Introduction
Much has been learned about carbohydrate digestion and absorption over the last 20 years, and this new knowledge has, in many ways, changed completely the way we think about dietary carbohydrates. We now know that starches are not completely digested, and, indeed, some are quite poorly digested. We have learned that the undigestible carbohydrates are not just neutral bulking agents, but have important physiologic effects, and even contribute energy to the diet. "Sugar" is not bad for health, and starches are not all equal in their effects on blood glucose and lipids. However, knowledge in all these areas is far from complete. In addition, there is unresolved controversy about how to define and how to measure dietary fibre and starch, and different methods are in use in different parts of the world. This presents a major challenge for those who have the responsibility of formulating policies and recommendations about dietary carbohydrates and how the energy value and carbohydrate composition of foods is determined.
Energy value of carbohydrates
Many different methods have been used to determine how much of the energy in foods is available for human metabolism, termed metabolizable energy (ME). The total amount of energy in a food (TE) can be determined by calorimetry, but ME is less than TE because not all the energy in food is absorbed and some is absorbed, but lost in the urine. Most of the energy not absorbed ends in the feces, but some is lost in the gases and heat produced during colonic fermentation.
The most common approach for determining the energy content of foods is the factorial method (68) in which the amount of energy contained in each of the various components of the food (ie. fat, protein, carbohydrate, alcohol) is calculated, and the sum of the resulting figures is taken as the amount of energy in the food. Determining the energy value of carbohydrate presents a conceptual challenge because carbohydrates vary in their gross energy content per gram, the degree to which they are digested and absorbed, and the fact that the undigestible carbohydrates provide an amount of energy which depends upon the degree to which they are fermented in the colon. This may vary from 0 to 100%. Alternative empirical models have been proposed based on regression equations developed from experiments where gross energy intake and energy excretion in urine and stool were measured on a variety of diets. Here, metabolizable energy in the diet is equal to gross energy intake minus energy losses, the latter being estimated from nitrogen and unavailable carbohydrate intakes. It has been argued that empirical models for determining the energy content of the diet are more accurate than the factorial approach because they have fewer and smaller sources of error (68). Nevertheless, it seems unlikely that the factorial approach will be replaced, at least in the near future, because it is ingrained in food labelling regulations and food tables.
Digestion and absorption of carbohydrates
Polysaccharides and oligosaccharides must be hydrolyzed to their component monosaccharides before being absorbed. The digestion of starch begins with salivary amylase, but this activity is much less important than that of pancreatic amylase in the small intestine. Amylase hydrolyzes starch, with the primary end products being maltose, maltotriose, and a -dextrins, although some glucose is also produced. The products of a -amylase digestion are hydrolyzed into their component monosaccharides by enzymes expressed on the brash border of the small intestinal cells, the most important of which are maltase, sucrase, isomaltase and lactase (69). With typical refined Western diets, carbohydrate digestion is rapid and carbohydrate absorption occurs primarily in the upper small intestine. This is reflected by the presence of finger-like villi in the mucosa of the upper small intestine, with wider and shorter villi in the lower half of the small intestine. However, carbohydrate digestion and absorption can occur along the entire length of the small intestine, and is shifted toward the ileum when the diet contains less readily digested carbohydrates, or when intestinal glucosidase inhibitors which may be used to treat diabetes are present. In this situation, the upper small intestine exhibits wide villous structures with leaf-like arrays, while in the ileum the villi become longer and more finger-like.
Monosaccharides
Only D-glucose and D-galactose are actively absorbed in the human small intestine. D-fructose is not actively absorbed, but has a rate of diffusion greater than would be expected by passive diffusion. The sodium dependent glucose transporter, SGLT1, is responsible for the active transport of glucose or galactose with an equimolar amount of sodium against a concentration gradient into the cytoplasm of the enterocyte. Fructose is taken up by facilitated transport by the glucose transporter 5 (GLUT5). Glucose is pumped out of the enterocyte into the intracellular space by the glucose transporter 2 (GLUT2) (70). The complete mechanism of fructose absorption in the human intestine is not understood. When fructose is given alone in solution, 40-80% of subjects have malabsorption, and some subjects can absorb less than 15g fructose. Flatulence and diarrhoea are common if doses of fructose over 50g are given by mouth. However, if fructose is given in combination with glucose or starch, fructose is completely absorbed, even in subjects who malabsorb fructose alone (71). Since fructose rarely occurs in the diet in the absence of other carbohydrates, fructose malabsorption is really only a problem for studies involving oral fructose loads.
Disaccharides
Intestinal brush border glucosidases tend to be inducible. For example, there is evidence that a high sucrose intake increases the postprandial insulin and the gastric inhibitory polypeptide responses to large loads of oral sucrose (72), which probably reflects an increased rate of absorption due to induction of intestinal sucrase activity. Lack of brush border glucosidases results in an inability to absorb specific carbohydrates. This occurs rarely, except for lactase deficiency which is common in non-Caucasian populations. The latter may be complete or partial and results in a reduced ability to digest and absorb lactose.
The Glycemic Index
The blood glucose responses of carbohydrate foods can be classified by the glycemic index (GI). The GI is considered to be a valid index of the biological value of dietary carbohydrates. It is defined as the glycemic response elicited by a 50g carbohydrate portion of a food expressed as a percent of that elicited by a 50g carbohydrate portion of a standard food (73). The glycemic response is defined as the incremental area under the blood glucose response curve, ignoring the area beneath the fasting concentration (i.e. the area beneath the curve) (74-76). The standard food has been glucose or white bread. If glucose is the standard, (ie. GI of glucose = 100) the GI values of foods are lower than if white bread is the standard by a factor of 1.38 because the glycemic response of glucose is 1.38 times that of white bread. GI values for several hundred foods have been published (77,78) (see Table 8).
The Glycemic Index and Mixed Meals
The validity of the GI has been the subject of much controversy, mostly because of supposed lack of application to mixed meals. Much of the controversy has been because of application of inappropriate methods to estimate the expected glycemic responses for mixed meals. When properly applied, the GI predicts, with reasonable accuracy, the relative blood glucose responses of mixed meals of the same composition but consisting of different carbohydrate foods (79).
Implications of the Glycemic Index
There are a number of long-term implications of altering the rate of absorption, or GI, of dietary carbohydrate. There is good evidence that reducing diet GI improves overall blood glucose control in subjects with diabetes (80) and reduces serum triglycerides in subjects with hypertriglyceridemia (81).
There is also some evidence that the glycemic index is relevant to sports nutrition and appetite regulation. Low GI foods eaten before prolonged strenuous exercise increased endurance time and provided higher concentrations of plasma fuels toward the end of exercise (82). However, high GI foods led to faster replenishment of muscle glycogen after exercise (83).
TABLE 8 Glycemic index of selected foods (continues on next page)
For further details log on website :
http://www.fao.org/docrep/W8079E/w8079e0k.htm#TopOfPage
Much has been learned about carbohydrate digestion and absorption over the last 20 years, and this new knowledge has, in many ways, changed completely the way we think about dietary carbohydrates. We now know that starches are not completely digested, and, indeed, some are quite poorly digested. We have learned that the undigestible carbohydrates are not just neutral bulking agents, but have important physiologic effects, and even contribute energy to the diet. "Sugar" is not bad for health, and starches are not all equal in their effects on blood glucose and lipids. However, knowledge in all these areas is far from complete. In addition, there is unresolved controversy about how to define and how to measure dietary fibre and starch, and different methods are in use in different parts of the world. This presents a major challenge for those who have the responsibility of formulating policies and recommendations about dietary carbohydrates and how the energy value and carbohydrate composition of foods is determined.
Energy value of carbohydrates
Many different methods have been used to determine how much of the energy in foods is available for human metabolism, termed metabolizable energy (ME). The total amount of energy in a food (TE) can be determined by calorimetry, but ME is less than TE because not all the energy in food is absorbed and some is absorbed, but lost in the urine. Most of the energy not absorbed ends in the feces, but some is lost in the gases and heat produced during colonic fermentation.
The most common approach for determining the energy content of foods is the factorial method (68) in which the amount of energy contained in each of the various components of the food (ie. fat, protein, carbohydrate, alcohol) is calculated, and the sum of the resulting figures is taken as the amount of energy in the food. Determining the energy value of carbohydrate presents a conceptual challenge because carbohydrates vary in their gross energy content per gram, the degree to which they are digested and absorbed, and the fact that the undigestible carbohydrates provide an amount of energy which depends upon the degree to which they are fermented in the colon. This may vary from 0 to 100%. Alternative empirical models have been proposed based on regression equations developed from experiments where gross energy intake and energy excretion in urine and stool were measured on a variety of diets. Here, metabolizable energy in the diet is equal to gross energy intake minus energy losses, the latter being estimated from nitrogen and unavailable carbohydrate intakes. It has been argued that empirical models for determining the energy content of the diet are more accurate than the factorial approach because they have fewer and smaller sources of error (68). Nevertheless, it seems unlikely that the factorial approach will be replaced, at least in the near future, because it is ingrained in food labelling regulations and food tables.
Digestion and absorption of carbohydrates
Polysaccharides and oligosaccharides must be hydrolyzed to their component monosaccharides before being absorbed. The digestion of starch begins with salivary amylase, but this activity is much less important than that of pancreatic amylase in the small intestine. Amylase hydrolyzes starch, with the primary end products being maltose, maltotriose, and a -dextrins, although some glucose is also produced. The products of a -amylase digestion are hydrolyzed into their component monosaccharides by enzymes expressed on the brash border of the small intestinal cells, the most important of which are maltase, sucrase, isomaltase and lactase (69). With typical refined Western diets, carbohydrate digestion is rapid and carbohydrate absorption occurs primarily in the upper small intestine. This is reflected by the presence of finger-like villi in the mucosa of the upper small intestine, with wider and shorter villi in the lower half of the small intestine. However, carbohydrate digestion and absorption can occur along the entire length of the small intestine, and is shifted toward the ileum when the diet contains less readily digested carbohydrates, or when intestinal glucosidase inhibitors which may be used to treat diabetes are present. In this situation, the upper small intestine exhibits wide villous structures with leaf-like arrays, while in the ileum the villi become longer and more finger-like.
Monosaccharides
Only D-glucose and D-galactose are actively absorbed in the human small intestine. D-fructose is not actively absorbed, but has a rate of diffusion greater than would be expected by passive diffusion. The sodium dependent glucose transporter, SGLT1, is responsible for the active transport of glucose or galactose with an equimolar amount of sodium against a concentration gradient into the cytoplasm of the enterocyte. Fructose is taken up by facilitated transport by the glucose transporter 5 (GLUT5). Glucose is pumped out of the enterocyte into the intracellular space by the glucose transporter 2 (GLUT2) (70). The complete mechanism of fructose absorption in the human intestine is not understood. When fructose is given alone in solution, 40-80% of subjects have malabsorption, and some subjects can absorb less than 15g fructose. Flatulence and diarrhoea are common if doses of fructose over 50g are given by mouth. However, if fructose is given in combination with glucose or starch, fructose is completely absorbed, even in subjects who malabsorb fructose alone (71). Since fructose rarely occurs in the diet in the absence of other carbohydrates, fructose malabsorption is really only a problem for studies involving oral fructose loads.
Disaccharides
Intestinal brush border glucosidases tend to be inducible. For example, there is evidence that a high sucrose intake increases the postprandial insulin and the gastric inhibitory polypeptide responses to large loads of oral sucrose (72), which probably reflects an increased rate of absorption due to induction of intestinal sucrase activity. Lack of brush border glucosidases results in an inability to absorb specific carbohydrates. This occurs rarely, except for lactase deficiency which is common in non-Caucasian populations. The latter may be complete or partial and results in a reduced ability to digest and absorb lactose.
The Glycemic Index
The blood glucose responses of carbohydrate foods can be classified by the glycemic index (GI). The GI is considered to be a valid index of the biological value of dietary carbohydrates. It is defined as the glycemic response elicited by a 50g carbohydrate portion of a food expressed as a percent of that elicited by a 50g carbohydrate portion of a standard food (73). The glycemic response is defined as the incremental area under the blood glucose response curve, ignoring the area beneath the fasting concentration (i.e. the area beneath the curve) (74-76). The standard food has been glucose or white bread. If glucose is the standard, (ie. GI of glucose = 100) the GI values of foods are lower than if white bread is the standard by a factor of 1.38 because the glycemic response of glucose is 1.38 times that of white bread. GI values for several hundred foods have been published (77,78) (see Table 8).
The Glycemic Index and Mixed Meals
The validity of the GI has been the subject of much controversy, mostly because of supposed lack of application to mixed meals. Much of the controversy has been because of application of inappropriate methods to estimate the expected glycemic responses for mixed meals. When properly applied, the GI predicts, with reasonable accuracy, the relative blood glucose responses of mixed meals of the same composition but consisting of different carbohydrate foods (79).
Implications of the Glycemic Index
There are a number of long-term implications of altering the rate of absorption, or GI, of dietary carbohydrate. There is good evidence that reducing diet GI improves overall blood glucose control in subjects with diabetes (80) and reduces serum triglycerides in subjects with hypertriglyceridemia (81).
There is also some evidence that the glycemic index is relevant to sports nutrition and appetite regulation. Low GI foods eaten before prolonged strenuous exercise increased endurance time and provided higher concentrations of plasma fuels toward the end of exercise (82). However, high GI foods led to faster replenishment of muscle glycogen after exercise (83).
TABLE 8 Glycemic index of selected foods (continues on next page)
GI*
|
n**
|
GI*
|
n**
| ||
BAKED GOODS
|
GRAINS
| ||||
| Cakes |
87±5
|
9
| Pearled barley |
36±3
|
4
|
| Cookies |
90±3
|
14
| Cracked barley |
72
|
1
|
| Crackers, wheat |
99±4
|
8
| Buckwheat |
78±6
|
3
|
| Muffins |
88±9
|
8
| Bulgur |
68±3
|
4
|
| Rice cakes |
123±6
|
2
| Couscous |
93±9
|
2
|
| Cornmeal |
98±1
|
3
| |||
BREADS
| |||||
| Barley kernel |
49±5
|
3
| Sweet corn |
78±2
|
7
|
| Barley flour |
95±2
|
2
| Millet |
101
|
1
|
| Rye kernel |
71±3
|
6
| Rice, white |
81±3
|
13
|
| Rye flour |
92±3
|
10
| Rice, low amylose |
126±4
|
3
|
| Rye crispbread |
93±2
|
5
| Rice, high amylose |
83±5
|
3
|
| White bread |
101±0
|
5
| Rice, brown |
79±6
|
3
|
| Whole-meal flour |
99±3
|
12
| Rice, instant |
128±4
|
2
|
| Other productsa |
100±4
|
5
| Rice, parboiled |
68±4
|
13
|
| Specialty rices |
78±2
|
5
| |||
BREAKFAST CEREALS
| Rye kernels |
48±4
|
3
| ||
| All bran |
60±7
|
4
| Tapioca |
115±9
|
1
|
| Cornflakes |
119±5
|
4
| Wheat keenelsa |
59±4
|
4
|
| Muesli |
80±14
|
4
| |||
| Oat bran |
78±8
|
2
|
DAIRY PRODUCTS
| ||
| Porridge oats |
87±2
|
8
| Ice cream |
84±9
|
6
|
| Puffed rice |
123±11
|
3
| Milk, whole |
39±9
|
4
|
| Puffed wheat |
105±3
|
2
| Milk, skim |
46
|
1
|
| Shredded wheat |
99±9
|
3
| Yogurt d |
48±1
|
2
|
| Other, GI³ 80b |
103±3
|
15
| Yogurt e |
27±7
|
2
|
| Other, GI<80c |
72±2
|
4
| |||
FRUIT
|
LEGUMES
| ||||
| Apple |
52±3
|
4
| Baked beans |
69±12
|
2
|
| Apple juice |
58±1
|
2
| Black-eyed peas |
59±12
|
2
|
| Apricots, dried |
44±2
|
2
| Butter beans |
44±3
|
3
|
| Apricots, canned |
91
|
1
| Chickpeas |
47±2
|
3
|
| Banana |
83±6
|
5
| Canned chickpeas |
59±1
|
2
|
| Banana, underripe |
51±8
|
2
| Haricot beans |
54±8
|
5
|
| Banana, overripe |
82±8
|
2
| Kidney beans |
42±6
|
7
|
| Kiwifruit |
75±8
|
2
| Kidney beans, canned |
74
|
1
|
| Mango |
80±7
|
2
| Lentils |
38±3
|
6
|
| Orange |
62±6
|
4
| Lentils, green |
42±6
|
3
|
| Orange juice |
74±4
|
3
| Lentils, green canned |
74
|
1
|
| Paw paw |
83±3
|
2
| Lima beans |
46
|
1
|
| Peach, canned |
67±12
|
3
| Peas, dried green |
56±12
|
2
|
| Pear |
54±4
|
4
| Peas, green |
68±7
|
3
|
| Other, GI<80f |
54±7
|
7
| Pinto beans |
61±3
|
3
|
| Other, GI³ 80g |
92±4
|
5
| Soya beans |
23±3
|
3
|
| Split peas, yellow |
45
|
1
| |||
PASTA
|
SNACKS
| ||||
| Linguine |
71±4
|
6
| Jelly beans |
114
|
1
|
| Macaroni |
64
|
1
| Lifesavers |
100
|
1
|
| Macaroni, boxed |
92
|
1
| Chocolate (various) |
84±14
|
2
|
| Spaghetti, white |
59±4
|
10
| Popcorn |
79
|
1
|
| Spaghetti, durum |
78±7
|
3
| Corn chips |
105±2
|
2
|
| Spaghetti, brown |
53±7
|
2
| Potato chips |
77±4
|
2
|
| Other |
59±3
|
8
| Peanuts |
21±12
|
3
|
POTATOES
|
SOUPS
| ||||
| Instant |
118±2
|
5
| Bean soups (various) |
84±7
|
4
|
| Baked |
121±16
|
4
| Tomato |
54
|
1
|
| New |
81±8
|
3
|
SUGARS
| ||
| White, boiled |
80±2
|
3
| Honey |
104±21
|
2
|
| White, mashed |
100±2
|
3
| Fructose |
32±2
|
4
|
| French fries |
107
|
1
| Glucose |
138±4
|
8
|
| Sweet potato |
77±11
|
2
| Sucrose |
87±2
|
5
|
| Yam |
73
|
1
| Lactose |
65±4
|
2
|
*GI = glycemic index (white bread = 100), mean ± SEM of mean values from various studies.
**Number of studies.a Bagel, stuffing mix, hamburger bun, rolls, melba toast.
b Bran buds, Bran chex, Cheerios, Corn bran, Corn chex. Cream of wheat, Crispix, Golden Grahams, Grapenuts, Grapenuts flakes, Life, Pro stars, Sustain, Team, Total (GI range, 83-127)
c Bran buds with psyllium, Red River, Special K (Australia), Sultana bran (Australia) (GI range 67-77)
d Sweetened with sugar
e Artificially sweetened
f Cherries, fruit cocktail, grapefruit, grapefruit juice, grapes, plum, pineapple juice
g Pineapple, raisins, rockmelon, sultanas, watermelon
For further details log on website :
http://www.fao.org/docrep/W8079E/w8079e0k.htm#TopOfPage
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