Low-fat diets have fallen out of style and are now a favorite punching bag in some corners of the popular media, the Internet, and the scientific literature. And there are reasons for this. As typically implemented, low-fat diets have largely failed to live up to their promise of reducing the risk of obesity, cardiovascular disease, and diabetes. Randomized controlled trials over the last two decades have increasingly supported the position that higher-protein, lower-carbohydrate, and sometimes higher-fat diets promote greater weight loss than low-fat diets over the short- to medium- term.
The truth is that this is a significant medical reversal. Researchers and public health authorities accepted a hypothesis-- and dispensed advice based on it-- to later learn that the hypothesis was not as well supported as they had thought. This is fair to point out.
Yet this idea often gets pushed a step further: by shifting our diets away from fat and toward carbohydrate beginning in 1980, the US Dietary Guidelines actually contributed to the obesity epidemic. This idea has been promoted in several popular books, and most recently, in an editorial published in the Journal of the American Medical Association (1). Here is the relevant quote:
As a result of [US dietary guidelines], dietary fat decreased to near the recommended limit of 30% total energy. But contrary to prediction, total calorie intake increased substantially, the prevalence of obesity tripled, the incidence of type 2 diabetes increased many-fold, and the decades-long decrease in cardiovascular disease plateaued and may reverse, despite greater use of preventive drugs and surgical procedures. However, other changes in diet (such as meals away from home) and lifestyle (such as physical activity level) may have influenced these trends.As usual, when someone makes a factual statement, they are stating a hypothesis. Whether or not the hypothesis is correct depends on whether its key predictions are supported by compelling evidence. So let's go through some key predictions of this hypothesis and see if the evidence supports them.
Recent research suggests that the focus on dietary fat reduction has directly contributed to this growing burden of chronic disease.
Prediction 1: There should be a correlation between the publication of the Dietary Guidelines and the obesity epidemic
The US government has been publishing dietary information for at least 120 years, and the 1980 version wasn't the first to recommend limiting the intake of fatty foods (2). Still, it could be argued that the 1980 version was more strongly worded and more influential of public eating habits.
Americans have been growing fatter for over a century (3), but the trend has accelerated over the past 3-4 decades, and we call this acceleration the "obesity epidemic". This epidemic was first clearly detected by the 1988-1994 NHANES survey. The previous survey was conducted between 1976 and 1980, so we can say that it began sometime between 1980 and 1988 (4).
Roughly speaking, it is true that there's an association between the publication of the 1980 Guidelines and the obesity epidemic. Yet by itself, this is very weak evidence, due to the large number of other things that also changed over that time period. For the hypothesis to be credible, it will need more support.
Prediction 2: Americans should have reduced our fat intake in response to the Guidelines
If the Dietary Guidelines caused us to gain weight by putting us on a low-fat diet, there should be evidence that we actually began eating less fat in response to the Guidelines. If not, the hypothesis cannot be correct.
Two independent lines of evidence suggest that our absolute fat intake did not decline after the publication of the Guidelines (5, 6). Proponents of the hypothesis invariably cite the fact that the percentage of fat in the US diet declined, which is true (although the change was rather small). The reason the percentage changed is not because our fat intake decreased, but because our carbohydrate intake increased, along with our total calorie intake. Does this count as a low-fat diet?
As an analogy, imagine a man named Jim who has obesity. Jim wants to lose weight, so he decides to eat a low-carbohydrate diet. Rather than reducing his intake of carbohydrate, Jim adds fat to all his meals so that the percentage of carbohydrate in his diet decreases. Jim's calorie intake increases from 3,000 to 4,000 Calories per day, and his absolute carbohydrate intake remains the same. Yet the percentage of carbohydrate in his diet decreases from 45% to 34%. Is Jim on a low-carbohydrate diet, and should we expect him to lose weight?
Of course not. Jim isn't eating a low-carbohydrate diet, and neither have Americans been eating a low-fat diet.
3. Low-fat diets should cause weight gain
For this hypothesis to be correct, low-fat diets have to cause weight gain. While low-fat diets as typically implemented may not be a very effective weight management tool, decades of research shows that they cause weight loss, not weight gain. This is true even when we only consider studies that didn't involve deliberate calorie restriction (7, 8).
The fact that low-fat diets cause less weight loss than low-carbohydrate diets over the short- to medium-term doesn't mean low-fat diets cause weight gain. Both diets are more slimming than eating as much as you want of everything.
4. People who followed the Guidelines should have gained more weight than people who didn't
If the Guidelines caused the obesity epidemic, then the people who actually followed the advice should have gained more weight than the people who didn't. Yet the evidence shows precisely the opposite (9). Here's the money quote:
Poor compliance with the Guidelines, rather than the guidelines themselves, is likely responsible for the weight gain observed in the American population. Adoption of an eating pattern consistent with the Dietary Guidelines should facilitate population weight control if sustained long term.5. Other countries with dissimilar dietary guidelines should not have a similar obesity problem
I'm not going to do an in-depth analysis of how national dietary guidelines relate to changes in weight, but I'll simply point out that nearly all affluent nations have been gaining weight rapidly in recent years, regardless of dietary advice or lack thereof (10). The US is only unique in the sense that we're on the leading edge of economic, dietary, lifestyle, weight, and health changes that are affecting the entire affluent world.
Although the low-fat diet doesn't appear to be a silver bullet for our national weight problem, the hypothesis that US Dietary Guidelines caused the obesity epidemic by reducing our national fat intake has more holes in it than Swiss cheese on baguette.