestine, indicating an absorption potential of up to 400 g of fat from a single meal. Wollaeger35 reported that over a wide range of fat intake, the quantity of fecal fat is related to the amount ingested. There was a linear relation between intake and absorption for 20 to 360 g fat intake per day. Others made similar observations. 36,37 Vol. 66, No. 2, March-April 1990
V. SIMKO
168
V.
800 M
0
700K
(I) o 0
I 't
L
600
H
400 .° 0 ci 300o g
FFG~~~~~0 0
0
200
