Particularities of certain carbohydrates

4.3.1 Galactose and cataract

Among all carbohydrates in the diet, galactose is that for which the pathoge-nic role has been the best demonstrated, for cataract induced experimentally in animals and that occurring in humans. The most striking example is congenital galactosemia, a disease in which cataract invariably appears between several weeks to 1 year after birth. It can be prevented or even cured by the elimination of galactose from the baby’s diet before the age of 1 month. In spite of some inconsistencies, several studies have shown that the partial deficit of one and/or the other of the enzymes of the galactose metabolic pathway is seen with a higher frequency in adults with an early cataract than in those without a cataract (see COUETet al., 1991 for review).

Dairy products are the essential source of galactose because of the lactose they contain. Lactose absorption depends on the activity of intestinal lactase.

The persistence of high lactase activity in adult has been incriminated in the occurrence of an early cataract, but conclusions from these studies are not una-nimous. If lactose is really a high risk factor for cataract, then there should be a relation between its incidence and the quantities of lactose consumed. The lite-rature contains data that are both favorable and unfavorable to a relation bet-ween the quantity of lactose consumed and the incidence of cataract. No definitive conclusion can be reached because a number of confusion factors were not taken into account in these works. Nevertheless, consumption of large quantities of lactose may be a risk factor for cataract when added to high acti-vity of intestinal lactase and/or a partial enzymatic deficit in the metabolic path-way of galactose.

4.3.2 Lactose

Lactose is a disaccharide (galactose + glucose) found in milk and dairy pro-ducts and is an important source of calories in young children. In France, the quantities of lactose ingested daily by children between 2 and 6 years of age is about 15 g·d^–1. These quantities progressively decrease with age, reaching about 7 g·d^–1beyond the age of 36 months (ASPCC). Its digestion depends on the presence of lactase, located in the brush border of intestinal cells. Lactase activity is the limiting factor in the absorption of lactose. Activity is maximal at birth and decreases sharply at weaning. In Asia and Africa, lactase deficit involves about 75% of the adult population. In Europe, the prevalence of the deficit increases from North (5-10%) to South (75%). This deficit involves 20 to 40% of the adult French population, again with a North-South gradient. Lactase deficit is accompanied by functional disorders (abdominal pain, meteorism, diarrhea) in only one case out of two. In the symptomatic intolerant subject, the use of fermented, hydrolyzed dairy products and yogurt, much better tolerated than whole milk, allows to maintain good sources of calcium.

4.3.3 Fructose

Fructose is the sugar present in the highest proportion in fruits, whether as the monosaccharide or as sucrose. Fruits containing the highest quantities are dates, figs and dried prunes, followed by apples, pears, cherries and grapes.

Most vegetables contain 1 to 2% free fructose and up to 3% as sucrose. Fruc-tose is present in some naturally sweet foods, such as honey that contains 40.5% free fructose, 34.2% glucose and 1.9% sucrose. Corn contains primarily sucrose (3%), cane sugar is composed of 99.5% sucrose, molasses contain 54% sucrose and cane contain up to 8% free fructose.

The two main industrial sources of fructose are cane sugar and high fructose corn syrups (HFCSs). Fructose can be obtained industrially by the hydrolysis of sucrose (also called inverted sugar) or by the isomerization of glucose obtained by the hydrolysis of starch (HFCSs). HFCSs, introduced in the US in 1967, is obtained by the isomerization of starch hydrolysates. This process is little used in Europe because of quotas limiting the use of isomerase.

The most thorough data on fructose consumption is found in the database of the Nationwide Food Consumption Survey, created in 1977-78 by the United States Department of Agriculture. In 1986, these data were processed by a Sugar Task Force created to establish the potential health risks of consuming sugars contained in carbohydrate sweeteners (GLINSMANN et al., 1986). By adding the individual consumptions of pure fructose, sucrose and HFCSs, total mean fructose individual consumption in the US is about 55 g·d^–1. The lack of hard data in France makes it impossible to determine real fructose consumption.

The only method is to make an indirect determination of fructose consumption based on the fact that mean consumption of total carbohydrates in 1997 by adults was 200 g·d^–1of which 20% is sucrose and 10% glucose-fructose-mal-tose. This leads to an approximate figure of 30 g of fructose per day.

Fructose is not an essential nutrient and so fructose deficiency does not exist. The essential fate of fructose is its conversion to glucose, whose supply in humans is covered by other metabolic pathways (gluconeogenesis, glycogeno-lysis) and/or by the dietary intake of other carbohydrates (DELARUE, 1996). The

high sweetening power of fructose makes it especially apt to provide a sensa-tion of pleasure related to the sweet taste, but this is also true for sucrose. Fruc-tose causes caries and so its replacement of sucrose is not justified in their prevention. The value of fructose in diabetes or states of hyperinsulinemia has not been shown. In these situations, the intake of simple sugars is justified only for reasons of taste (desire to maintain a sweet taste). In addition, there may be harmful effects to the lipid balance (hypertriglyceridemia). The risk of dyslipide-mia does not increase in healthy subjects consuming about 50-70 g·d^–1or 27%

of total calories. On the other hand, fructose consumption should be limited to fruits in subjects with a type IV dyslipidemia, whether or not they are diabetic.

The hypertriglyceridemic effect of fructose in this population is similar to that of sucrose (HOLLENBECK, 1993).

4.3.4 Carbohydrate sweeteners

Other carbohydrate sweeteners, in particular polyols or sugar-alcohols, are used to manufacture candies, chewing gum, pastry, biscuits and ice cream. In general, they have a low sweetening power, a reduced energy content (but variable depending on the polyol) and a “freshness” effect in the oral cavity (variable depending on their heat of solution). Polydextrose is composed of 9 glucose molecules for 1 sorbitol molecule. Polydextrose is highly resistant to intestinal digestion and behaves more like a “fiber” than a sweetener. The same is true for fructose oligosaccharides. Considerable consumption of polyols (>

40 g·d^–1) may cause digestive discomfort resulting from their fermentation (flatu-lence, diarrhea). Nevertheless, adaptation mechanisms kick in to increase the digestive tolerance of these sweeteners as they are ingested repeatedly.

In France, the use of polyols was first authorized for the manufacture of pro-ducts designed for special diets, except propro-ducts for babies and children (J.O.

(Federal Register) of July 24, 1987). By extension, this authorization is also applicable to the use of polyols in all types of foods. It is prohibited to use poly-ols in sweet beverages in order to limit the risk of excessive consumption.

4.3.5 Rare sugars

Sugars other than glucose play an important role in the organism as consti-tuents of circulating and tissue glycoproteins or proteoglycans of connective tis-sue and cartilage: galactose, mannose, fucose, sialic acids, N-acetylglucosamine and N-acetylgalactosamine. All these sugars can theoretically be synthesized in humans from glucose, but they are also present in the diet, free or bound and in unknown quantities (except for galactose, most of which is supplied by dairy products). There may be a nutritional and metabolic interest in the exogenous supply of these rare sugars, but this remains to be confirmed (MARTIN et al., 1998). At the present time, it is thus not possible to propose recommendations on their use, except for cautionary warnings in the case of supplementations:

galactose is involved in cataract pathogenesis (see above) and glucosamine in that of insulin resistance.