Beet juice boosts endurance for many (but not all) people, according to a series of studies over the past five years or so. The active ingredient appears to be nitrate, which somehow reduces the oxygen cost of muscle contractions. How does this happen? There are a number of overlapping theories related to blood circulation, neurotransmission, and even the contractile properties of the muscle fibers themselves. A new study from researchers at Loughborough University, recently published in Medicine & Science in Sports & Exercise, takes a look at this last theory. There have been some studies of nitrate and muscle fibers in mice, but this one uses real live humans.
The study design was fairly straightforward. A group of 19 volunteers (untrained men, average age 21) supplemented with beet juice or placebo for 7 days; everyone did both protocols in a double-blind, randomized design. The amount of beet juice was 1.5 shots of Beet It concentrated beet juice (total 600 mg of nitrate). After 7 days, they did a series of muscle tests, including maximal voluntary contractions and various electrically stimulated muscle contractions. There are a number of subtleties in the measurement of electrically evoked muscle contractions, but the basic result is that the involuntary contractions were enhanced but the voluntary contractions weren't.
Here's a graph showing the increase in twitch force as a function of time; the difference between placebo and nitrate at peak is statistically significant with p<0.01:
How does nitrate affect muscle contraction? Based on the mouse experiments, the researchers suggest that it has to do with the muscle fiber's response to calcium, which is an important signalling molecule. By some estimates, calcium handling eats up 30-50% of the ATP used in muscle contraction, so a change in calcium sensitivity could indeed explain why nitrate makes endurance exercise more efficient. Is this the "secret" to nitrate's ergogenic powers? Hard to know, but it bolsters the idea that muscle contractile properties play a role.
Why, then, was there no effect on voluntary muscle contractions? When you use electricity to make a muscle twitch, you're just testing the properties of the muscle. When you ask a volunteer to contract as hard as possible, you're testing a much more complex system that includes the brain and the network of nerves that connect the brain to the muscles. As a result, voluntary contractions have a huge amount of variability (especially in untrained subjects), so it may simply be that the effects of nitrate on muscle are too subtle to show up in voluntary contractions.
Last point: the volunteers kept dietary logs during the weeks of supplementation to assess how much nitrate they were getting from other sources. Here's how the increases in muscle twitch force were related to the amount of nitrate the volunteers were getting from other sources:
Not surprisingly, those who ate the most nitrate (e.g. from leafy green vegetables) got the least benefit from adding beet juice. Perhaps a reminder that, rather than thinking of beet juice as a magic supplement, we should think of nitrate-rich foods as something to be incorporated into our regular diets.