PubMed · 6579997
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1983-11-19. Sweet reason.. https://doi.org/10.1038/sj.bdj.4805228
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Many factors potentially influence the digestion, absorption, and metabolism of the various species of sugars occurring in the human diet. Experimental evidence indicates that the source of sugars in foods does not in itself affect the rate of absorption or the metabolism of the sugars. However, the form in which the sugars are ingested and the physical and chemical properties of the food matrices do have significant effects on the rates of absorption. Food matrices influence gastric emptying and through their physical properties affect the rate of transport across the small intestinal mucosa. Disaccharides form the major proportion of ingested carbohydrates in the small intestine and the digestion and transport systems for these sugars, except for lactose, are the most efficient. After absorption, the pathways of the different dietary sugars converge and the original dietary source has only minimal effects on metabolism.
Physical activity in the form of exercise requires the metabolism of bodily fuel reserves to provide energy for muscle contraction. Under normal circumstances, very little protein is metabolized to provide the energy for muscle contraction. At rest and at low exercise intensities, the metabolism of fat provides a considerable proportion of the energy for resting metabolic processes and muscle contraction. However, at exercise intensities at which athletes train and compete, the metabolism of bodily carbohydrate reserves (eg, blood glucose and liver and muscle glycogen) provides the predominant fuel for muscle contraction. Furthermore, when these substrates reach critically low amounts or are decreased by some amount, fatigue occurs. There is a significant body of literature examining the effects of ingestion of various types of sugars at various times during exercise and during recovery from exercise on carbohydrate fuel reserves and on physical performance.
I describe recent advances in assessing the amount of energy available from diets of varied composition. Empirical models of food energy prediction have grown in preference to factorial models over the past 20 y and knowledge of the quantitative aspects of energy salvage in the colon have modified our thoughts on how to best calculate food energy values. In contrast, food regulatory practices have been limited mostly to changes in the way in which carbohydrate is defined or measured and there has been little change in the basic approach to energy evaluation or the energy conversion factors used with food components. The empirical systems have advantages over factorial models of food energy assessment and it is proposed that such empirical systems should be used in food regulations, food tables and databases, and in nutritional studies in which knowledge of metabolizable energy intake is desirable but cannot be determined directly.