How many hours have you spent wondering what type of protein you should consume post workout to maximize your gains?
Well a paper published in the American Journal of Clinical Nutrition attempts answer this question by studying whether a milk protein source or a beef protein source leads to greater muscle protein synthesis.
Differences in postprandial protein handling after beef compared with milk ingestion during postexercise recovery: a randomized controlled trial
Often times research studies don’t control their variables very well, making solid conclusions difficult. This is a prime example. On the surface, the study appears to look at whether milk protein or beef protein is better at promoting muscle synthesis post exercise.
Unfortunately, I think they fall short of answering the “protein” aspect directly. When we dig into it we realize we can’t draw a solid conclusion.
Shortcomings aside, the paper is quite interesting and there are some interesting findings that provide further credence to the notion that carbs and protein are better for muscle gain than protein alone.
Like the meal frequency study, this one also had a relatively small sample size (n=12). It was likely small because of the invasive nature of the study. Taking chunks out of your leg with a biopsy needle is not pleasant.
The participants were in their early 20’s, were fairly lean (13% body fat) and exercised at least 2 times a week.
The Experimental Design
Also like the meal frequency study, this one utilized a randomized, 2-way crossover design with a washout period of 8-20 days.
Essentially, each subject came in to get familiarized with the exercise equipment and tested for max strength, then came in two weeks later, consumed one of the two protein types, did the testing, waited between 8-20 days, consumed the other protein type, and repeated the testing.
This study used isotope labeled tracers to track how and when certain substrates were being used.
The Actual Intervention
Participants came in early in the morning and were infused with labeled amino acids, specifically phenylalanine and tyrosine.
Muscle biopsies were then taken from the vastus lateralis (the muscle on the outside of your thigh).
The subjects then performed 4 sets of 8-10 repetitions to volitional fatigue of a leg-press and a knee extension exercise.
Then another biopsy was taken.
The subjects then consumed either labeled beef or skim milk (the nutrition breakdown is below)*. Then blood was sampled every 30 or 60 minutes and two more biopsies were taken at 2 hours and 5 hours post exercise.
*They did this by infusing a cow with labeled phenylalanine… and used its milk, that is pretty epic. Customizing a cow for an exercise study.
The blood was used to measure glucose, insulin, and amino acid concentrations in each trial.
Western blotting allows you to look at proteins so they used this method to check phosophorylation( (i.e. activity) of proteins involved in the signaling of muscle protein synthesis, mTOR and p70S6k1, and proteins involved in creating the actual proteins, 4E-BP-1 and rpS6.
Muscle Protein Synthesis
The authors also measured whether beef or milk resulted in greater muscle protein synthesis post exercise.
Blood measures from intrinsic Amino Acids
The authors did the baseline infusion with labeled amino acids so they could see how ingesting milk or beef affected amino acids already in the system.
Milk lead to greater increases in plasma glucose and insulin than beef did. Plasma levels of the intrinsic phenylalanine increased with both milk and beef but were higher in with the beef. Plasma leucine followed the same trend with increases in both, but greater plasma levels of leucine during the first 30 minutes with milk, but greater peaks at 135 minutes with the beef. Suggesting different temporal effects with the different foods.
Amino Acid Kinetics From Ingested Food
The milk and beef were labeled differently than the basal infusion so the authors could see how the amino acids directly from the post-exercise food showed up in the blood.
It appears from the data that beef led to greater overall levels of phenylalanine in the blood, and greater overall “disappearance” when compared to milk. Interestingly, there was no difference in amino acid oxidation; meaning the amount of amino acids “burned” was the same.
Anabolic Muscle Signals
Both milk and beef led to similar activity (phosphorylation) of the mTOR pathway proteins. The translational proteins (E4-PB1 and rpS6) both saw increases in the milk and the beef.
Both milk and beef showed increased muscle protein synthesis with milk having an earlier peak (likely due to the insulin peak), yet the overall amount was statistically similar between groups. However, when you look at the data it appears that milk might have the slight edge, again likely due to the fact that milk had an insulin peak while beef did not.
The Wrap Up
One important aspect of this study is the fact that the beef meal had virtually all protein and some fat while the milk contained equal amounts of protein and carbohydrates. The milk also had a higher calorie count. This makes a direct “protein content to protein content” comparison impossible.
We often agonize over what protein choice is best to maximize gains post workout. This study showed that milk and beef have similar effects on overall muscle protein synthesis, with the caveat that milk might have a slight edge. That edge likely comes from the carbohydrate content of the milk, although it is impossible to say definitively. The authors would have to include another group with just milk derived proteins (i.e. whey/casein)
I think one of the things to take away from this study is the face that the insulin spike from the milk associated with increased muscle protein synthesis over a short window (0-2 hours). This provides mechanistic data to corroborate previous studies showing that consuming carbohydrates and protein together leads to greater muscle protein synthesis than protein alone.