Remember when the sun was good and wine was bad? If you’re like me, the constantly evolving research that alters our understanding of health and fitness can be daunting. The temptation is to believe what you want to believe because, eventually, research will come out to support that belief! Who would have thought, for instance, that there would be health benefits to eating dark chocolate? Well, besides chocoholics. But recent research has shown clearly that our old hackneyed beliefs about lactic acid—that it’s bad for you and must be removed from your muscles—is not only wrong, it’s really wrong. So what does lactic acid do?
As you probably know, increased levels of lactic acid in muscles have long been believed to play a role in muscle fatigue, cramping, and reduced performance. Such beliefs about lactic acid also led to the notion that if you reduce you exercise activity slowly—that is, cool down following a workout—you would give lactic acid a chance to leave your muscles. In fact, however, thirty years of research by George Brooks at the University of California at Berkeley have shown that lactic acid is one of the most important energy sources for high-intensity muscle activity. Before I get any further into discussion of this research, though, two other issues need sorting out.
First, “lactic acid” is a misnomer in any case. What the body actually produces is lactic acid minus one proton, or “lactate.” If “lactate” sounds like something a pregnant woman might do, that’s because, when the acid was first discovered by a Swedish chemist in 1789, he found it in some sour milk. Since “lactic” means “relating to milk,” Carl Whilhelm Steele named his newly discovered acid “lactic acid,” though in fact lactic acid is not confined to milk but can also be found in meat and blood. And again, it is not lactic acid that is produced in the body—it’s lactate. Lactate is the product of a side reaction to glycolysis, which is a process in which the energy stored in glucose is converted to a form the body can use.
The second issue I wanted to address is, if lactic acid (or lactate, as I’ll refer to it here on out) isn’t the culprit in muscle soreness, why have we all—including fitness professionals and educators—believed it was for all these many years? This prevailing understanding of the role of lactate came about because of incomplete observations of cell metabolism made by Nobel Prize–winning researchers Otto Meyerhoff and Archibald V. Hill in 1922. As noted exercise physiologist Robert A. Robergs puts it, the “research showed that lactic acid was produced during conditions of low oxygen content (hypoxia), and that muscle contraction also became impaired. This research led to the assumption that lactic acid production caused decreases in cellular and blood pH, which in turn caused the symptoms of muscle and body fatigue during intense exercise. This evidence was not cause-and-affect, but rather guilt by association [emphasis mine].” This incomplete description of acidosis—the low pH in body tissues and blood that arises when cells receive too little oxygen—then continued to appear in fitness and biochemistry textbooks, leading to our ongoing misconceptions about lactate and exercise.
That said, what actually does happen in the muscle during exercise? Without getting too technical, it has become clear that lactate production is not the cause of cellular acidosis but, rather, a consequence of it. But more than that, elevated levels of lactate—which occur when people exercise regularly and/or during high-intensity workouts—lead muscle cells to make adaptations that give them the ability to use lactate as an energy source rather than a mere waste product. It turns out that interval training, with its short bursts of intense activity, is particularly effective in causing spikes in lactate levels that increase the number of molecules in mitochondria that transport lactate. (The mitochondria, as you may recall from biology classes, is considered the “powerhouse” of the cells in your body because of its role in breaking down sugars into basic chemicals that the cell can then use for energy.)
So what does cause your muscles to be sore if not lactic acid? During high-energy exercise, our muscles use adenosine triphosphate, or ATP (a form of energy), which results in the release of hydrogen ions or protons that then accumulate and stimulate pain receptors. This is the “burn” we sometimes get during exercise. (In contrast, if you are doing low- or moderate-intensity exercise, you will seldom experience this burn because the protons are absorbed by other chemical reactions taking place.) This burning sensation generally disappears as soon as exercise is stopped or shortly thereafter. As for delayed onset muscle soreness, which generally occurs a day or two after a period of heavy exercise, it is partly caused by structural damage to muscle fibers, which causes an inflammatory response. This response factors into muscle soreness in two ways: (1) it produces a buildup of fluid (that is, swelling) that results in pressure being exerted on the muscle fibers and (2) the body’s defense system kicks in, producing white blood cells that “enter the muscle fibers and secrete chemicals that activate pain receptors,” according to fitness expert Deborah Riebe.
The short and sweet of all this? The production of lactate during exercise actually provides the muscles with fuel and is not the bad seed we always thought it was.
If you’d like more detail on the science of all of this, here are some websites you can check out: