Carbohydrates

By the mid 1990s it was generally accepted that carbohydrates are absorbed, metabolized, and stored with less energetic efficiency than dietary fat and were protective against weight gain. Indeed, a general perception was developing that because de novo lipogenesis appears limited when humans feed on Western diets, carbohydrate ingestion does not promote fat storage. At the same time the notion that carbohydrate metabolism or stores exert powerful negative feedback on EI became quite firmly established. By the same reasoning, diets high in carbohydrates were deemed to be more satiating, specifically because they were high in carbohydrates. High-fat diets were seen to promote overconsump-tion because they are relatively low in carbohydrate.

Recently, doubts have surfaced about the paramount role of carbohydrates as the central nutrient around which energy balance is regulated and body weight controlled. Several rigorous tests of carbohydrate-specific models of feeding have suggested that carbohydrate oxidation or stores do not exert powerful negative feedback on EI. Rather, as macronutrients come in the diet (where fat is disproportionately energy dense) there appears to be a hierarchy in the satiating efficiency of the macronutrients protein, carbohydrate, and fat. Per megajoule of energy ingested protein induces supercaloric compensation, carbohydrate generates approximately caloric compensation, and fat precipitates subcaloric compensation, and hence often excess EI. When energy density is controlled protein is still far more satiating than carbohydrates or fats (at least when ingested in excess of 1-1.5 MJ loads). Under these conditions differences in the satiating efficiency of carbohydrates and fats become subtle. Some studies are now showing that it is possible to overeat when consuming a high-carbohydrate, energy-dense diet. Furthermore, in recent studies where both fats and sugars are added to the diet, there is no evidence that increasing sugar intake levers fat out of the diet and protects against weight gain.

The foods most capable of limiting EI (both voluntary and metabolizable) are those rich in unavailable complex carbohydrates. However, humans are not too fond of these foods. The average Western adult's fiber intake is spectacularly low.

It has recently been suggested that carbohydrates with a high glycemic index are especially conducive to weight gain. The evidence relating to the glycemic index of carbohydrates and appetite control is currently very inconclusive. It is likely several factors associated with readily absorbed carbohydrates can promote higher energy intakes. These include their sweetness, ready solubility, and ease with which they can be added to foods and absorbed across the gut wall. It may be a coincidence that these traits also determine the high glycemic load of these carbohydrates. Thus, while certain high-glycemic-index carbohydrates may promote higher energy intakes the effect may not be due to their glycemic index per se.

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