Why High-Fat Diets Are Not Fattening
It is easy to believe that fat is fattening. Compared to carbohydrate or protein, fat is rich in calories. However, the evidence does not link eating fat with obesity.
Eating low-fat diets is quite ineffective for weight loss. The evidence suggests that fat is not fattening. Understanding how this can be helps us understand what influences the accumulation of fat in the fat cells.
One key player is insulin. This hormone predisposes the body to fat accumulation through a variety of mechanisms, including enhanced uptake of fat into the fat cells (through activation of the enzyme lipoprotein lipase) and suppression of fat release (by inhibiting the enzyme hormone-sensitive lipase).
Fat does not stimulate insulin secretion directly. Is it possible that someone could eat a diet of nothing but fat and lose weight? This concept may sound far-fetched, but there is evidence for it.
One study was made on individuals who were allowed no food or were fed with fat into a vein. The study was published in May 1992 in the American Journal of Physiology. Individuals were fasted for a total of 84 hours on one occasion.
During fasting, insulin levels normally fall, and levels of fatty acids in the bloodstream rise. Levels of substances called ketones also rise. Ketones are formed in the liver from fat and can be used for energy and fuel for the brain.
Ketone production is a normal, healthy mechanism that allows the body to fuel itself when food is in short supply or when carbohydrate is severely restricted in the diet.
During the fasting phase of this study, levels of glucose and insulin fell, while levels of fatty acids and ketones rose. The rate of lipolysis (fat breakdown) also increased.
The really interesting part of this study came when the same individuals were fasted on another occasion. The difference was that, on this occasion, individuals were fed with an intravenous drip containing mostly fat. The calorific value of this fat matched the calorie needs of the individuals taking part in the study.
On this occasion, glucose and insulin levels fell, while fatty acid and ketone levels rose. Lipolysis also rose. The extent of these changes was the same as during complete fasting. In other words, feeding the body pure fat induced a metabolic state in the body that was, to all intents and purposes, the same as the state induced by fasting.
While infusing significant quantities of nothing but fat into the body, the body still readily gave up its fat. And it gave it up as readily as it did when eating nothing at all.
This study provides some evidence that fat is not inherently fattening and reminds us of the critical role that hormones, not mere caloric balance, have to play in body-fat management.
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