Most people who work out regularly know lactic acid as the metabolic waste product that builds up in muscles, leading to the characteristic burning sensation you experience during an intense session. While you associate this lactic acid with feeling tired, it's not the lactic acid itself responsible for the fatigue; neither will lactic acid in food make you tired.
Lactic Acid
Lactic acid is a small molecule that has both biochemical significance and occurs in many food products. While it's not something you want building up in your body -- when this happens, you feel fatigued, though the fatigue isn't a direct result of the lactic acid -- there's no harm in eating lactic acid-containing foods. In fact, the lactic acid you eat in food is of virtually no biochemical significance.
Metabolism
When you burn nutrient molecules such as proteins, fats and carbohydrates for energy, you generally produce only carbon dioxide and water as byproducts; however, if you're not getting enough oxygen -- if you're doing sprints or lifting heavy weights, for instance -- you have to use another energy-production strategy. This results in production of the waste product lactic acid, which accumulates in muscles, explains Dr. Lauralee Sherwood in her book "Human Physiology." The lactic acid can make muscles burn.
Lactic Acid In Food
The most common source of lactic acid in food is in fermented dairy products like yogurt. Bacteria can thrive in milk and break down the sugars for energy. As byproducts, they produce lactic acid. Since the source of the lactic acid isn't your own cells, you won't feel fatigue if you eat yogurt or other similar products. The lactic acid in food doesn't really have any activity in or effect on the body, for good or for ill.
Lactic Acid Effects on Food
Lactic acid in food isn't a nutrient from which you derive nutrition; however, its presence in food does change a food product. When bacteria produce lactic acid in dairy, the acid imparts a characteristic sour flavor, which you notice when you eat yogurt or other fermented dairy products. Further, the lactic acid causes proteins to clump together, producing a creamy texture. There's no reason, however, that any of the changes caused by the lactic acid would contribute to feeling tired.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
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