Here's what you need to know...

While it's biochemically possible for protein to turn into fat by ingesting extremely high numbers of calories or extremely large amounts of protein, it's unlikely you'll ever be in that situation. You can pretty much eat as much protein as you want and it won't turn to fat. That old chestnut about only being able to absorb 30 grams of protein in one sitting is bunk.

Aside from building muscle, protein provides essential amino acids that serve as the building blocks for other proteins, enzymes, and hormones within the body that are vital for normal functioning. Without this steady supply of amino acids, the body resorts to breaking down its own proteins – typically from muscle – in order to meet this demand.

Protein has its share of misconceptions. It's not uncommon to hear claims that dietary protein eaten in excess of some arbitrary number will be stored as body fat. Even those who are supposed to be reputable sources for nutrition information propagate this untenable dogma. While paging through a nutrition textbook I came across a section in the protein chapter regarding amino acids and energy metabolism (1). To quote the book directly:

"Eating extra protein during times of glucose and energy sufficiency generally contributes to more fat storage, not muscle growth. This is because, during times of glucose and energy excess, your body redirects the flow of amino acids away from gluconeogenesis and ATP-producing pathways and instead converts them to lipids. The resulting lipids can subsequently be stored as body fat for later use."

This is, more or less, supported by another textbook I own (2):

"In times of excess energy and protein intakes coupled with adequate carbohydrate intake, the carbon skeleton of amino acids may be used to synthesize fatty acids."

While these passages do take into account the metabolic state of the person, I still find these explanations to be lacking. Indeed, more recent evidence is needed when talking about amino acid conversion to fatty acids. While the metabolic pathways to convert amino acids to fatty acids do in fact exist in the human body, the fact of the matter is that under almost no circumstance will this ever happen.

Protein Digestion Begins in the Stomach and Ends in the Small Intestine

While the physical breakdown of proteins does take place in the mouth, it's not until the protein reaches the stomach that any appreciable chemical breakdown occurs. This breakdown is facilitated by hydrochloric acid (HCl) and the enzyme pepsin (converted from its inactive form, pepsinogen). Once the initial protein denaturing and peptide cleaving is complete, the product polypeptides pass through the pyloric sphincter of the stomach and into the proximal small intestine (i.e., duodenum).

The proximal small intestine is where most of the digestion of proteins and virtually all of the absorption of amino acids occurs (~90% is absorbed with a very small amount excreted in the feces). Here even more digestive enzymes are present to break down the remaining polypeptides into their individual amino acids along with some trace amounts of di- and tri-peptides. Once broken down completely, the free amino acids and di-/tri-peptides can then enter the cells of the small intestine where some (like glutamine) are used for energy within the intestinal cells, while the remaining pass through into circulation and head for the liver.

Protein Absorption Claims

Before we head on over to the liver and discuss amino acid metabolism with regards to the initial claims, I'd first like to touch upon another related claim that you may have heard in the lay media or from your local gym guru. It usually reads:

"The average person can only absorb 30 grams of protein at one sitting. Anything above that will be stored as fat."

Unlike the previous claims, this one offers no context whatsoever. Moreover, it's downright moronic. While this claim looks like a straw man argument readily poised for the takedown, it actually comes from an online article written by a Registered Dietitian.

Let's, for example, take someone who eats 40 grams of protein in one sitting. If we're to assume only 30 grams can be absorbed at a time, then it's safe to say that the extra 10 grams will be excreted in the feces. If this were the case, most people would be egesting tiny sirloin steaks on a daily basis. Moreover, based on the initial argument, how are you supposed to convert and store 10 grams of excess dietary protein as body fat if you can't even absorb it in the first place? Nevertheless, while I agree that most processes in the human body don't proceed with 100% efficiency, excreting 25% of one's ingested protein content is far from the 90% efficiency supported in the literature (3).

Theoretically, it could be possible for intestinal absorption of amino acids to be drastically impaired, resulting in excess amino acids passing through into the colon. In reality, this process does occur but only to an extremely limited extent. More severe malabsorption would require the absorptive capacity of the small intestine to be greatly surpassed.

However, in the face of larger protein and caloric boluses, the stomach just reduces its rate of gastric emptying in order to more slowly supply the gut with incoming amino acids (larger meals take longer to digest). Moreover, the gut itself will slow motility as to increase the time nutrients are available to be absorbed (4).

The bottom line is that the stomach will take its sweet time to release amino acids into the gut where they can be further detained and subsequently absorbed into the body/circulation. Without this tightly orchestrated process, we as a species would have died off long ago. Having to consume intricately planned out 30g protein meals, multiple times per day in order to effectively derive the nutrients from our food just wouldn't have cut it.

Liver, the Primary Site for Amino Acid Metabolism

Let's circle back to the initial claims that excess protein, during times of adequate energy and carbohydrate intake, is converted to fatty acids and stored as body fat.

The amino acids absorbed and 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's 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

In 2012, George Bray and colleagues (5) 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 force fed 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 and 140g (1.79g/kg) and 230g (3.0g/kg) for the normal and high protein groups, respectively.

Carbohydrate was kept constant between the groups (41-42%), 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 (DXA, 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. This, however, 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).

As you can see, the additional ~3 kg of body weight gained in the higher protein groups (15% and 25%) 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 protein was converted to glucose (via gluconeogenesis) and stored subsequently as glycogen along with some accompanying water weight. Either way, it wasn't body fat.

Regroup

Let's 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, 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 – it's clear that those in the higher protein groups didn't succumb to any of those dire predictions. In reality, compared to the lower protein group, they gained very little (if any) additional body fat. 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: what's 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, etc. Unfortunately, we just don't have the answers to these questions right now.

Final Thoughts

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 didn't 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. Quite the contrary to what textbooks and gurus might preach.

In reality, the chances that excess protein contributes to body fat stores are insignificant, and arguably physically impossible under normal or even slightly hypercaloric conditions that most athletes face on a daily basis. Only until theoretical extremes – either for protein intakes or calories or both – are achieved will there be any significant contributions to body fat from excess protein intake.

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