 The same lactic acid that makes yogurt tangy is the same lactic acid that builds up in our muscles when we exercise strenuously. Instead of bacteria fermenting the sugar and milk to make energy for themselves, our muscles ferment sugar in our diet to produce energy to contract. If, like when we're sprinting, lactic acid builds up in our muscles faster than it can be removed, we can end up with a burning sensation in our muscles forcing us to stop. Now, if we train, we can increase the number of blood vessels in our muscles and clear out the lactate faster. For example, if you take some overweight sedentary women and start them on an aerobic training program of running and walking, at the end of three months, their lactate levels during exercise decrease 17%. But those on the same program who drank two cups of orange juice a day decreased their levels 27%. They did the same exercise program, but the citrus group experienced a significant decrease in blood lactate concentration, signifying improvement in physical performance with less muscle fatigue. I don't recommend drinking juice because you're losing all that wonderful fiber that slows the rate of fruit sugar absorption into our system. Here's the blood sugar spike one might see after drinking Coca-Cola. Compare that to the spike you see with orange juice. No difference. But now what if you ate the same quantity of sugar in the form of orange slices, significantly lower? So the whole fruit is nearly always better than fruit juice. Now this is not to say OJ isn't better than Coke. OJ has those citrus phytonutrients like hasperidin, which may be why the women's triglycerides didn't go up, even though they were drinking two cups of fruit juice every day. Hasperidin may actually help lower our digestion of fats. But once you get up to three cups a day of juice, you can really start bumping up your triglycerides.