Stability and bioavailability of vitamins

2.3.1 Stability

Vitamin stability is an important issue when considering the nutritional value of a food. Processing and storage losses depend upon conditions such as pH, temperature and moisture content. Niacin and biotin are relatively stable, but the other water-soluble vitamins are labile to varying extents and under different conditions. Riboflavin is notoriously susceptible to decomposition by light. During domestic cooking the water-soluble vitamins are easily leached out into the cooking water or exuded from meat, but are not lost if the cooking fluids are consumed. In the case of vitamin C, rapid heat treatment, such as the blanching of fruits and vegetables or the pasteurization of fruit juices, actually serves to prevent vitamin losses during post-processing storage by inactivating enzymes that promote the direct oxidation of ascorbic acid.

2.3.2 Bioavailability

The term 'bioavailability', as applied to vitamins in human nutrition, refers to the proportion of the quantity of vitamin in the food ingested that undergoes intestinal absorption and utilization by the body. Utilization encompasses transport of the absorbed vitamin to the tissues, cellular uptake and the subsequent fate of the vitamin. The vitamin may be converted to a form which can fulfil some biochemical function. Alternatively, the vitamin may be metabolized within the cell to a nonfunctional form for subsequent excretion or simply stored within the cell for future use. Any definition must be viewed as an operational definition within the context of the method used to determine bioavailability.

Bioavailability should not be confused with nutrient stability. Whereas food processing can result in the loss of a labile vitamin, the bioavailability of the remaining amount of vitamin is not necessarily altered. Bioavailability is influenced by a diverse range of interacting parameters and therefore the amount of bioavailable vitamin in a diet or individual food can vary considerably.

Absorption of a vitamin depends on the chemical form and physical state in which the vitamin exists within the food matrix. These properties may be influenced by the effects of food processing and cooking, particularly in the case of niacin, vitamin B6 and folate. In foods derived from animal and plant tissues, the B-group vitamins occur as their coenzyme derivatives, usually associated with their protein apo-enzyme. In addition, niacin in cereals and vitamin B6 in certain fruits and vegetables occur largely as bound storage forms. In milk and eggs, which are derived from animal secretions, the B-group vitamins occur, at least to some extent, in the underivatized form, a proportion of which is associated with specific binding proteins. Vitamins that exist naturally as chemically bound complexes with some other material in the food matrix exhibit lower efficiencies of digestion and absorption compared with the free (unbound) vitamin ingested, for example, in tablet form.

Certain dietary components can retard or enhance a vitamin's absorption, therefore the composition of the diet is an important consideration. For example, the presence of adequate amounts of dietary fat is essential for the absorption of the fat-soluble vitamins. Carotenoids exhibit low bioavailability relative to vitamin A due to the poor digestibility of fibrous plant material. Other ingested substances such as alcohol and drugs may interfere with the physiological mechanisms of absorption.

Biological factors can influence the absorption of a vitamin from a particular food or diet. For example, the absorption mechanism in intestinal epithelium can be adapted physiologically to meet changing metabolic requirements and food deprivation. Malabsorption may occur in the presence of gastrointestinal disorders or disease. Other general factors that influence absorption include the plane of nutrition, metabolic requirements, age and state of health.

It is impracticable to determine true or absolute bioavailability, and therefore almost all methods for determining vitamin bioavailability in foods yield a measurement of relative bioavailability. This is the observed response obtained when the animal is fed the test food or diet expressed as a percentage of the response obtained by feeding a reference material of high bioavailability. Rats or chicks have been used extensively as experimental animals, but these animal studies are now thought to have relatively little value in predicting vitamin bioavailability for humans. This is because of problems such as intestinal synthesis of water-soluble vitamins by gut microflora, coprophagy (faecal recycling) and metabolic differences between animals and humans. The main emphasis nowadays in the field of nutrient bioavailability has turned to the use of protocols with human subjects in order to avoid the uncertain relevance of animal models (Gregory, 1988).

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