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In this article we will discuss about:- 1. Meaning of Specific Dynamic Action 2. Facts About Specific Dynamic Action 3. Causes.
Meaning of Specific Dynamic Action (S.D.A):
All foodstuffs, after being ingested, increase the rate of heat production though the subject is in a basal state. This increased heat production due to ingestion above the basal metabolic rate (B.M.R) is known as Specific Dynamic Action (S.D.A) or calorigenic action of foods. It can be expressed at cost of metabolism of the various food substances.
The highest Specific Dynamic Action (S.D.A.) is shown by protein and is about 30%, whereas others, e.g., carbohydrate 6%, fat about 4%. By this it is meant that, if a quantity of protein, carbohydrate or fat, each having energy value of 100 calories, be given separately to an animal (whose basal metabolism is 100 cal) the actual heat production will be 130,106 and 104 calories respectively.
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The extra heat comes from the combustion of the tissue substances causing loss of body weight. To prevent this loss of weight, the subject must be supplemented extra calorific allowance for the S.D.A. of the food itself.
Facts About Specific Dynamic Action (S.D.A):
The following facts about S.D.A. are noteworthy:
i. Specific Dynamic Action (S.D.A.) starts within an hour of taking food becomes maximum in about the third hour and is maintained at this level for several hours.
ii. Liver is the site of S.D.A. of proteins, because proteins fail to exert S.D.A. after removal of liver. But it is not the seat of S.D.A. of carbohydrates and fats. Because the removal of liver does not affect it.
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iii. In fasting and under-nutrition the S.D.A. of all foodstuffs increases.
iv. During positive nitrogen balance, S.D.A. of protein does not seem to occur.
v. In order to exhibit S.D.A the animal must have a temperature above 33°C. Below this temperature the S.D.A. of foodstuffs becomes less and less apparent.
vi. The S.D.A. of protein cannot be utilised in any other way. It is a waste heat and lost. But the case is different with fats and carbohydrates. Their S.D.A. can be utilised in the performance of work. Hence, when exercise is done with a food containing these substances, the extra heat due to S.D.A. is not seen, because it becomes used up in the energy exchanges underlying muscular action. For this reason high protein diet is not suitable for heavy muscular work. S.D.A. of protein is useful in maintaining heat balance in cold climates.
Cause of Specific Dynamic Action:
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1. S.D.A. of Proteins:
The following observations are significant:
i. That S.D.A. is not due to the synthesis of body proteins from the amino acids of food. Because proteins do not exert any S.D.A. upon growing infants or upon those subjects where nitrogen balance is positive.
ii. That S.D.A. is not due to any specific-stimulating action of some amino acids. Although it is seen that S.D.A. of protein depends upon the following amino acids only-glycine, alanine, glutamic acid, tyrosine and phenylalanine, yet it is not due to the effects of these amino- acids. Because they exert no S.D.A. in infants.
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iii. S.D.A. of protein is exerted only when proteins are breaking down. Breakdown of amino acid include three processes—first deamination, secondly, the changes undergone by the non-nitrogenous part, and thirdly, the changes undergone by the nitrogenous part. It has been shown that the first two processes are not responsible for S.D.A.
S.D.A of proteins is shown during the change undergone by the nitrogenous part. S.D.A. is due to largely associated with the nature of deamination, i.e., oxidative deamination [transformation into urea). ATP also helps in the urea formation. For the synthesis of ATP energy is required which leads to increased metabolism.
2. S.D.A. of Carbohydrates:
It is suggested that it represents the energy liberated in excess of that required for the conversion of glucose into glycogen. After a fast, when glycogen store is depleted, ingested glucose (which is mostly converted into glycogen) exhibits pronounced S.D.A. Glucose with thiamine exerts more S.D.A., since thiamine promotes and accelerates fat synthesis from carbohydrate.
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3. S.D.A. of Fats:
According to Lusk (Plethora theory), it is due to more rapid oxidation of fats in the cells owing to their increased concentration in the tissue fluids (plethora) after absorption due to rate of digestive product reaching the tissue exceeds the rate of utilisation.
Conclusions:
Three applications may be made of these facts:
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i. The high S.D.A. of proteins should be made use of in cold climates by giving a protein rich food.
ii. While prescribing adequate diet, proper allowance should be made for the S.D.A. of proteins, otherwise tissue substances will burn and the subject will lose weight.
iii. B.M.R. of a subject should be determined avoiding the period during which S.D.A. develops.
