New study on protein and weight is interesting, but let’s get to the nub of the issue…

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There’s a general notion in nutrition that, as far as body weight is concerned, ‘a calorie is a calorie’. In other words, the impact that food has on body weight is ultimately down to the number of calories we consume, and has nothing to do with the form that they come in. However, as I explain in my just-published book, Escape the Diet Trap, there are several reasons why different forms of calories may have different ‘fattening’ effects. I also explore the science behind this as well as detail more than a handful of studies which appear to provide evidence for the notion that a calorie is not necessarily a calorie after all.

This week, I came across another study which provides evidence for the idea that different forms of calorie can have differing impact on body weight (as well as body composition) [1]. Here, in brief, is what the study entailed:

25 adult men and women spent some time being assessed to ascertain the number of calories required to maintain a stable weight. Then they were randomised to one of three diets, each of which contained a different amount of protein (low, normal and high). The number of calories contributed by protein, carbohydrate and fat respectively for the three diets was:

  • Low protein – 6:42:52
  • Medium protein – 14:41:44
  • High protein – 26:41:33

You’ll notice that percentage of calories from carbohydrate was essentially the same for all three diets – only protein and fat intakes were different. The individuals were fed these diets in a very controlled environment for a period of 8 weeks.

One other thing: these diets supplied about 40 per cent more calories than estimated to be required for weight maintenance. This equated to overfeeding of an average of about 950 calories a day.

The researchers assessed a number of body parameters including body weight, fat mass and lean mass (muscle, essentially). Here’s a summary of the findings (all figures are kilograms):

weight change lean mass change fat mass change
low protein +3.61 -0.70 +3.66
medium protein +6.05 +2.87 +3.45
high protein +6.51 +3.18 +3.44

In summary, the most notable findings were, I think:

  • The amount of fat gained was essentially the same (irrespective of dietary composition).
  • The normal and high protein diets both led to an increase in lean mass (muscle) that was roughly equal in weight to the fat that was accumulated.
  • The low protein diet led to less weight gain overall, principally because it did not contribute to lean mass gain (actually some lean mass was lost).

The main conclusion drawn by the authors was that when people are overfed, the fat they accumulate is not influenced by the amount of protein they consume (only the total number of calories).

That observation is supported by their data, but another way of interpreting the same data would be to say that when people over-consume food, the percentage of calories coming from fat have no bearing on fatness. This perhaps contrasts with what most people might imagine, as many believe is easier for dietary fat to end up as fat in our tissues than carbohydrate or protein.

Another thing that this study appears to show is that when protein is low and fat is high, less weight is gained overall than when, relatively speaking, protein is higher and fat is lower. This finding does appear to provide evidence that contradicts the idea that all calories influence body weight identically whatever their form (a calorie is a calorie).

What is also true is this study also found that if we’re looking to maintain (or perhaps build) muscle, it makes sense to eat adequate amounts of protein. The diets richer in protein led to improved body composition compared to the low-protein diet.

OK, one could argue there’s some interesting stuff here, but I would also argue that this study tells us very little indeed that has real relevance to people seeking to lose weight or maintain a healthy weight. And that’s because in this study, the individuals were essentially compelled to eat much more than they ordinary would. In the real world, when people overeat they usually do it all by themselves (without being asked to or remunerated for it).

So, what is it that causes people to overeat? There are many factors here, but let me list just a few:

1. Too little protein

Study after study shows that for a given number of calories, protein sates the appetite more effectively than either carbohydrate or fat.

2. Too little fat

Quite a lot of people find that for food to be satisfying, it helps for it to contain a decent quantity of fat. I have this going on: a chicken leg leaves me feeling much better sated than a dry breast.

3. Too much carbohydrate

Eating a carbohydrate rich diet can cause cycles of blood sugar high and lows. Lows in blood sugar (or even just rapidly reducing sugar levels) can cause hunger and food cravings (particularly for sugary/sweet/starchy foods).

4. Too much carbohydrate again

A lot of carbohydrate can cause elevated levels of insulin – the hormone chiefly responsible for fat storage in the body. This can make fat hard to shift from the fat cells. That’s a shame, because fat liberated in this way can provide fuel for the body, and is essentially food. It is possible that liberated fat helps quell the appetite through this mechanism, but that’s not going to happen so well if the diet is rammed full of carbohydrate and causing chronically elevated levels of insulin.

Let’s put this in reverse, though. Imagine what would happen if less carbohydrate was eaten. Insulin levels may well come down and now fat can get out of the fat cells, where upon it can ‘feed’ us and keep us nicely satisfied. The proportionally more fat and protein in the diet might help here too. Time and again I’ve seen individuals who adopt a lower-carb diet relatively rich in protein and fat they automatically eat less (often several hundred calories less each day) than they ordinarily do, and the research bears this out too.

Overfeeding studies like the one described above can be interesting, but they don’t really help people who want to lose weight. What we really need to know if what to eat to facilitate fat loss and keep hunger at bay.


1. Bray GA, et al. Effect of Dietary Protein Content on Weight Gain, Energy Expenditure, and Body Composition During Overeating – A Randomized Controlled Trial. JAMA 2012;307(1):47-55

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