OF NUTRITION, HEALTH & AGING©
4.1 Carbohydrates and health
4.1.1 Carbohydrates and blood sugar
Carbohydrates are the sources of glucose but different carbohydrate-contai-ning foods do not increase blood sugar levels to the same degree. The distinc-tion between “slow sugars” and “rapid sugars” was used for a long time. In the 1970-1980s, the analysis of the hyperglycemic power of carbohydrate-contai-ning foods gave rise to the concept of the «glycemic index» (GI).
The GI enables foods to be ranked as a function of their hyperglycemic effects in comparison to a reference sugar. The reference sugar is either glu-cose (in aqueous solution) or white bread starch (FOSTER-POWELLand BRAND -MILLER, 1995; BRAND-MILLER, 1997). The GI is the ratio of the area under the curve of blood sugar levels for the food tested over that obtained with the refe-rence sugar. The conditions of measurement and interpretation have been stan-dardized.
The GI of the food is primarily dependent on its composition in simple and complex carbohydrates. The GI of glucose is high, that of fructose is low and that of sucrose in intermediate (FOSTER-POWELL and BRAND-MILLER, 1995;
BRAND-MILLER, 1997). The effects of starch depend on the proportion of amylo-pectin and amylose that in turn depend on its botanical origin. The higher the amylose content (15 to 30% for cereals, 17 to 22% for tubers, 30 to 66% for leguminous plants), the lower the GI. The presence of lipids tends to reduce the GI, which is the case for chocolate, potato chips or ice cream, for example.
Vis-cous dietary fibers such as pectins (fruits and vegetables) or guar gum and anti-nutritive substances (phytates, lectins, saponins and tannins) limit the hypergly-cemic effect of carbohydrates by decreasing the action of digestive enzymes.
Mechanical treatments modify the physical form of the food and thus its bio-availability. The smaller the particle, the higher the GI. Hydrothermal treatments change the structure of starch and make it more easily hydrolysable by amy-lases. Thus, the GI of gelatinized starch in instant mashed potatoes increase the GI. Other processes such as bread making, on the other hand, reduce the GI by limiting the action of amylases. This is the case for the gelatinized starch of durum wheat semolina, which remains encapsulated in a mass of gluten.
The total GI of a meal can be calculated using the GI values of its constituent foods, but the presence of other macronutrients should also be taken into account. Finally, most studies have been short-term, and even on a spot basis.
Adaptation mechanisms could annul dietary contributions made on the basis of GIs.
The concept of GI can be useful in the study of mechanisms of satiety or of the action of insulin. Several authors have also reported than insulin indices generally vary in the same direction as GI values. Athletes are recommended to eat foods with a low GI within several hours before an event and to choose those with a high GI during the activity or in the recovery phase to better reconstitute glycogen reserves (see below). The concept of the GI applied to diabetics limits abusive prohibitions concerning the consumption of sweet foods, since the hyperglycemic power of sucrose is lower than that of bread and the addition of sugar to natural or manufactured products generally does not cause in increase of the GI. Finally, foods with a low GI could be of value in preventing type 2 diabetes in high-risk subjects.
The practical application of the concept of GI is relatively complicated.
Some foods with a low GI are not to be recommended because they are rich in lipids. Others have a high GI (e.g. carrots) but have a low carbohydrate content and thus a low hyperglycemic effect.
4.1.2 Carbohydrates and satiety
The energy content of a food is the most significant determinant of its satiety-inducing capacity (BLUNDELLet al., 1994). Satiety is a behavioral pheno-menon that is a proportional response to a nutritional pre-load and ensures energy regulation. Carbohydrates, whether or not sweet, act on satiety to an extent that depends on their energy content.
The time line of the satiety effect differs depending on the nutrient in ques-tion. Satiety caused by carbohydrates is apparently maximal immediately after a meal and during the following several hours. The satiety-inducing capacity of carbohydrates occurs earlier than that of lipids, that becomes more evident several hours (8 to 12) after a meal. The early effect of carbohydrates results from a number of factors: rapid gastric emptying, physicochemical simplicity of some sugars, etc. It is not known whether the satiety-inducing capacity of sweet carbohydrates is more or less than that of starches, or whether fructose is more filling than sucrose because of its hepatic metabolism.
Studies have suggested that satiety is inversely proportional to the glycemic index of a food. In particular, according to this hypothesis, satiety lasts longer
as the postprandial insulin secretion peak is low, and as the hyperglycemic effect is low and prolonged (BLUNDELL et al., 1994). There is in fact a positive correlation between blood sugar levels and satiety for values close to basal levels (HOLTand MILLER, 1995).
Fibers increase the sensation of satiety of carbohydrates. Passage of carbo-hydrates in the digestive tract triggers the secretion of several hormones, in par-ticular glucagon whose satiety-inducing role has been proven. In addition, a carbohydrate-rich diet and thus relatively depleted in lipids accounts for a large volume of food that, at least in the short term, can increase satiety and modify certain digestive functions.
4.1.3 Carbohydrates and physical activity
Skeletal muscle oxidizes not only glucose but also fatty acids and branched chain amino acids (LEVERVE et al., 1996). Fatty acids, the practically exclusive energy substrate of resting muscle, are also consumed during exercise, espe-cially if it is prolonged and/or of moderate effort. The contribution of glucose oxidation to the energy supply increases with the intensity of exercise. For intensities close to maximal aerobic intensity (VO2max), glucose from the blood-stream (especially muscle glycogen) becomes the exclusive energy substrate of aerobic metabolism of skeletal muscle. At intensities higher than VO2max, anae-robic glycolysis — whose main substrate is glucose obtained from muscle gly-cogen, contributes to the energy supply and whose proportion increases as the exercise — is more intense. Thus, even if skeletal muscle is not strictly glucose-dependent, it can work at high intensities only by using the glucose/glycogen pair in high proportions, even uniquely. This is why performance during prolon-ged (60-90 min and more) and intense efforts depends on the initial level of muscle glycogen reserves (WILKINSONand LIEBMAN, 1998). It can thus be impro-ved by ingesting carbohydrates immediately before or during the exercise (COG -GANand COYLE, 1991). On the other hand, increasing muscle glycogen reserves has no apparent beneficial effect for performance in very short and very intense exercises (WILKINSONand LIEBMAN, 1998).
The organism has only a low capacity for producing glucose from amino acids and glycerol (probably less than 0.1 g·min–1; DIETZE, 1982). Such low capa-city does not lead to the constitution of large liver or muscle glycogen reserves during quiescent periods or to the production of the quantity of circulating glu-cose used by the muscle during a high intensity effort (> 1 g·min–1; KATZ et al., 1986). The ingestion of carbohydrates is thus necessary to constitute good muscle and liver glycogen reserves. Ingested carbohydrates are easily oxidized, reducing the quantities available for glycogen synthesis. In order to build up gly-cogen reserves before an endurance exercise, the dietary carbohydrate supply (primarily in the form of starch) should account for a high energy percentage in the diet (70-90% of energy) and high absolute value (8-10 g·kg–1·d–1vs. about 4 g·kg–1·d–1 for a sedentary subject) (WILKINSONand LIEBMAN, 1998).
The ingestion of carbohydrates within the hour preceding the effort (50-60 g) and during the effort itself (0.75 to 1.2 g·min–1) can delay the onset of fatigue, especially if the effort continues for more than one hour (COGGANand COYLE, 1991). Glucose, maltose or even better glucose polymers, generally given in solution in water, are preferred to fructose or galactose, that are less rapidly available for oxidation (LEIJSSEN et al., 1995; MASSICOTTE et al., 1986). In
contrast to longstanding conventional wisdom, it is not contraindicated to ingest carbohydrates immediately before an exercise. Reaction hypoglycemia during effort is not a frequent phenomenon (BROUNS et al., 1989) and can be prevented by ingesting carbohydrates during exercise.
It is recommended to ingest carbohydrates immediately after the effort to favor glycogen resynthesis (about 50 g of carbohydrates per hour until the first meal following the exercise). Carbohydrates with a high glycemic index are pre-ferred, since they cause a higher response of blood sugar and circulating insulin levels, that favor the storage of glycogen reserves (MURRAY and HORSWILL, 1998). To the same end, carbohydrates should be favored in the meal following the exercise.
4.1.4 Carbohydrates and the colon
A variable fraction of some dietary carbohydrates (primarily supplied by die-tary fibers and resistant starches) escapes digestion-absorption in the small intestine of healthy humans (FLOURIE, 1992). These carbohydrates are almost totally digested by bacteria of the colon flora (MCFARLANE and CUMMINGS, 1991). The dominant flora is composed of obligate anaerobic bacteria that hydrolyze carbohydrates with a particular metabolism, yielding short chain fatty acids (acetic, propionic and butyric acids) and gases (carbon dioxide, hydrogen and methane). A fraction of these metabolites is used by the colon bacteria themselves. Another is eliminated in the stools or as gas. The majority of the metabolites are absorbed and it is admitted that these carbohydrates fermented in the colon provide an energy supply of 2 kcal·g–1 or about 5% of the daily energy supply in a healthy subject eating a Western diet.
The effect of indigestible carbohydrates on colon physiology depends on their nature and the quantities ingested. Mono-, di- and oligosaccharides (fruc-tose, lac(fruc-tose, sugar-alcohols) have a high osmotic power. They retain water and electrolytes in the intestinal lumen proportional to the quantity ingested, leading to an acceleration of intestinal transit. The functions of the colon (capacitance, fermentation, resorption of water and electrolytes) can be overwhelmed by the inflow of carbohydrates, water and electrolytes from the small intestine, resul-ting in diarrhea. This happens when healthy subjects who cannot digest milk (lactase deficit) absorb a large quantity of milk, but the intensity of the effect varies with the individual (FLOURIEet al., 1987). The occurrence of diarrhea has also been reported in children after the excessive ingestion of beverages contai-ning fructose or sugar-alcohols. Inversely, if the supply of carbohydrates, water and electrolytes from the small intestine is lower, colon bacteria will have the time to degrade carbohydrates to short chain fatty acids. They are absorbed in the colon and lead to the absorption of water and electrolytes, preventing diar-rhea. Whether or not diarrhea will occur thus depends on the balance between the quantity of carbohydrates ingested and the capacity of the colon to ferment them. The bacterial flora can adapt to the presence of carbohydrates in the colon, explaining the low prevalence of diarrhea in subjects who regularly consume low molecular weight indigestible carbohydrates (FLORENTet al., 1985;
HILL, 1983). When the incompletely digested carbohydrate is a polysaccharide (resistant starches and dietary fibers), the volume of water retained is low and depends on the water retention capacity of the ingested carbohydrate. Transit in the small intestine is unchanged or only slightly accelerated and the
polysac-charide arriving in the colon will be fermented. The clinical consequences of consuming an indigestible polysaccharide are pain and meteorism (abdominal ballooning), as well as excessive flatulence. These symptoms are explained by an increased production of fermentation gases and their intensity depends on both individual capacity of the flora to produce gases and individual factors of tolerance to gas distensions. Finally, it is also possible that adaptation pheno-mena occur and that the intensity of symptoms decreases when polysaccharide consumption is prolonged.
The insufficient supply of indigestible carbohydrates is the main cause of chronic, so-called idiopathic constipation, which involves 10% of the French population. Its treatment involves increasing the quantity of carbohydrates in the diet, since stool weight depends on them (CUMMINGSet al., 1992). An addi-tional daily supply of 10 to 20 g of indigestible carbohydrates is required to return decreased transit to normal levels.
Some indirect arguments strongly suggest that insufficient indigestible car-bohydrates in the diet participate in the occurrence of colon cancer (leading cancer in France with 35,000 new cases every year) and colonic diverticulosis that afflicts half the population after the age of 80.
In conclusion, for the “health” of the colon, it can only be recommended to increase the dietary supply of indigestible carbohydrates in order to render tran-sit regular and avoid certain colon pathologies. Based on epidemiological stu-dies showing a decrease in the relative risk above an intake of 25 g·d–1, the recommendation is to reach this value and, if possible 30 g·d–1. For children, the recent American proposal of “age + 5” g·d–1 is appropriate. Practical recom-mendation is to vary fiber sources using fiber-rich foods, such as fruits, vege-tables, cereals and leguminous. A diet too rich in indigestible carbohydrates, however, especially with purified fibers, can lead to occasionally unpleasant symptoms, often transitory and whose intensity depends on the state of adap-tation of colon bacterial flora.
4.1.5 Carbohydrates and body weight
In addition to their effects on satiety (see above), the metabolic particularities of carbohydrates have several theoretical advantages over lipids in the preven-tion of weight gain and obesity. The storage compartment is very small (0.5 to 1 kg maximum of liver and muscle glycogen) and the energy expenditure for storage is about 10% of carbohydrate energy in vivo. They are oxidized rapidly and practically proportionally to the quantities ingested up to a maximum of 250 mg·min–1at rest, but can exceed 5 to 6 g·min–1 during intense effort (see above). There is little transformation of carbohydrates to lipids (de novo lipoge-nesis), even if 500 g of carbohydrates are consumed in a single meal, and the energy cost for this transformation is high (about 25% of carbohydrate energy ingested in vivo). Nevertheless, there are no epidemiological data to substan-tiate that for the same level of energy supply and physical activity, a carbohy-drate-rich and lipid-poor diet has any advantage in preventing weight gain and obesity compared to a lipid-rich and carbohydrate-poor diet (see WILLETT, 1998 for review). The results of randomized nutritional studies have shown that the relative increase of daily carbohydrates leads to modest weight losses that often are non-significant. Weight and mass losses resulting from a hypocaloric diet (4.2 to 5.0 MJ·d–1) do not increase when the contribution of carbohydrates
to energy supplies increases from 25 to 75% at the expense of lipids (GOLAYet al., 1996). In a situation of excessive supplies, excess energy is stored regard-less of the quantities of carbohydrates and/or lipids in the excess energy (HOR -TON et al., 1995). Nevertheless, when the excess energy is provided by carbohydrates (75-85%) rather than lipids (90-95%), the proportion of energy stored is lower, at least during the first days of the imbalance. It is thus impor-tant to bear in mind that any excess energy supply, whether it be from carbohy-drates, lipids or mixed, leads to weight gain when it is not compensated by increased energy expenditures, e.g. more physical activity.
The differences in energy density between carbohydrates (4 kcal·g–1) and lipids (9 kcal·g–1) give the former a potential advantage in the regulation of the energy balance and body weight. The quantities of ingested energy increase with the energy density of the diet (see PRENTICE, 1998 for review). In addition, the energy density is lower as the quantity of carbohydrates per gram of food ingested is higher and that of lipids reduced. BELLet al. (1998) showed that the energy density of the diet is an important determinant in energy supplies to humans. This finding was based on the observation that the weight of ingested food is perfectly reproducible in the same subject for diets whose energy den-sity varies from 1.02 to 1.34 kcal·g–1. Carbohydrates thus reduce the quantity of energy ingested by providing a lower energy density to the diet. This effect is indirect, since the quantities of energy spontaneously ingested do not vary for carbohydrate levels between 45 and 65% of total energy once the energy den-sity of the diet is held constant (STUBBSet al., 1996).
4.1.6 Carbohydrates and mental functions
Ingesting carbohydrates has no apparent effect on the behavior and cogni-tive functions of children (WOLRAICH et al., 1995). Their beneficial effect on memory, on the other hand, is incontestable. Thus, POLLITTet al. (1998) showed that performance in memory tests and discrimination of stimuli carried out in children in the late morning were higher if they had eaten a breakfast rich in car-bohydrates instead of not eating at all, undoubtedly because of the better stabi-lity of blood sugar levels.
The effect of carbohydrates on memory, attention span and cognitive func-tions of young adults is a controversial subject (KOROLand GOLD, 1998). In the elderly, the deterioration of glucose metabolism is apparently correlated with progressive deficits of mental functions. Glucose improves performance in memory function tests to a greater extent than in children and adults (MESSIER and GAGNON, 1996; MESSIER et al., 1997). Improvement vs. dose administered follows an inverted U-shaped curve with a maximum effect at the dose of 25 g (PARSONSand GOLD, 1992). Some non-memory cognitive functions may also be improved (ALLENet al., 1996). The mechanisms of action by which the adminis-tration of glucose improves some mental functions including memory are poorly understood. Glucose could stimulate the synthesis or release of acetylcholine in some regions of the brain, such as the hippocampus (KOROLand GOLD, 1998).
4.1.7 Carbohydrates and sleep
The beneficial effect of the simultaneous administration of glucose and insu-lin on the quality of sleep has been observed in rats (ischymetric theory of DAN -GUIR, 1980). Injecting glucose improves paradoxical (REM) sleep that normally is
degraded in rats during the aging process (STONEet al., 1992). The effect is greater when acetylcholine metabolism in the hippocampus has deteriorated.
This was not observed in young rats. Data on the beneficial role of dietary car-bohydrates on human sleep remain to be confirmed.
4.1.8 Carbohydrates and altitude
Dietary carbohydrates can be useful at high altitudes where altitude sickness is a frequent occurrence. Symptoms are nausea and a loss of appetite, espe-cially for lipid-rich foods. If this condition continues and if it is combined with increased energy expenditures, it contributes to a loss of weight, especially lean mass, that is a frequent consequence of prolonged exposure to high altitude (ASKEW, 1998). Supplementing with carbohydrates, better accepted by the body than lipids (ROSE et al., 1988), can reduce the symptoms of altitude sickness (CONSOLAZIOet al., 1969), help to maintain sufficient caloric ingestion and limit weight loss (EDWARDSet al., 1994). The ingestion of carbohydrates also leads to an improvement in ventilation and the PaO2 in situations of hypoxia (DRAMISEet al., 1975).
4.1.9 Carbohydrates and nutrient absorption
Lactose, glucose, galactose, fructose, xylose, and sucrose increase the absorption of inorganic ions such as calcium, magnesium, sodium, strontium, barium, radium, manganese, cobalt, zinc, copper, lead and iron (FOMON, 1993;
DEBRY, 1996a). Considerable work has been done on calcium in preterm infants (STATHOSet al., 1996), adults (WOODet al., 1987) and postmenopausal women (SCHUETTEet al., 1991). A beneficial effect of carbohydrates on calcium absorp-tion was shown, but the mechanisms involved are not fully understood. Calcium is absorbed primarily in the ileum and only glucose disaccharides and polymers,
DEBRY, 1996a). Considerable work has been done on calcium in preterm infants (STATHOSet al., 1996), adults (WOODet al., 1987) and postmenopausal women (SCHUETTEet al., 1991). A beneficial effect of carbohydrates on calcium absorp-tion was shown, but the mechanisms involved are not fully understood. Calcium is absorbed primarily in the ileum and only glucose disaccharides and polymers,