Why high fat diets may not be fattening

Share This Post

Because it’s called fat, it makes sense and is intuitive to believe that fat is fattening. It’s also rich in calories compared to carbohydrate or protein. The thing is, though, the evidence does not strongly link fat-eating with obesity, and eating low-fat diets are, on the whole, spectacularly ineffective for the purposes of weight loss. Never mind what common sense may dictate, taken as a whole the evidence suggests fat is not particularly fattening after all.

To understand how this can be it helps to understand a little about what influences the accumulation of fat in the fat cells. One key player here is the hormone insulin. This hormone predisposes the body to fatty 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). Here’s the thing: fat does not stimulate insulin secretion directly. Is it possibly then that someone could eat a diet of nothing but fat and lose weight, even if they were not in calorie deficit?

This concept may sound far-fetched to some, but there is some evidence for it. One piece of evidence comes in the form of a study in which individuals, on separate occasions, were allowed no food or fed with fat into a vein.

In the study in question [1], individuals were completely fasted for a total of 84 hours on once 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, in particular, to fuel the brain. Their production is a normal, healthy mechanism that allows the body to fuel itself when food is in short supply or if carbohydrate is quite several restricted in the diet. During the fasting phase of this study, as expected, levels of glucose and insulin fell, while levels of fatty acids and ketone bodies all 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 little else but 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 at least that fat is not inherently fattening, and reminds us of the critical role that hormones, not mere caloric balance, have a key role to play in body fat management.


1. Klein S, et al. Carbohydrate restriction regulates the adaptive response to fasting. American Journal of Physiology – 1992;262(5):E631-E636]

More To Explore

Walking versus running

I recently read an interesting editorial in the Journal of American College of Cardiology about the relative benefits of walking and running [1]. The editorial

We uses cookies to improve your experience.