March 20, 2017

Protein will not make you fat

Here's what you need to know...

Although it is biochemically possible for protein to be converted to fat if you consume large amounts of calories or extremely large amounts of protein, it is unlikely that you will ever find yourself in this situation.
You can eat more or less as much protein as you want and it won't make you fat.
The old myth that it is not possible to absorb more than 30 grams of protein during a meal is nonsense.

Aside from building muscle, protein provides essential amino acids that serve as building blocks for other proteins, enzymes and hormones in the body that are essential for normal body function. Without this continuous supply of amino acids, the body is forced to break down its own proteins - typically from muscle tissue - to meet its amino acid needs.

There are a lot of misconceptions about protein. It's not uncommon to hear claims that protein intake in excess of an arbitrarily chosen amount will result in storage as body fat. Even those who are supposed to be reliable sources of nutritional information propagate this unsupportable dogma. While flipping through a nutrition textbook, I stumbled across a section in the protein chapter regarding amino acids and energy metabolism (1). Here is the quote from that book:

"Consuming supplemental protein during periods of adequate glucose and energy intake generally contributes to higher fat storage rather than muscle growth. This is because during times of excess glucose and energy, your body directs the flow of amino acids away from gluconeogenesis and ATP-producing pathways and converts them into lipids instead. The resulting lipids can then be stored as body fat for later use."

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

"During periods of excess energy and protein intake combined with adequate carbohydrate intake, the carbon skeleton of amino acids can be used to synthesize fatty acids."

Even if these passages take into account the metabolic state of the person, I still don't think this explanation is complete. In fact, more up-to-date studies are needed when it comes to the conversion of amino acids into fatty acids. While the metabolic pathways for converting amino acids to fatty acids do indeed exist in the human body, the fact is that under most circumstances this conversion will not occur.

Protein digestion begins in the stomach and ends in the small intestine

Even though the physical "breakdown" of protein takes place in the mouth, it takes until the stomach for any significant chemical breakdown to occur. This breakdown is promoted by hydrochloric acid and the enzyme pepsin (which is formed from its inactive form pepsinogen). After the initial protein denaturation and protein cleavage is complete, the polypeptides pass through the gastric pyloric muscle and enter the duodenum.

The duodenum is where most of the protein digestion and virtually all of the absorption of amino acids takes place (about 90% of amino acids are absorbed, while a small amount is excreted in the stool. Here, even more digestive enzymes are present to break down the remaining polypeptides into their individual amino acids along with traces of di- and tripeptides. Once fully broken down, the free amino acids and the di- and tripeptides can enter the cells of the small intestine, where some of these (such as glutamine) are used as an energy source within the cells of the intestine, while the remaining amino acids enter the bloodstream and are transported to the liver.

Claims regarding protein absorption

Before we move on to the liver and discuss amino acid metabolism in relation to the initial claims, I would first like to address another related claim that you may have heard about in the media or through your local gym. This claim usually goes along the following lines:

"The average person can only absorb 30 grams of protein per meal, anything above this is stored as fat."

Unlike the previous claims, there is no context for this one. In fact, it is completely idiotic. While this claim may sound like a straw man argument, it actually comes from an online article written by a registered dietitian.

Take, for example, a person who consumes 40 grams of protein during a meal. If we were to assume that only 30 grams of protein can be absorbed at a time, then it is safe to say that the extra 10 grams would be excreted in the stool. If this were the case, then most people would be excreting small steaks on a daily basis. Going back to the original assertion, there is also the question of how we can store 10 grams of excess protein as body fat if we can't absorb it in the first place. While I agree that most processes in the human body do not operate at 100% efficiency, excreting 25% of the protein consumed is a far cry from the 90% efficiency supported by the literature (3).

Theoretically, it could be possible that the absorption of amino acids in the gut is drastically impaired, which would result in excess amino acids passing through the gut. In reality, this process is extremely limited. A serious absorption disorder would require the absorption capacity of the small intestine to be greatly exceeded.

With large amounts of protein- and calorie-rich food, the stomach simply reduces the rate at which it empties in order to supply the intestine more slowly with the amino acids supplied (which means that larger meals take longer to digest). In addition, the gut itself will slow down motility (rate of movement of food mash in the gut) to increase the time nutrients are available for absorption (4).

The bottom line is that the stomach will allow sufficient time for the amino acids to be released into the intestine, where they can be further retained and subsequently absorbed into the body/bloodstream. Without this finely tuned process, we would have died out as a species a long time ago. Consuming precisely planned meals with 30 grams of protein several times a day in order to efficiently isolate the nutrients from our food would simply have been too much to ask in our evolutionary past.

The liver, the primary site of amino acid metabolism

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

The amino acids that are absorbed and released from the small intestine are transported to the liver. More than half of all amino acids supplied in the form of protein are absorbed by the liver. The liver acts as a kind of controller for absorbed amino acids and adjusts their metabolism (breakdown, synthesis, catabolism, anabolism, etc.) based on the metabolic state of the body and the body's needs (2).

This is where the initial assertion comes into play. Even though the pathways for fatty acid synthesis from amino acids exist - there is no room for debate here - the statement that all excess protein is stored as fat under the specified circumstances ignores current research findings.

Welcome to one of the most rigorously controlled studies of our time

In 2012, George Bray and colleagues set out to investigate whether dietary protein levels affected body composition, weight gain and/or energy expenditure in subjects who followed one of three hypercaloric diets (i.e. consumed more calories than they expended): low protein (5% protein), normal protein intake (15%) or high protein (25%).

After the subjects were randomly assigned to one of the three groups, they were placed in a metabolic ward and had to consume 140% of their maintenance calorie intake (+ 1000 kcal per day) every day for a period of 8 weeks. Protein intake averaged 47 grams (0.68 g/kg) in the low-protein group, 140 grams (1.79 g/kg) in the normal-protein group and 230 grams (3.0 g/kg) in the high-protein group.

Carbohydrate intake was the same in all groups (41 to 42%), while fat intake ranged from 33% in the high-protein group to 44% in the normal-protein group and 52% in the low-protein group. Over the course of the 8 weeks, the subjects' body composition was measured every 2 weeks using Dual X-Ray Absorptiometry (DXA, the "gold standard" for body composition measurements).

Results

At the end of the study, all subjects from all groups showed almost identical gains in body fat. (Strictly speaking, the group on the high-protein diet had gained slightly less body fat than the group on the low-protein diet. However, this difference was not statistically relevant). The low-protein group gained the least amount of weight (3.16 kilograms), while the subjects in the normal and high-protein groups gained about twice as much weight (6.05 and 6.51 kilograms).

As you can see above, the extra 3 kilos gained by the subjects in the two higher protein groups (15% and 25%) was lean body mass and not body fat. The conclusion of the authors of the study was as follows:

"Calories alone contributed to the increase in body fat. In contrast, protein contributed to changes in lean body mass but not to body fat gain.

Although we can't say for sure what the exact composition of lean body mass built up was, we can say with a fair degree of certainty that the extra protein was not primarily used for fat storage. My sense is that the protein was converted to glucose (via gluconeogenesis) and subsequently stored as glycogen in conjunction with some water weight. Either way, it wasn't body fat.

Let's rethink our ideas

Let's let this sink in for a second. The subjects were literally forced to eat about 1000 kcal more than they needed to maintain their body weight for 8 full weeks, and even then, protein was observed to contribute to an increase in lean body mass rather than body fat.

Considering the initial claim - that once energy, glucose and protein needs are met, all excess amino acids are converted to fatty acids and stored as body fat - it is clear that members of the higher protein intake groups did not succumb to any of these silly predictions. In reality, they gained very little (if any) additional body fat compared to the low protein intake group. This is in stark contrast to what is commonly believed.

In the end, however, the essential question underlying the whole concept still remains: what is the maximum amount of protein (amino acids) that the body can effectively use before it starts converting amino acids into fatty acids and storing them as body fat?

Considering the results of this study, it appears that this number is either significantly higher than three times the officially recommended daily protein intake with an accompanying hypercaloric diet for weeks in a row, or that it requires a similar protocol of excessive caloric intake over an extended period of time before the gains in lean body mass reach a plateau and fat mass increases more.

Either way, both situations are highly unlikely for the general public and even those who are consciously trying to gain weight through higher protein and calorie intake. Furthermore, this upper extreme is likely to be highly individualized and dependent on other factors such as genetics, lifestyle, training status, etc. Unfortunately, we don't have the answers to these questions yet.

Final thoughts

So even if we have the biochemical processes required to convert amino acids to fatty acids, the likelihood of this ever happening to any significant degree - even in the face of adequate energy and carbohydrate intake - with a slightly increased protein intake is minimal, given what we know about the extreme measures required for protein to lead to a significant increase in fat.

In fact, a caloric surplus of 1000 kcal per day for 8 weeks combined with a higher protein intake compared to a hypercaloric diet with less protein did not lead to additional gains in body fat. Instead, excess protein in the face of excess calorie intake contributed to gains in lean body mass. This is the opposite of what textbooks and gurus preach.

In reality, the likelihood of excess protein contributing to body fat storage is insignificantly low and undoubtedly virtually impossible under normal or slightly hypercaloric circumstances, as can be observed in most athletes. Only when theoretical extremes - either for protein intake or calorie intake or both - are reached will excessive protein intake contribute significantly to body fat gain.

References

McGuire M, Beerman, KA.: Nutritional Sciences: From Fundamentals to Food. 2nd edn. Belmont, CA: Wadsworth Cengage Learning; 2011.
Gropper S, Smith, JL, Groff, JL: Advanced Nutrition and Human Metabolism. 5th edn. Belmont, CA: Wadsworth Cengage Learning; 2009.
Ten Have GA, Engelen MP, Luiking YC, Deutz NE: Absorption kinetics of amino acids, peptides, and intact proteins. Int J Sport Nutr Exerc Metab 2007, 17 Suppl:S23-36.
Maljaars PW, Peters HP, Mela DJ, Masclee AA: Ileal brake: a sensitive food target for appetite control. A review. Physiol Behav 2008, 95:271-281.
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.