Metabolism and calorific value
The official EU calorific value
The ED has agreed that in calculating the energy value of foods the calorific value of all polyols shall be 2.4 kcal g-1. This is to be compared with a value of 4 kcal g-1 for sugars and other carbohydrates.
Intestinal metabolism. Sugars are completely absorbed in the intestine through an active transport mechanism for glucose. In their subsequent metabolism they contribute a calorific value of 4 kcal g-1. With polyols, the majority is not absorbed and what reaches the colon is fermented by the microflora to give volatile fatty acids. This fermentation represents a calorific value of about 2 kcal g-1.
There are some minor differences between various polyols. Only a small proportion of ingested maltitol, mannitol, sorbitol and xylitol is absorbed through the small intestine wall by a slow passive diffusion. This fraction, therefore, only contributes a small part to the calorific value of polyols. Part of the mannitol which is absorbed is eliminated by urinary excretion. For maltitol to cross the intestine wall it must first be split by hydrolysis which very slowly generates glucose and sorbitol.
Polyols and diabetic diet
The enzyme sorbitol dehydrogenase (found mainly in the liver, the prostate, and the kidneys) enables D-sorbitol to enter the normal metabolic pathway for sugars at the level of D-fructose. The oxidation of D-sorbitol and the subsequent oxidation of D-fructose are hepatic reactions that allow these compounds to enter the glycogenolytic pathways through an insulin-independent mechanism. The enzymes involved, D-sorbitol (or L-iditol) dehydrogenase, fructokinase, and hepatic aldolase, are not insulin dependent in contrast to glucokinase which converts D-glucose into Glucose-6-P. This explains why D-sorbitol may replace D-glucose and sucrose in the diet for diabetics.
In contrast to D-glucose, there is no active transport mechanism for the absorption of D-sorbitol by the body. Passive absorption has been estimated at 10-20 g h-1 , but the value varies widely according to the physiological state of the individual. Sorbitol has the advantage of not giving rise to elevated blood glucose levels when eaten. Glucose may be generated during the metabolism of sorbitol although, owing to the delay in its production, it does not lead to any appreciable hyperglycaemia.
Mannitol and xylitol also induce very low glycaemia and are therefore suitable for diabetics.
Maltitol and maltitol syrups yield glucose and sorbitol when hydrolysed, thus their metabolism relates to that of these materials. The oligosaccharide content of maltitol syrups is hydrolysed in the intestine into glucose and maltitol as a first step, and then the maltitol is slowly hydrolysed as a second step. Owing to the slow rate of the maltitol hydrolysis, the glycaemic index is low and there is a low insulin response.
Polyols – dental aspects
Dental caries is caused by the fermentation of sugars by the buccal flora which in turn leads to acid production and decalcification. Many studies and official reports have been published advising the use of sugar substitutes. Polyols are not fermented by oral bacteria and are therefore not acidogenic or cariogenic. Replacement of the fermentable sugars by polyols is thus a sound option, especially in confectionery and liquid medicines.
The San Antonio Integration of Methods Working Group (A.D.A., 1986) reached the conclusion that some food could be categorized as having no cariogenic potential. The group proposed that measurements of the pH of dental plaque (pH telemetry) should be an approach capable of identifying food possessing no cariogenic potential. Specifically, food that results in pH profiles statistically equivalent to those generated by sorbitol would be deemed as possessing no cariogenic potential. It is relevant to note that sorbitol was chosen as the control.
Professor Muhlemann at the Zurich Dental Institute developed a test to measure the acidogenic potential of food in vivo. In Switzerland, it is now possible legally to claim that a product is ‘safe for teeth’ only if it has undergone tests to prove that the plaque pH does not fall below 5.7 within 30 min of consuming the product. Sorbitol, mannitol, xylitol, maltitol, maltitol syrup and Lycasin have all passed the test (Imfeld, 1983).
Having passed this test it is possible to use these polyols to manufacture sugar-free confectionery to make use of the logo of the tooth-friendly sweets association, Sympadent.
Polyols – safety and tolerance aspect
Polyols are completely safe for use in foods. The acceptable daily intake (ADI) for all polyols is established as ‘not specified’ by the JECFA (Joint FAOIWHO Expert Committee on Food Additives). This signifies that on the basis of the available data (chemical, biochemical and toxicological), the total daily intake of the substance arising from its use at the levels necessary to achieve the desired effect and from an acceptable background in food, does not, in the opinion of the committee, represent a hazard to health.
Polyols may induce mild flatulence or laxation. This effect is not limited to polyols but is characteristic of many other natural ingredients and foodstuffs, such as fruits, vegetables and other sources of dietary fibre. Different individuals have differing degrees of tolerance. Regular consumption can lead to adaptation and increased tolerance.
The report of the EEC Scientific Committee for Food (SCF) on sweeteners (Opinion of 14 September 1984, updated in December 1987) noted that “consumption of the order of 20 g of polyols per day is unlikely to cause any undesirable laxative symptoms”. The SCF further noted that this figure should not be interpreted as a legislative restriction. In the USA a warning about a possible laxative effect is needed only if over 50 g per day are likely to be consumed.