I’ve discussed in a previous article how studies on multiple different species of animal show that protein is the primary factor determining body weight. Put a mouse, a cockroach, or a monkey on a high protein diet (i.e. one where a high percentage of calories comes from protein), and it becomes lean. Switch it to a low protein diet, and it becomes obese.
Since this pattern is seen so clearly in widely varying species, it seems reasonable to think that it would also apply to humans. But what we really want is to confirm that hypothesis with randomised trials that put people on either a high protein or a low protein diet, and then follow them over time to see what happens to their body weight.
There are actually lots of trials that have tested a high protein diet on humans. Unfortunately, almost all of them are crap, so flawed in their design or execution that they can’t actually tell us anything useful. Two major flaws stick out in almost all of these studies, immediately invalidating them. The first is that they restrict calorie intake, usually in both the high protein group and the control group, but sometimes just in the control group.
Why is this a problem? Because the whole theory behind a high protein diet is that it works by reducing our appetite and thus our calorie intake. Basically, the thinking goes that protein is the most satiating food substance, so a higher protein content in the diet means less calories eaten overall. If you consciously restrict calories then you’re accomplishing the same result (less calorie intake), but through a different means (i.e. starvation).
It’s been shown many times over that a calorie reduced diet is effective over the short term, regardless of what the diet consists of. People are able to starve themselves temporarily, and thus lose weight, regardless of whether their diet consists entirely of red meat or vegetables or Doritos. But starvation doesn’t work long term (at least not if you’re living in a first world country with easy access to food). People can only consciously starve themselves for so long. Then they give up. And when they do, the weight rapidly piles back. That’s why conscious calorie restriction isn’t an effective long term diet.
A high protein diet isn’t magical. If it works, then it works by reducing appetite, which reduces calorie intake. In other words, it causes unconscious calorie restriction. That is why it shouldn’t just work short term, it should also work long term – it’s easy to maintain.
If you do a study where the control group reduces calorie intake by consciously starving itself, and the intervention group reduces calorie intake by eating a greater percentage of protein, so that both the control group and the intervention group are consuming the same reduced amount of calories, then both groups will lose a similar amount of weight. The study will thus not be able to show a meaningful difference between the groups, at least over the short term (and diet studies are almost invariably short term).
This flaw in thinking invalidates most so-called trials of a high protein diet. A properly designed test of the high protein diet doesn’t restrict calories at all in either group. Both groups need to be allowed to eat as much as they need to in order to feel full.
The second major flaw that is seen in almost all so called “high protein” trials is that the trials are not able to achieve a meaningful difference in protein intake between the groups. Almost all only achieve a difference of a few percent, so that the “high protein” group perhaps consumes 21% of calories as protein, while the “low protein” group consumes 18%. That difference is so small that it is unlikely to produce a meaningful weight loss effect over the course of a few months (although it probably would produce a meaningful difference over several years, if any diet trials ever ran that long).
In order to expect a meaningful difference after just a few months, the difference in protein intake needs to be at least 10%, and preferably more.
When I’ve gone looking, I’ve only found a single high protein weight loss trial that didn’t restrict calories, that did achieve a meaningful difference in protein intake between the high protein group and the control group, and that on top of those two conditions had at least 50 participants (a bare minimum for meaningful results), and ran for at least three months (again, a bare minimum for meaningful results).
It was published in the International Journal of Obesity in 1999. It was funded by companies that sell meat and dairy products (as usual in nutrition science, no-one funds a study for the betterment of humankind – they do it because they hope it will help them sell more product). It was carried out in Copenhagen, Denmark.
65 overweight and obese adults were randomised to one of three different groups and followed for six months.
The first group was provided with a high protein diet (25 individuals), the second a low protein diet (25 individuals), and the third was told to just keep doing what it was already doing (15 individuals). In other words, there were two control groups – an “active” control group and a “passive” control group.
This is actually quite useful. The active control group is a necessity, because it allows you to see how much of any weight loss is actually due to the high protein diet and how much is due to the intervention effect (i.e. the fact that when people take part in an intervention it causes them to start thinking and acting differently in lots of different ways), but the passive control group is also nice to have, because it allows you to see what would have happened if the participants hadn’t taken part in the study and had just gone on about their lives.
When I said before that the participants in the two active groups were “provided” with their respective diets, that is exactly what I meant. Twice a week the participants went to a “store” at the university where they were provided with all their food for the next few days, free of charge. When they went to the “store” they also returned any uneaten food from their previous visit, so that a detailed record could be kept of what they had eaten.
The high protein group was given a mix of foods that provided 25% protein, 45% carbohydrate, and 30% fat, while the low protein group was given a mix of foods with 12% protein, 58% carbohydrate, and 30% fat. If you give less of one micronutrient, you have to give more of another. In this case, protein was replaced with carbohydrate, while fat was the same in both groups. One could therefore argue that it’s the increased carbohydrate content in the diet that’s producing any difference between the groups, and not the decreased protein content.
The only way to settle that issue conclusively would be to do another almost identical study where the protein is replaced with fat, not carbs, and see if the same effect is seen. However, if it was the case that it was the different proportions of carbs that were generating any difference in weight loss, and not the different proportions of protein, then that would make humans pretty unique in the animal kingdom. As mentioned earlier, it has been shown conclusively in many different animals that it’s the protein content that drives weight change.
The fact that the food was provided free of charge to the participants by the researchers from their own “store” is a major strength of the study, because it allowed the researchers to continuously keep track of what the participants were actually eating, and thus to ensure that there was a meaningful difference in protein intake between the groups. Most diet trials just give the participants some rough guidance on what to eat and then send them on their merry way and hope for the best, which is a recipe for a failed study.
Ok, so what were the results?
At six months, the high protein group had lost 10% of their starting weight. The low protein group (a.k.a. the active control group) had lost 6% of their starting weight. And the passive control group had actually gained a little weight, weighing 2% more at the end of the study than they did at the beginning. The differences between the groups were highly statistically significant (p=0.0002 for you nerds, like me, who want to know the exact number).
What does this tell us?
Well to begin with, the fact that the passive control group gained weight is as we would expect. Without any intervention, people who are overweight or obese tend to continue to gain weight over time.
Then we come to the two active groups. Here we see a significant weight loss in both groups, which shows that just getting people to do something active, which gets them to start thinking consciously about what they’re eating, will have a beneficial effect on their weight. However, the high protein group was clearly superior, losing almost twice as much weight as the low protein group.
The study does have one major weakness, which is that it was unblinded. The researchers knew who was in which group, and this might have influenced their behaviour in some way that increased the difference in weight loss between the high protein group and the low protein group. Lack of blinding will tend to introduce biases in to a study, making the benefits of an intervention seem bigger than they really are. However, the size of the difference between the groups in this study was so large that it’s hard to see how it could purely be due to any biases introduced by the lack of blinding.
Other than protein, the factor that has been shown to influence body weight most is fiber. So we want to know what the differences in fiber intake were between the groups. If the high protein group was consuming much more fiber than the low protein group, then that could explain some of the difference in weight loss. But when we look, we see that the low protein group was actually consuming more fiber than the high protein group (23 grams per day vs 19 grams), so if anything the differences in fiber intake should have reduced the size of the difference in weight loss between the groups, not increased it.
Overall, in spite of some flaws, this was a well designed trial and it confirms what’s been found in animals – that a high protein diet is effective for weight loss. It’s worth noting that high protein in this study was only 25%. There have been traditional societies around the world, such as the Inuit and Masai, that have been getting more like 30-40% of their calories from protein for their entire life span without any obvious negative health effects. So 25% protein isn’t really that high, and it’s likely that an even more rapid and effective weight loss could be achieved with an even higher protein intake.