HMB-elly be gone

Muscle-building supplements are among the most popular supplements on store shelves and on the internet, often accompanied by some of the most intensive marketing campaigns. When looking at the evidence, muscle-building supplements tend to fall into three major categories: staples with ample amount of evidence behind them (e.g. creatine), new and ‘exciting’ supplements that lack evidence to support the marketing claims (too many to count), and the middle group of supplements that have enough evidence to make them interesting yet not enough to make them staples. Time tends to make this middle group fall by the wayside and out of consumer thought but every now and then a new study comes by which may revitalize the supplement. Maybe it should be a staple, maybe there are benefits which might not be significant enough to justify the price, or maybe a few more drops can be squeezed out of the cash cow before it’s forgotten about again?

β-Hydroxy-β-methylbutyrate (HMB) is a metabolite of the amino acid leucine that, along with KIC (α keto-isocaproate) and isovaleryl-CoA, mediate the effects of leucine. Approximately 5% of dietary leucine is oxidized into HMB, and HMB appears to be the main metabolite of leucine that prevents the breakdown of muscle protein. It has been studied specifically for human consumption for numerous decades, and there is no doubt it is reasonably safe and that it could affect human biology. There is plenty of in vitro data to support its mechanisms and plenty of sources available (both scientific and supplemental) to keep it on the shelves, but it has never reached ‘staple’ status.

Recently, however, two^[1] studies^[2] using the HMB free acid form were published and found very impressive results in trained weight lifters, which seems to have renewed public interest in the compound. These studies are not the ones to be discussed today but rather a recent study suggesting HMB supplementation can reduce body fat in the elderly. Previous^[3] studies^[4] have found that HMB increases total-body fat loss in older adults. However, no studies previous to this one have taken a look at the effects of HMB and resistance training on abdominal fat in elderly. Abdominal fat is important as it is at least as strongly correlated with mortality in the elderly^[5] as body mass index, if not more so.The purpose of this study was to examine the effects of 12 weeks of HMB supplementation with or without resistance training on abdominal fat in elderly men.

Both obesity and waist circumference are associated with bad health outcomes, namely death^[6]. So why are the authors of this study so concerned about abdominal fat as opposed to fat mass in general?

Because it seems that abdominal fat may be a primary culprit in a lot of metabolic problems leading up to death. There are two kinds of belly fat: subcutaneous fat, which lives beneath the skin, and visceral fat, which is packed deep in the gut and surrounds the organs. It’s been known for a while that visceral fat is correlated with metabolic problems such as high cholesterol and glucose intolerance^[7]. We’ve also known for a while that hypertension is sometimes more responsive to loss in visceral fat than subcutaneous fat. So it seems that visceral fat can cause more health problems than subcutaneous fat.

The mechanism for this difference is slowly being worked out, and generally outlined in Figure 1. One reason why visceral fat may be more deadly than subcutaneous fat is because visceral fat inhibits the secretion of a molecule known as adiponectin^[8], and reduced circulating adiponectin has been linked to many of the metabolic problems seen in those with central obesity.

Figure 1: How visceral fat promotes cardiovascular disease

The take-home here is that fat just doesn’t sit around and store energy; it’s also a metabolically active tissue. As visceral fat accumulates, one’s metabolism can be thrown out of balance. This is at least part of the reason why abdominal fat is of particular importance.

HMB is a metabolite of leucine that helps prevent muscle breakdown and reduces total body fat in the elderly when combined with resistance training. However, previous to this study under review, no one has examined the effect of HMB plus resistance training on abdominal fat in the elderly, which is a strong predictor of mortality.