The bottom line


The bottom line is that your body will take its sweet time absorbing protein, no matter the amount. And this will vary from source to source. In the end, however, 30g is just an arbitrary number that has no roots in scientific evidence.

At this point let us circle back to the initial claim, that excess protein, which has already been absorbed, during times of adequate energy and carbohydrate intake, is predominantly converted to fatty acids and stored as body fat.

Liver, the primary site for amino acid metabolism

As we’ve already covered, the amino acids released from the small intestine are destined for the liver. Over half of all the amino acids ingested (in the form of protein) are bound for and taken up by the liver. The liver acts almost as a monitor for absorbed amino acids and adjusts their metabolism (breakdown, synthesis, catabolism, anabolism etc.) according to the body’s metabolic state and needs [2]. It is here the initial claim comes into play.

While the pathways for fatty acid synthesis from amino acids do exist, no argument there, the statement that all excess protein, under specified conditions, will be stored as fat ignores recent evidence.

Enter one of the most tightly controlled studies of our time.

Bray GA, et al. 2012

In 2012, George Bray and colleagues [3] sought to examine whether the level of dietary protein affected body composition, weight gain, and/or energy expenditure in subjects randomized to one of three hypercaloric diets: low protein (5%), normal protein (15%), or high protein (25%). Once randomized, subjects were admitted to a metabolic ward and were fed diets at 140% (+1,000kcals/day) of their maintenance calorie needs for 8 weeks straight. Protein intakes averaged ~47g (0.68g/kg) for the low protein group, 140g (1.79g/kg) for the normal protein group, and 230g (3.0g/kg) for the high protein group.

Carbohydrate was kept constant between groups (~41-42% of total calories), with dietary fat ranging from 33% in the high protein group to 44% and 52% in the normal and low protein groups, respectively. Lastly, during the course of the 8-week overfeeding period, subjects’ body composition was measured bi-weekly using dual x-ray absorptiometry (DEXA; i.e. the “Gold Standard” for measuring body composition).

Results

At the end of the study, all subjects gained weight with near identical increases in body fat between the three groups (in actuality, the higher protein groups actually gained slightly less body fat than the lower protein group, however, this wasn’t significant). The group eating the low protein diet gained the least amount of weight (3.16 kg) with the normal and high protein groups gaining about twice as much weight (6.05 and 6.51 kg, respectively).

See the chart below.

The effects of over-consuming ( ~ 1,000 kcal day) diets composed of low (5 %), normal (15 %), or high (25 %) amounts of energy from protein are shown. Over-consuming diets composted of 15-25 % energy from protein resulted in significant lean mass accretion compared to over-consuming a diet composed of 5 % energy from protein. Note there are no differences between groups in fat mass gain over the overfeeding intervention; the additional weight gain (body mass) in the 15-25 % groups is accounted for by lean body mass accretion. Redrawn from Bray et al. (2012)

However, the additional 3 kg of body weight gained in the normal and high protein groups was shown to be due to an increase in lean body mass and not body fat. To quote the conclusions of the authors:

“Calories alone […] contributed to the increase in body fat. In contrast, protein contributed to changes in […] lean body mass, but not to the increase in body fat.”

While we can’t say for sure the exact composition of the lean mass that was gained, we can assuredly say that the extra protein was not primarily used for fat storage. My hunch is that the excess protein that was not used for synthetic purposes was mostly converted to glucose (via gluconeogenesis) and stored subsequently as glycogen with the accompanying water weight.

Either way, it wasn’t body fat.


Regroup

So before we continue, let’s just take a second for this to sink in. These subjects were literally forced to eat ~1,000kcals more than what they needed to maintain their body weight for 8 full weeks (not an easy task, even for the strong-willed) and even then it was seen that the protein contributed to increases in lean body mass rather than body fat. Given the initial claim – that once energy, glucose and protein requirements are met all excess amino acids will get converted to fatty acids and stored as body fat – I find it hard to argue that those in the normal and high protein groups did not meet any of the aforementioned requisites. In reality, they far and away surpassed them, yet still did not gain additional body fat compared to the lower protein group. This is in stark contrast to what is thought.

In the end, however, we’re still left with the quintessential question underlying the whole concept, and that is: what is the maximal amount of protein (amino acids) that the body can effectively utilize before being converted to fatty acids and stored as body fat?

Given the results of this study; it appears that this number is either way higher than three times the current RDA with concomitant hypercaloric intakes for weeks on end, or; it requires a similar overfeeding protocol drawn out over a longer time period at which point lean mass gains would plateau and fat mass would accrue. Either way, both situations are highly unlikely for the general public and even those consciously trying to gain weight with higher intakes of protein and calories. Moreover, this upper extreme is likely to be highly individual and contingent upon other factors such as genetics, lifestyle, training status (if an athlete), etc.

Unfortunately, we just don’t have the answers to these questions right now.


Conclusions

So, while we do biochemically possess the pathways needed to convert amino acids to fatty acids, the chances of that ever happening to a significant degree during slightly higher protein intakes, even in the face of adequate energy and carbohydrates, are irrelevant given what we know about the extreme measures that need to be surpassed in order for any appreciable fat gain from protein to take place.

Indeed, overeating by ~1,000kcals/day for 8 weeks in combination with higher protein intakes did not amount to any additional gains in body fat compared to a lower protein, hypercaloric diet. Rather, excess protein in the face of overfeeding actually contributed to gains in lean body mass (be that what it may); quite the contrary to what textbooks and classrooms teach. In reality, the chances that excess protein contributes to body fat stores are insignificant, and arguably physically impossible under normal and/or even slightly hypercaloric conditions that most people/athletes face on a daily basis.

Only until theoretical extremes are achieved, either for protein intake or calories or both, will there be any significant contributions to body fat from excess protein intake.

Author: Dylan Klein
References

1. McGuire M, Beerman, KA.: Nutritional Sciences: From Fundamentals to Food. 2nd edn. Belmont, CA.: Wadsworth Cengage Learning; 2011.
2. Gropper S, Smith, JL., Groff, JL.: Advanced Nutrition and Human Metabolism. 5th edn. Belmont, CA.: Wadsworth Cengage Learning; 2009.
3. Bray GA, Smith SR, de Jonge L, Xie H, Rood J, Martin CK, Most M, Brock C, Mancuso S, Redman LM: Effect of dietary protein content on weight gain, energy expenditure, and body composition during overeating: a randomized controlled trial. JAMA 2012, 307:47-55.

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