A new study published in The American Journal of Clinical Nutrition is reporting an association with eating meat and weight gain. This is a fairly robust epidemiological study, but at the same time is a good example of how such information is poorly reported in the media, leading to public confusion.

The data is taken from the European Prospective Investigation into Cancer and Nutrition–Physical Activity, Nutrition, Alcohol, Cessation of Smoking, Eating Out of Home and Obesity (EPIC-PANACEA) project. This is a long term epidemiological study involving hundreds of thousands of individuals, and is therefore a great source of data. We are likely to see many publications from from it. This one looked at the association of meat eating – poultry, red meat, and processed meat – with total weight.  From the methods:

A total of 103,455 men and 270,348 women aged 25–70 y were recruited between 1992 and 2000 in 10 European countries. Diet was assessed at baseline with the use of country-specific validated questionnaires. A dietary calibration study was conducted in a representative subsample of the cohort. Weight and height were measured at baseline and self-reported at follow-up in most centers. Associations between energy from meat (kcal/d) and annual weight change (g/y) were assessed with the use of linear mixed models, controlled for age, sex, total energy intake, physical activity, dietary patterns, and other potential confounders.

They found that an increase in 240 grams per day of meat in the diet was associated with a 2kg increased weight after 5 years (that’s about 5 pounds, or 1 pound per year). The BBC reported this study as finding:

A European study of almost 400,000 adults found that eating meat was linked with weight gain, even in people taking in the same number of calories.

and

Although it is not clear why meat would lead to weight gain in people eating the same number of calories, one theory is that energy-dense foods like meat alter how the body regulates appetite control.

I find that conclusion problematic in several ways. Let’s look at the study design. One primary weakness is that weight (after the initial weighing) was self reported in most centers. This is a odd study design, and I can only assume this was a matter of practicality. Regardless of reason, self-reported weight is a major weakness. However it pales in comparison to the fact that total caloric intake was estimated, not rigorously controlled. To put this into perspective, 1 pound per year is 3500 Calories, or 67 Calories per week on average. There is no way someone can estimate their caloric intake within 67 Calories per week – that’s less than 10 calories per day.

The notion that appetite control was responsible for the findings also contradicts the assertion that total caloric intake was the same – appetite can only affect weight by increasing caloric intake. The correlation itself is in question because of the self-reported weight. But if we take the correlation as a given, the easiest explanation is that people who consume more meat also tend to consume slightly more calories, which add up over the years. Another possibility is that increased consumption of meat might also correlate with slightly less physical activity.

Assigning a cause and effect is difficult because slight changes that are difficult to measure accurately can result in modest weight differences over years.

Also, the authors concluded:

Our results suggest that a decrease in meat consumption may improve weight management.

“Suggest” and “may” are appropriate in that statement, but were largely lost in the secondary reporting. Again – even if we take the correlation as a given, this kind of data cannot be used to assign cause and effect. It cannot be concluded, in other words, that reducing meat will help reduce weight. Perhaps people who are more hungry for other reasons consume more meat, and if they cut down on their meat consumption they will just replace those calories with other sources. Other studies show that it is the consumption of calorie dense foods that correlate with weight gain, which can either be high fat and protein or high sugar. Calorie density seems to be the common element – which makes sense as increased calorie density can easily lead to overeating total calories, and it only takes a small amount to result in the kind of weight differences typically reported by these studies.

What we don’t have is evidence that decreasing meat intake as an intervention aids in weight control.

Conclusion

This study is interesting, but ultimately does not add much to our knowledge of diet and weight. It is not evidence that diets with the same calories but of different types lead to different weight outcomes, as has been reported. It does add to the literature that suggests that calorie dense foods correlate with weight gain, and this is likely due to increased overall caloric intake. There may be other factors as well, such as total activity, effects on hunger, and even calorie efficiency – how efficiently our bodies extract calories from certain foods.

But I am also struck in such studies, even intervention studies, by how small the difference are among the various diet types. This leads me to the conclusion that varying the ratios of macronutients (protein, carbohydrates, and fats) is of little ultimate utility in weight control. These studies get much attention in the media, but it is often much ado about nothing.

Meanwhile, the more significant factors are basic things like portion control and regular exercise. For health reasons other than weight control eating more vegetables is also a good idea, and this is also a good way to reduce total caloric intake.