In 2000, the International Journal of Obesity published a nice review article of low-fat diet trials. It included data from 16 controlled trials lasting from 2-12 months and enrolling 1,910 participants (1). What sets this review apart is it only covered studies that did not include instructions to restrict calorie intake (ad libitum diets). On average, low-fat dieters reduced their fat intake from 37.7 to 27.5 percent of calories. Here's what they found:
Low-fat intervention groups showed a greater weight loss than control groups (3.2 kg, 95% confidence interval 1.9-4.5 kg; P < 0.0001), and a greater reduction in energy intake (1,138 kJ/day, 95% confidence interval 564-1,712 kJ/day, P = 0.002). Having a body weight 10 kg higher than the average pre-treatment body weight was associated with a 2.6 +/- 0.8 kg (P = 0.011) greater difference in weight loss.In other words, low-fat groups reduced their calorie intake by an average of 271 calories per day, and lost 7.5 pounds (3.2 kg). When they considered only people who started off overweight, they lost 12.8 pounds (5.8 kg). The investigators noted that the results were similar no matter what the duration of the trial, because weight loss plateaued fairly quickly. This is all without any instruction to reduce calorie intake, therefore we can assume these dieters were eating to fullness. The investigators concluded:
A reduction in dietary fat without intentional restriction of energy intake causes weight loss, which is more substantial in heavier subjects.Their conclusion was modest compared to certain other low-fat diet trials, which stated outright that eating fat leads to obesity. Their findings mirrored those of a longer trial that lasted two years (2).
The best low-carbohydrate diet study I've seen was published in 2008 in the New England Journal of Medicine (3). 322 "moderately obese" participants were placed on a low-carbohydrate diet, a calorie-restricted low-fat diet, or a Mediterranean diet, for two years. The low-carbohydrate group's carbohydrate intake decreased by 130 grams per day, which is about half of a typical person's total intake, and neatly corresponds to the reduction in calories of 561 per day, despite not being instructed to reduce calorie intake.
At two years, the low-carbohydrate group had lost 10.4 lbs (4.7 kg), which is similar to the average weight loss seen in long-term low-fat diet trials that do not impose calorie restriction. At that point, they had been relatively weight stable for nearly a year. Markers of metabolic health improved, and total and LDL cholesterol decreased.
Which Macronutrient Causes Obesity?
Trick question! Overall, these studies show that both fat and carbohydrate can be fattening, and each can also be slimming-- it just depends on the context. Food reward is the concept that makes sense of these seemingly contradictory findings, as fat and carbohydrate are both major reward factors. Reducing one or the other appears to decrease the body fat setpoint, leading to reduced hunger and a new lower weight plateau. This plateau effect is remarkable in all the diet groups of the low-carb study mentioned above (4):
If food reward causes obesity, then diets that are restricted in other ways should also cause weight loss...
In 2002, the journal Obesity published a paper on the long-term results of a diet trial comparing a vegan diet to the standard "heart-friendly" low-fat, low-cholesterol NCEP diet in overweight women (5). After one year, vegan dieters had lost 10.8 lbs (4.9 kg), although after two years, the loss had diminished to 6.8 lb (3.1 kg). The vegan diet outperformed the NCEP diet in terms of weight loss, although to be fair, I don't think the NCEP diet is really designed to be a weight loss diet.
A separate 74-week vegan diet trial in diabetics came to a similar conclusion, and also showed that the vegan diet improved metabolic function somewhat (6). In fact, markers of metabolic function improved in all the weight loss diets I mentioned in this post, suggesting that energy balance is a critical factor in metabolic health. I believe that garden-variety insulin resistance is probably mostly due to energy imbalance, i.e. energy intake exceeding what the body can constructively handle. It starts off as a reversible phenomenon but can become semi-permanent over time. I also suspect that long-term energy imbalance plays a role in elevated blood cholesterol and other factors leading to heart attacks.
For those who aren't familiar, "paleo" diets only allow categories of foods that were available before the advent of agriculture. That means no processed foods, no grains, no beans, no refined sugar, no dairy. Paleo diets are mostly composed of some combination of meat/organs, seafood, starchy tubers/roots, vegetables, nuts, fruit and eggs.
Unfortunately, the paleo diet trials published to date lasted no longer than 12 weeks. However, Dr. Staffan Lindeberg's group showed that 12 weeks of a paleo diet caused a substantial decrease in calorie intake, a weight loss of 11 lbs (5 kg), and a remarkable improvement in metabolic function in overweight patients with diabetes and pre-diabetes (7).
Again, participants were not instructed to reduce calorie intake. Yet, a separate analysis of the data showed that according to questionnaires, they felt just as full as the comparison group that ate 31 percent more calories on a Mediterranean diet (8).
One of the things I like about the paleo diet is it reduces food reward (especially industrially processed items) and other potentially problematic foods, while remaining highly nourishing and not necessarily imposing limits on macronutrients.
Putting it All Together
So here we have four diets that are diametrically opposed to one another. On one hand, we have low-fat vs. low-carbohydrate diets, and on the other, we have vegan vs. high-meat diets. All four cause a spontaneous decrease in calorie intake in overweight people, all four cause fat loss, and all four improve metabolic markers in overweight people with diabetes risk factors. I feel that food reward provides a compelling explanation for the evidence as a whole. Reduce the rewarding quality of the diet, and you get changes that are consistent with a lowered body fat setpoint. I think once we take a broad view through the lens of food reward (while not forgetting other potential mechanisms), we can start designing fat loss strategies that are highly effective because they're based on a deeper understanding of the underlying principles involved.