An exercise
recovery shakes salesman recently posted up outside of our gym, offering free
samples to interested members. Although
I do not particularly subscribe to post-workout reconstituted beverages, he piqued
my interest. When I told him that I
would like to try the “protein” formulation, he responded, “That has too much
protein in it for you. I’m only going to
give you 2/3 a scoop.” While he measured
out my soon-to-be drink, he recommended the “women’s” formulation instead. I was somewhat taken aback, but his proclaim
got me thinking, what does happen when you consume too much protein? Is my effort to build more muscle instead
detrimental to my health?
Proteins
play many roles in biology. They facilitate
biochemical reactions (enzymes). They
serve structural and mechanical functions such as forming scaffolding for cellular
structure and executing muscle contraction.
In addition, they often act as signaling molecules that relay messages
between cells. Proteins are synthesized
in the cell by linking together building blocks known as amino acids. There are 20 standard amino acids that
represent a variety of different chemical functionalities – acidic, basic,
aromatic, polar, and hydrophobic – that can be combined to form proteins with
diverse structures and functions. Humans
can create all but 9 of these amino acids themselves, and therefore rely upon
their diet to supplement these essential protein building blocks.
The
National Academy of Sciences Institute of Medicine Food and Nutrition Board recommends
that both men and women consume “0.8g of good quality protein/kg body
weight/d.” For a 67kg person, like
myself, this amounts to roughly 54g of protein/day. In other words, I should eat 1 cup of cottage
cheese and a 3.5 ounce piece of chicken breast (about half of a large chicken
breast) per day. Prior to joining the
aerobic/weightlifting/gymnastics cult known as CrossFit, I definitely did not
make it a priority to consume protein. Now
I try to consume protein at every meal, which puts me over my daily-recommended
intake.
After eating
your favorite meat/cheese/protein source, the food makes its way from your
mouth down to your stomach. Its acidic
environment, combined with enzymes secreted by the cells lining the stomach,
break down the proteins into individual amino acids. As the food travels from the stomach, to the
small intestine, and then to the large intestine, different proteases (enzymes
that break down proteins) are released to further digest the proteins. These free amino acids are then absorbed by
the epithelial cells lining the respective organ and taken up into the blood
stream, through which they make their way to the liver. Here, transaminases and deaminases (enzymes
that transfer and remove amino groups respectively) break down the amino acids
into alpha-keto acids and ammonia. The
alpha-keto acids can be utilized for energy production, whereas the ammonia
provides a nitrogen source for the biosynthesis of new proteins, nucleotides
(DNA and RNA building blocks) or other biological amines. If there is excess ammonia, it is excreted,
for example as urea in urine. Consumption
of protein most importantly provides a nitrogen source for a variety of
biomolecules. When other more easily
metabolized sources of energy are low (e.g. carbohydrates or fats), proteins
can be used instead.
But what if
not all of the free amino acids are absorbed into the bloodstream? What if some slip through and are instead
exposed to the trillions of bacteria that inhabit our guts? Our commensal bacterial friends feed on the
nutrients that are not readily absorbed by our gastrointestinal tract and
generate byproducts that are both beneficial and detrimental. For example, we cannot readily digest
resistant starches and fiber, leaving them for our gut microbes. They consume these carbohydrates and produce
short chain fatty acids (SCFAs), which are an energy source for the cells
lining the large intestine. These SCFAs also
have been shown to regulate inflammation and even to help fight off pathogenic
bacteria. In contrast, bacterial
fermentation of a subclass of amino acids produces toxic hydrogen sulfide and
aromatic compounds (phenols and indoles).
Our liver and kidneys, if functioning properly, neutralize these toxins. However, these compounds are linked to
diseases such as ulcerative colitis and inflammatory bowel disease.
Protein
sources contain other molecules besides proteins and amino acids that have been
shown to have adverse health effects. Choline,
a byproduct of a lipid found in plants and animals, is converted by our gut
microbiota into trimethylamine (TMA). Once
TMA is absorbed into our liver, it can be oxidized to form trimethylamine-N-oxide (TMAO), which is then released
into the bloodstream. A high
concentration of blood plasma TMAO is correlated with cardiovascular disease. Recently, L-carnitine has also been shown to
be converted to TMA and thus TMAO by gut bacteria. L-carnitine (a compound synthesized from the
two amino acids lysine and methionine) is found in high levels in red
meat. However, increased L-carnitine in
the blood was only correlated with increased risk of cardiovascular disease
when accompanied by high levels of TMAO, which suggests that only when bacteria
that can degrade L-carnitine to TMA are present, is there an increased risk of
atherosclerosis.
The
CrossFit community is not the only group increasing their protein intake. With the popularity of the Atkins and Paleo
diets, many Americans have shifted their focus from low-fat foods to consuming
more protein. Although these diets seem
to help people lose weight, overconsumption of protein may result in the
production of toxic compounds with detrimental effects. After an intense workout, the body requires
amino acids in order to rebuild muscle.
Therefore, the post-workout recovery shake is most likely
beneficial. However, chronic
overconsumption of protein may indeed lead to an increased risk of
cardiovascular and gastrointestinal diseases.
Only more research and time will tell, but until then perhaps I should
only buy protein shakes formulated specifically for women.
Further Reading:
Daily Recommended Protein Intake:
http://www.cdc.gov/nutrition/everyone/basics/protein.html#How%20much%20protein
Protein metabolism/catabolism:
http://www.nature.com/scitable/topicpage/nutrient-utilization-in-humans-metabolism-pathways-14234029
Gut microbiota and protein metabolism:
Russell WR et al. Colonic bacterial metabolites and human
health. 2013. Curr
Opin Microbiol. 3: 246-54
Nyangale EP et al. Gut Microbial Activity, Implications for
Health and Disease: The Potential Role of Metabolite Analysis. 2012. J. Proteome Res. 12: 5573-5585
Willyard C. Pathology: At the heart of the problem. 2013. Nature 493: S10-11
Koeth RA et al. Intestinal microbiota metabolism of
L-carnitine, a nutrient in red meat, promotes atherosclerosis. 2013. Nat Med. 19: 576-585
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