I'll get right to the point: humans living in a non-industrialized setting tend to be lean, regardless of how much meat they eat. This applies equally to hunter-gatherers, herders, and farmers.
One of the leanest populations I've encountered in my reading is the 1960s Papua New Guinea highland farmers of Tukisenta. They ate a nearly vegan diet composed almost exclusively of sweet potatoes, occasionally punctuated by feasts including large amounts of pork. On average, they ate very little animal food. Visiting researchers noted that the residents of Tukisenta were "muscular and mostly very lean", and did not gain fat with age (1, Western Diseases, Trowell and Burkitt, 1981).
|!Kung man gathering mongongo fruit/nuts.
From The !Kung San, by Richard B. Lee.
Traditionally, the arctic Inuit ate more meat than almost any other population on Earth, due to the scarcity of edible plant foods in their arctic ecosystem. We often perceive the Inuit as fat, but in fact this is a misconception caused by their voluminous clothing and broad faces. Accounts of traditionally-living Inuit suggest that they were in fact lean underneath all that fur. Arctic explorer and anthropologist Vilhjalmur Stefansson spent a number of years with the Inuit of Alaska and Canada in the early days of European contact, reporting that they were characteristically lean and fit (My Life with the Eskimo, Vilhjalmur Stefansson, 1912).
Consistent with Stefanssons's account, Mouratoff and colleagues reported that in Western Alaska in 1962, only four percent of Inuit men, and ten percent of Inuit women, were overweight (2). Among middle-aged and older men and women, even fewer were overweight, in contrast to the gradual weight gain that characterizes affluent industrialized societies today. By 1972, the prevalence of overweight had increased substantially, as diets and lifestyles continued to modernize in the area.
Meat eating doesn't seem to have much of an impact on body weight in traditionally-living societies, suggesting that it may not be a major determinant of body weight. However, those societies differ from ours in many ways. What is the relationship between meat eating and body weight in affluent societies today?
Vegans don't eat any animal foods. The stereotypical vegan body type is extremely lean with low fat and muscle mass-- and this stereotype is fairly accurate. Although it's possible to be vegan and obese, most vegans are indeed quite a bit leaner than the typical omnivore, and vegetarians are somewhere in between. Data from the 38,000-person EPIC-Oxford study illustrate this well, using body mass index as a measure of fatness (3):
Omnivores are the fattest, pescetarians (fish eaters) and vegetarians are tied, and vegans are the leanest. The study suggests that these differences in body weight were linked with both dietary and non-dietary factors (e.g., physical activity level). This study also illustrates the difficulty of disentangling the effect of meat eating from other diet and lifestyle factors, since the vegan group exercised much more, ate more fiber, smoked fewer cigarettes, and was much less likely to be married and have children.
In 2010, Bernstein and colleagues published a paper based on the massive Nurses' Health Study, reporting the association between the intake of different types of meat and cardiovascular risk (4). Although the study wasn't designed to investigate body fatness, the paper does report the association between meat intake and BMI at baseline (Table 1). In this cohort, people who ate the most red meat or poultry did have a somewhat higher BMI than those who ate the least. There was little association between fish intake and BMI. Surprisingly, the strongest association was actually with poultry. As usual, meat intake was associated with a number of other diet and lifestyle factors.
In 2011, Mozaffarian and colleagues published a paper on diet and weight changes that received a huge amount of attention (5). Drawing from three large US cohorts collectively representing 120,877 people, they reported the association between changes in dietary habits and changes in body weight over 4-year intervals. They reported a number of associations*, including for unprocessed red meat and processed meat, both of which were associated with weight gain. The associations were among the stronger of those that they identified. Unfortunately, they didn't report data for poultry or fish.
These observational studies use relatively inaccurate dietary measurement tools, and suffer from a number of potentially serious confounding factors, so we have to be careful in interpreting them. Still, vegetarians and vegans clearly tend to be leaner than omnivores, and I believe meat is part of the explanation. Here's why: people eat fewer calories when they eliminate a food category from the table. Also, certain meats, such as fatty and cured meats, are calorie-dense and highly palatable, potentially contributing to overeating. For what it's worth, low-fat vegan diet advocates are consistently among the leanest diet advocates.
Now, let's move beyond observational studies. What happens to body weight when omnivores stop eating animal foods?
Vegan diet interventions
Physician, researcher and vegan diet activist Neal Barnard has conducted a number of vegan diet studies. In 2007, he co-authored a two-year randomized weight loss study in overweight post-menopausal women "comparing a vegan diet to a more moderate low-fat diet" (6). Unfortunately for our purposes, the study didn't isolate the effects of animal foods specifically, because the vegan diet was also low-fat.
However, the low-fat vegan diet did cause weight loss. At one year, vegan participants had lost 11 pounds (5 kg), and they managed to keep most of it off for two years. As expected, those who adhered closely to the diet did better than those who were less adherent. This degree of weight loss is similar to what is reported from low-carbohydrate dietary interventions, although we don't have body composition data so we don't know how much of the loss came out of muscle tissue.
Vegetarian diet interventions seem to have a more limited effect on body weight. A 2008 randomized trial comparing weight loss diets with or without meat found that both diets produced a similar degree of weight loss over 18 months (7).
Together, these studies suggest that avoiding meat doesn't enhance weight loss, but avoiding all animal foods might. We don't yet have a very good sense of how much of this weight loss comes from fat vs. muscle tissue.
High-protein diet interventions
Low-fat vegan diets cause weight loss, but guess what else does too? High-meat diets! Or, more precisely, high-protein diets in which the protein comes mostly from meat.
Protein is the most satiating macronutrient: more filling than carbohydrate or fat, per unit calorie (8). Lean meat is the most concentrated and convenient source of protein. A number of studies have shown that high-protein diets cause people to eat fewer calories per day, and lose fat, without experiencing increased hunger (9, 10).
Not only do high-protein diets reduce calorie intake and promote fat loss, they attenuate the loss of muscle mass and reduction of metabolic rate that occurs during dieting (11, 10). Not surprisingly, this makes protein one of the most effective ways to support long-term fat loss maintenance (12, 13), along with physical activity. Protein is a powerful tool in the fat loss toolbox, which is why it's a central part of the Ideal Weight Program. We designed the Ideal Weight Program to accommodate omnivores, vegetarians, and everything in between, but meat is certainly an easy and effective way to meet your daily protein goal.
Some researchers believe that increased protein intake, rather than reduced carbohydrate intake per se, accounts for the fat loss effectiveness of low-carbohydrate diets (14, 10). The case isn't closed yet, but there is some evidence to support that view.
Together, the evidence suggests that meat, particularly fresh lean meat, is a powerful fat loss tool.
Synthesis and conclusion
So, does eating meat contribute to weight gain? Yes and no!
Meat intake has a complex relationship with body weight. Vegetarian and vegan diets are associated with leanness, but at the same time, high-meat diets are one of the most effective fat loss tools we have.
I believe the effects of meat on body weight may be explained by three simple principles:
- Taking an entire food category off the table tends to reduce total calorie intake.
- Fatty and cured meats are calorie-dense and highly palatable, and can therefore contribute to overeating.
- Fresh lean meat is rich in protein, which tends to increase satiety and suppress calorie intake.
* This was the study that caused the media to proclaim that potatoes are fattening. The reason has to do with a quirk of one of the figures: in Figure 1, the most widely reproduced figure, the authors lumped the data for French fries together with baked, boiled, and mashed potatoes! Since French fries had by far the strongest association with weight gain of any of the factors they studied (implausibly strong), lumping it together with non-fried potatoes made it appear as if all potatoes share the same strong association. It also had the added benefit of making Figure 1 look nice-- if they hadn't lumped them together, the figure would have been dominated by the massive, implausible, French fry data. The association they found for baked, boiled and mashed potatoes is much less impressive. People who increased intake of baked, boiled and mashed potatoes gained 0.6 lbs over 4 years, compared with 3.4 lbs for French fries, 1.7 lbs for potato chips, and 1 lb for sugar-sweetened beverages, unprocessed red meat, and processed meat. Also, keep in mind that people slather all kinds of calorie-dense toppings on their potatoes, so the effects of the potatoes themselves will be difficult to disentangle from the toppings. When is the last time you saw someone eat a plain potato?