Muscle-building effects of adding leucine or HMB to a protein drink

No studies have compared the effects of consuming different protein sources with added leucine or HMB on muscle protein balance in the same subjects, until now. Danish researchers compared four different drinks in healthy men. This was a cross-over study where all men consumed each of the four drinks after a 36-hour fast in order to induce a catabolic state. The drinks contained approximately 400 kcal. One-third of the drink was consumed after the 36-hour fast and the rest was consumed evenly distributed every 20 minutes for the next 4 hours.

The four drinks included:
1) carbohydrate only
2) whey protein with added leucine to provide approximately 10 grams of leucine
3) soy protein
4) soy protein plus 3 grams of HMB

The net muscle protein balance improved for all drinks after the fasting period with the carbohydrate only and soy protein treatments achieving similar low responses.

The net muscle protein balance was highest for the whey protein+leucine and soy protein+HMB drinks. Only the whey+leucine drink activated the mTOR protein synthesis signaling pathway in muscle. These results show that whey with added leucine is significantly more anabolic than soy protein. However, adding 3 grams of HMB, a leucine metabolite, to soy protein resulted in a similar response as whey+leucine.

The take home message is that whey protein is an excellent protein source to reverse the catabolic state (tissue breakdown) caused by fasting. Soy protein can be an effective choice as well for those who want to avoid or limit dairy proteins. But to achieve similar effects as whey, consider adding HMB to soy or adding leucine to soy as it is likely to perform in a similar fashion.


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