In the fight over best diet for health and weight loss, it's protein lovers vs. vegetarian zealots. So far, a clear winner has not emerged. Only one loser: you, the victim of biased research. Here is an example of why you should keep your bullshit alarm on high alert when reading about weight loss diets.
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Ellen M. Evans and colleagues wanted to know whether overweight men and women differ in their body composition responses to different weight loss diets [1]. So they enrolled 58 men and 72 women with a BMI greater than 26, and randomized them into two diet groups.
One group was instructed to follow a high-protein low-carbohydrate diet, which delivered 1.6 g of protein per kg bodyweight per day. The high-carb group received only half that amount of protein, and both groups' fat intake was capped at 30% of total energy intake. Both diets contained the same amount of fiber. Women received a daily total of 1700 calories, men 1900 calories. The intervention lasted for 4 months, followed by an 8-months weight maintenance period. Fast forward to the 12-months results:
One group was instructed to follow a high-protein low-carbohydrate diet, which delivered 1.6 g of protein per kg bodyweight per day. The high-carb group received only half that amount of protein, and both groups' fat intake was capped at 30% of total energy intake. Both diets contained the same amount of fiber. Women received a daily total of 1700 calories, men 1900 calories. The intervention lasted for 4 months, followed by an 8-months weight maintenance period. Fast forward to the 12-months results:
Both diet groups and both genders lost about 10% of their body weight. But expressing weight loss in kilos of body weight can be a deceptive thing. Ideally we want that loss to be fat loss rather than loss of lean mass, that is, muscle mass. In the study at hand, for men on the high-carb diet, a little over one third of their weight loss came from lean body mass. Meaning, of the 14 kilos, which they lost on average, 5 Kilos came from a reduction in muscle tissue. The high-protein guys maintained their muscle mass to a greater extent: only 20% of their weight loss came from wasted muscle. For the women the picture looked almost identical: muscle mass contributed 37% to the weight loss of the high-carb women, compared to 23% in the high-protein group.
You would be forgiven if you now agreed with the authors' statement that the high-protein diet "...was more effective in reducing percent body fat...". Or in other words, a high-protein diet is superior to a high-carb alternative, as losing lean mass isn't a good thing in weight loss. I'll get to that point shortly in a little more detail.
Before we go there, let me state, that, being a firm supporter of the high-protein low-carb dietary philosophy, I loved to read this study. But I'm an equally firm supporter of proper scientific methods. And they have been prostituted in this case, which is why I love this study a lot less than its results.
Here is why: When I read the tables in which the authors present the results, I was impressed by the fact that both groups not only managed to rescue the 4-months weight loss to the 12-months finish line, but even increased this weight loss a little. When you have read literally hundreds of studies on weight loss interventions, as I have done, you'll find this observation to be in stark contrast to what we typically see: a reversal of weight loss. That is, at least a partial post-intervention regain of the weight lost during the dietary period.
We find the explanation for this miraculous exception in the number of participants. Or rather in the number of disappearing participants. Of the 66 participants who started in the high-carb group, only 30 made it to the finish line 12 months later. That's a drop-out rate of more than 50%! And of the 64 participants in the high-protein group 23, or 36%, had dropped out by month 12.
High drop-out rates are nothing unusual in weight loss trials, but it is good practice for researchers to tell their readers, how they accounted for these drop outs in the statistics, with which they interpret the data. Nothing of that in this paper. So, we don't know whether the drop-outs simply did not show up for their measurements, or whether the researchers did not consider the data of those participants, who failed to achieve some arbitrary weight loss threshold. The latter is an absolute no-no. It enables researchers to skew the results every which way they want. And the former is reason to investigate whether the drop-outs differed in some way significantly from the adherent participants. Such differences often affect the interpretation of the results.
One interpretation emerges right away, when checking the differences of relative fat loss while considering the drop-out rates: the smaller relative loss of muscle mass in the high-protein diet is not significantly different from the loss observed in the high-carb group. That does not mean, there is no difference between these two diet types. It only means, the study was underpowered to detect such difference, if there was any. And if it was underpowered to detect the difference between diet groups, it was certainly underpowered to differentiate between men and women in this respect.
If you still want the final verdict on high-carb vs. high-protein, I'm afraid I can't give it to you, even though I'm heavily leaning in favor of the high-protein version. I base my judgment on a 2009 systematic review of all randomized controlled trials, which were performed between 2000 and 2007, and which had pitted high-carb vs. high-protein strategies [2]. This review demonstrated that high-protein diets are more effective with respect to weight loss and probably with respect to cardiovascular risk factors than high-carb diets. At least over observation periods of 6 to 12 months.
Only long-term observations, comparing hard endpoints, can decide which diet may be better. Those studies are a long way off. To complicate matters, we might find that different people react differently to the same type of dietary strategy. Until we know better, we need to go with what we know:
The preservation of lean body mass certainly is a key aspect. Muscle tissue is an important endocrine organ, which, when exercised, produces potent anti-inflammatory substrates and hormones. These are the key elements of physical activity's protection against the initiating step of heart disease: atherosclerosis. Muscle tissue is also the body's primary site to store dietary carbohydrate in the form of glucose. The other site being the liver. With a high-carb diet, these storage sites are easily overwhelmed, which leads to conversion of carbs to fat. When, ironically, a high-carb diet nibbles away at the body's carb storage sites, you can imagine what this means to the body's relative fat content. Another aspect is that muscle tissue consumes energy, even at rest. The loss of this "burner" during weight loss makes weight rebound more likely.
So, if all these matters are known and understood, why perform a study, which is underpowered and fraud with questionable interpretations? Why produce the food equivalent of a scientology propaganda piece?
Beats me. Maybe because part of the study's funding came from the National Cattlemen's Beef Association and The Beef Board. Both of which are, of course, entirely neutral to the outcome of research funded by them, and unbiased to its interpretation.
It also beats me, why a respected journal and its peer reviewers facilitate the publication of such a study. Maybe because its senior author, Professor DK Layman, is a leading researcher in nutrition science, and...
...the Egg Nutrition Center's director of research.
As much as my dietary preferences place me in the protein camp of this contest, my bullshit alarm is set to high-sensitivity. And so should yours be.
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1. Evans, E., et al., Effects of protein intake and gender on body composition changes: a randomized clinical weight loss trial. Nutrition and Metabolism, 2012. 9(1): p. 55.
2. Hession, M., et al., Systematic review of randomized controlled trials of low-carbohydrate vs. low-fat/low-calorie diets in the management of obesity and its comorbidities. Obesity Reviews, 2009. 10(1): p. 36-50.
Hession, M., Rolland, C., Kulkarni, U., Wise, A., & Broom, J. (2009). Systematic review of randomized controlled trials of low-carbohydrate vs. low-fat/low-calorie diets in the management of obesity and its comorbidities Obesity Reviews, 10 (1), 36-50 DOI: 10.1111/j.1467-789X.2008.00518.x
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What about exercise, did the participants exercise following a plan? I think the quantity and type of exercise would be at least as important as the diet type. What about the types of carbohydrates? There is a big difference between, say, beans and sugar! Were the participants free to chose the carbohydrates they wanted (and to add fiber from isolated sources)? And would you not say that your nutritional needs vary with your daytime routine? Carbohydrates on your mountain bike and protein at your desk? I, for my part, could not exercise much on a "paleo" type diet without complex carbohydrates.
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