How dietary fat affects obesity
Researchers have followed several leads in an attempt to discover how diet composition, and especially dietary fat, may influence obesity. As always happens in research, studies designed to answer one question have inadvertently answered others and led to still more questions. Some studies have asked whether dietary fat encourages overeating and thus increases total energy intake, which in turn leads to obesity. After all, fats make foods taste delicious.
High-fat diets' effects on food intake
One study allowed average weight women to eat freely from 3 different plans: a low-fat diet (15 to 20 percent of kcalories from fat), a medium-fat diet (30 to 35 percent), and a high-fat diet (45 to 50 percent). The women followed each plan for two weeks. The foods in each plan were similar in taste and appearance; they differed only in the percentages of kcalories contributed by fat and therefore carbohydrate as well.
As might be expected, the more fat in the diet, the more kcalories of energy the women received. Somewhat more surprisingly, the more fat in the diet, the less food the women consumed. In other words, the women did adjust their food intakes somewhat, but not enough to compensate fully for the high-fat diet's energy-rich composition. For example, a person drinking whole milk instead of non-fat milk would need to drink 40 percent less milk to receive the same number of kcalories, and the women failed to make that adjustment fully.
Body weight changes at the end of each two-week session were consistent with fat and energy intakes. The average weight change on the low-fat diet was a loss of almost 1 pound, whereas the average weight change on the high-fat diet was a gain of almost 0.75 pound. A 0.75-pound weight gain seems small, but repeated every two weeks, it would amount to some 20 pounds a year. Such a course is the path to obesity.
Carbohydrate's role in providing satiety
Another study of dietary fat as a contributor to energy intake discovered, by surprise, a role played by carbohydrate. In this experiment, eight non-obese men ate freely from either a high-fat or a mixed diet. When the men consumed the high-fat diet, they overate, taking in over 1000 kcalories per day more than when they were eating the mixed diet. Not only were their fat intakes high, but their carbohydrate intakes were also high; they seemed to continue to eat until they attained a desired level of carbohydrate intake, as if they were deriving their satiety from carbohydrate.
If people do, indeed, eat to obtain a certain amount of carbohydrate, then the higher the diet is in fat, relative to carbohydrate, the more fat and total energy (kcalories) they will have to eat before they are satisfied. Thus a person reducing carbohydrate intake may unwittingly head toward obesity by increasing fat intake in order to get enough carbohydrate.
High-fat diets' effects on body composition
Using seven-day diet records, researchers found that 155 obese men were consuming the typical American diet - about 16 percent of total kcalories from protein, 38 percent from carbohydrate, 41 percent from fat, and 6 percent from alcohol. Their total food energy intakes (2570 kcalories per day) fell short of current energy recommendations (2900 kcalories per day). This might indicate that their intakes were appropriate to their needs and would not lead to weight gain.
When the researchers compared the dietary data to body weight and body composition measures, they found two particularly interesting results. They found no correlation between total food energy intakes and the men's body fat measurements. They did, however, find a positive correlation between the men's dietary fat intakes and their body fat measurements. They also found a negative correlation of carbohydrate, plant protein, and fiber intake with body fat. These findings suggest that when people eat diets high in fat, they tend to preserve their body fat efficiently, even with moderate food energy intakes.
Other studies have shown that the same relationship holds true for women: those who ate higher-fat diets had higher body fat contents than total energy intake alone would predict. Many studies comparing body compositions and the diets of adults have found, that high-fat intakes are associated with high percentages of body fat. In still another study, researchers provided either high-fat or mixed diets to men who were attempting to gain weight and found that those eating the high-fat diets gained the same amount of weight, but in less time and with fewer kcalories than those eating the mixed diets. In addition, the men eating the mixed diets needed to continue consuming high-kcalorie diets to maintain their weight gains, whereas those who gained weight on the high-fat diets were able to maintain their weight gains at the same energy intakes as they had consumed before the study.
In general, it seems that high-fat diets push body composition toward fatness. Apparently, even with moderate food energy intakes, people can convert large percentages of their dietary fat to body fat. The picture that comes to mind is that immediately after a meal, when blood lipids are high, fat-storage cells eagerly take their fill. Later, once deposited in storage, fat becomes a less readily available energy source than it was before.