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Weight regulation part 2: Does dietary fat make us fat?

The calorie theory of weight regulation holds that in order to maintain a stable weight we must eat the same number of calories as we expend over time.  If we want to lose weight then we must eat fewer calories than we expend.  Since fat contains 9 calories per gram and carbohydrate and protein each contain 4 calories per gram, theoretically we can eat a larger volume of food if we eat a low fat diet.  Most obesity researchers and health authorities recommend the low-fat low-calorie diet as the best method of weight control.  However, there are problems with this scenario.  I will begin to address them in this post.

As background, epidemiological studies show no evidence linking obesity to dietary fat intake either in the same population or between different populations. Our dietary fat intake in the US has decreased since the 1970s but we have gotten fatter. Studies showing that certain strains of rats and mice gain weight on high fat diets actually compared diets high in carbohydrates with diets high in both fats and carbohydrates.  But some strains of rats do not grow obese on high fat diets, and those that do get much fatter on high-fat high-carbohydrate diets than on high-fat low-carbohydrate diets.  These studies also used far higher proportions of fat than rodents would normally consume, and most of them used vegetable oils as their fat source.  Also, when rats are allowed to eat all they want of the standard American diet of supermarket foods they grow very fat, but the foods they consume in excess are sugars and carbohydrates; high fat foods like cheese and peanut butter are self-limiting.   Finally, studies that have attempted to cause weight gain in human subjects have found that it is impossible to cause people to overeat on diets high in fat and protein, but easy to cause them to eat several thousand extra calories a day when the food contains mainly carbohydrate.  How much meat and butter can one eat without losing appetite?  How many cookies, chips, crackers and other carbohydrate foods can one eat and still be hungry for more?  Diets that limit carbohydrates but allow unlimited protein and fat work for weight control for these very reasons.  I’ll go into this topic when we get to carbohydrates in my next post.

There are physiological reasons for the satiating qualities of fat.  Hormones in the small intestine sense the presence of fat and turn off hunger.  Leptin is up-regulated, telling our bodies that we have eaten enough; ghrelin is down-regulated, telling us that we are no longer hungry.  Fat is also slower to digest, causing us to feel full longer. Lack of fat in our diet tells our bodies that food is scarce and we had better hold on to our fat stores.  We get hungry sooner and may crave fried food.  Evolutionarily, fat is our preferred fuel.  It signals the body that we are not starving, that food is abundant, and that our bodies can use our own stored fat for fuel. When our bodies are deprived of fat they get very efficient at converting carbohydrates to fat.

Some interesting recent studies are pointing to one type of fat as a contributor to obesity.  While only investigated in mice and rats so far, it appears that a disproportionally high ratio of omega-6 to omega-3 polyunsaturated fatty acids causes rodents to become obese, grow a larger number of adipocytes (fat cells), and suffer liver damage.  Linoleic acid, an omega-6 fatty acid, is converted to endocannabinoids that trigger appetite in both rats and humans.  We have been encouraged to substitute vegetable oils and margarines, which are disproportionately high in omega-6 fatty acids, for traditional fats like butter and lard, which have only small amounts of omega-6 fatty acids.  Could the same process be happening in humans? We have been subject to a vast experiment in non-traditional eating.  Is it making us fat?  These rodent studies are certainly suggestive of a reason for weight gain in those who indulge in foods high in vegetable oils or fried in them, but the actual mechanism is not known, nor have human studies been done.

All fats are mixtures of fatty acids.  In general the vegetable oils have a very small amount of omega-3 fatty acids and large amounts of omega-6 fatty acids. Natural fats like butter, lard, coconut oil, olive oil, beef and chicken fat have higher amounts of saturated and monounsaturated fatty acids and very little omega-6 or omega-3 fatty acids.  However, if animal fats come from grass fed animals they have higher amounts of omega-3 fatty acids than if they come from grain-fed animals.  Do not be afraid of saturated fatty acids—they have nothing to do with heart disease (more on this later).  While both linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are dietary requirements (essential fatty acids), they are required in very small amounts and will be retained and used more efficiently if the diet includes plenty of saturated fatty acids.

The bottom line is that we need fats in our diet.  They play vital roles in the body and form important structures; from the walls of our cells to the sheaths around our nerves to most of our brains.  We cannot live without them and we are much healthier if we eat them in adequate amounts.  They are calorie dense but at the same time are satiating and self-limiting if they are from the traditional sources listed above.  So if fat is not the culprit in weight gain, with the possible exception of vegetable oils, what is?  Stay tuned for my next post on carbohydrates.


Taubes, Gary.  2007.  Good Calories Bad Calories. Challenging the Conventional Wisdom on Diet, Weight Control, and Disease. Alfred A. Knopf, New York.  601 pp.

Hanbauer, Ingeborg, et al. 2009.  The decrease of n-3 fatty acid energy percentage in an equicaloric diet fed to B6C3Fe mice for three generations elicits obesity.  Cardiovascular Psychiatry  & Neurology. Publ. online 9/16/09.

Ailhaud, Gerard, et al. 2008. An emerging risk factor for obesity: does disequilibrium of polyunsaturated fatty acid metabolism contribute to excessive adipose tissue development? British Journal of Nutrition 100:461-470.

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