New JAMA Study: Effect of Dietary Protein on Body Composition During Overeating

[hailtotheking]

Effect of Dietary Protein Content on Weight Gain, Energy Expenditure, and Body Composition During Overeating, January 4, 2012, Bray et al. 307 (1): 47

This study doesnt present a whole lot of radically new ideas, but it does go some way towards more rigorously validating some ideas and questioning others when it comes to the 'magic' of protein

Its important is because of how rigorously controlled it was (metabolic ward) and its length (3 months). The video is worth a watch / listen:

Here is a good laymans overview, with a decent video commentary at the bottom by one of the lead scientists involved.
Varying dietary protein with overfeeding in a metabolic ward | | nutscinutsci

From what i understand of it, the take home points are:

- Fat gain is a direct result of how much of a calorie excess you consume, and changing protein intakes does not vary how much fat is gained.

- The amount of weight gained on the same calorie excess can vary significantly depending upon how much protein is consumed - the higher the proportion of protein, the more weight gained.
Because fat gain was the same in all groups, this was therefore all down to an increase in lean mass - i.e. increased % protein intake lead to greater lean mass gains.

- Increasing protein intake does increase thermogenesis (energy wasted during metabolism and ‘lost’ as heat). But even though thermogenesis went up with protein intake, it didnt stop the same amount of fat being gained as was also seen on the lower protein diets.

Basically, if you took a low, high, and medium protein diet all at the same calorie excess, you would gain the same amount of fat regardless of protein intake. Increased thermogenesis from increased protein doesnt change that. The idea that eating proportionally more protein to limit fat gain is a myth.

[NU_nutrition_TS]

I was recently reminded that protein being the most satiating macronutrient is also a myth!

[stavrog1]

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I was recently reminded that protein being the most satiating macronutrient is also a myth!

I take it the argument was that fat is the most satiating macro then?

[Matt Strong]

Protein is quite clearly the most satiating gram for gram....

[NU_nutrition_TS]

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I take it the argument was that fat is the most satiating macro then?

Depends on which study or studies you believe. Perhaps I was being inaccurate by saying protein was NOT the most satiating (in the sense that 'satiety' means the sensation of being 'full' or 'satisfied'). However, that is only worth a damn if it means you eat less at subsequent meals. The best study I have seen shows that protein does not limit appetite or intake at subsequent meals better than fat.

[NU_nutrition_TS]

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Protein is quite clearly the most satiating gram for gram....

Like I said, that is what is popularly believed. But is the evidence really that strong? Are the study protocols designed in such a way that they can show a clear advantage without the influence of confounding variables muddying the waters?

The study most often cited in support of protein being the most satiating macronutrient is this one by Weigle et al:
A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations

However, if you work out the absolute mass of food consumed on each diet you will see that the higher protein diet allowed for more food to be consumed for the same number of total calories.

One of the physiological determinants of 'fullness' is the presence of 'stretch receptors' in the stomach - the more food your stomach contains the greater the signal from these receptors and the more 'full' or 'satiated' you are likely to feel after eating.

In their discussion, the study authors state:

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It is likely that a reduction in dietary fat by 15% of total energy contributed to weight loss in the present study. However, a larger 20% reduction in dietary fat with no change in the percentages of calories from protein produced less weight loss (3.7 ± 0.6 compared with 4.9 ± 0.5 kg; P = 0.13) in our previous study.

The previous study they refer to is here:
Roles of Leptin and Ghrelin in the Loss of Body Weight Caused by a Low Fat, High Carbohydrate Diet

Again, a similar study design results in the lower fat diet having more absolute grams of food for the same total calories. Therefore, feelings of satiety could be attributed to the effects on stomach stretch receptors of the greater mass of food consumed.

In this previous study the percentage of total calories from protein was kept constant (at 20%) while the proportions of fat to carbohydrate was varied (35:45 & 15:65). Now read that quote (above) again. They state that, although they credit fat restriction as an important factor in weight loss, the larger reduction in dietary fat in their previous study resulted in less weight loss. They go on to infer that this may have been due to protein being held constant in both diets. But they ignore the reciprocal (and confounding) factor that carbohydrate was significantly increased in the lower fat diet!

Now, the question is how is dietary fat likely to be more satiating than protein given that you can eat far less of it, in absolute terms, while keeping calories constant? And, perhaps more importantly, does it not only satiate you immediately after you have eaten it but limit your intake at subsequent meals?

For these questions you have to consider what other mechanisms may exist - besides stretch receptors - to signal satiety. In the case of dietary fats, SI mucosal cells produce a lipid messenger called OEA, which may then engage PPAR-alpha to reduce meal frequency. Neither proteins nor carbohydrates have this effect.