Sunday 9 August 2015

Avidin: Raw Eggs and Biotin

Biotin is a member of the B vitamin family of vitamins. Biotin is required in the diet as it is used as a cofactor in the acetyl CoA carboxylase, propionyl CoA carboxylase and pyruvate carboxylase enzymes. Plant sources such as grains are good sources of biotin, but animal foods with the exception of liver and eggs are poor sources. The fact that eggs contain a good supply of biotin is interesting because raw egg white may be potent inhibitors of biotin absorption. Initial observations about the interactions of raw egg whites and dietary effects were made when it was observed that laboratory animals on healthy diets did not do well if raw egg white was added to their diets. In particular, experiments showed that animals lost some of their hair, and developed dermatitis, particularly around their eyes. It was observed that addition of high amounts of B vitamins to the diets reversed these symptoms, and by carefully changing the B vitamin administered, it was found that biotin was the vitamin responsible for this effect.
Further tests of chick, rats, rabbits, monkeys and guinea pigs showed similar dermatitis and skin lesions in the animals when fed raw egg whites at about 50 % of their total diet. These symptoms were similar to the deficiency symptoms of biotin that had previously been established by feeding animals biotin deficient diets. Observations of eczema in children eating raw eggs have also been made as far back as 1939. In 1942 human studies were performed whereby humans were fed 30 % of their energy intake as raw eggs. Only a handful of subjects completed the study, and these subjects developed dermatitis, a grey pallor to the skin, as well as a dryness and scaliness to the skin. Other symptoms including depression, lassitude, sleepiness, muscle pains, numbness, and appetite suppression also developed. Some of the subjects also developed heart irregularities, and cholesterol concentrations increased in the blood. Addition of biotin to the diet reversed all of the symptoms quickly.
The symptoms of egg white induced biotin deficiency were referred to as ‘egg white injury’. Subsequently, the discovery that egg white inhibit the absorption of biotin lead to the identification of a protein in egg whites called avidin. In chicken eggs, avidin makes up around 0.05 % of the total protein in egg whites, a reasonably large amount. However, avidin is secreted into the egg white of other birds as well as the eggs of amphibians, although the function of avidin is not understood. Each avidin molecule can bind up to four molecules of biotin. Because avidin is a protein it is able to be denatured, and this removes its biotin binding capacity, something that is dependent on the shape of the molecule. Heating egg white will denature avidin, and so cooked egg whites are not able to bind biotin and has no effect on the absorption of biotin. Some denaturation of avidin may occur in the hydrochloric acid of the stomach, although clearly this does not occur quickly enough to prevent the binding of most biotin.

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