Scientists have discovered that lactose, a byproduct of intense muscular activities, can be used to fuel the brain with energy. When glucose, the natural fuel of the brain, is no longer present in sufficient quantities, the cell tissue can “switch” to alternative energy, to prevent any damage to the brain on account of the lack of energy.

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Consequently, by consuming the lactose in the blood, the brain clears the way for glucose, the main powering substance in the body, to reach the muscles and provide them with energy for the hard work they are doing. This research is very important because it explains why the brain is able to operate even when the body demands unusually high amounts of energy and oxygen. In fact, our mind actually goes into a higher “gear,” in order to be able to cope with any situation.

Lactate

Lactate is a negatively charged molecule formed from a compound called lactic acid. Some bacteria make lactate when they metabolize sugars. Humans also produce lactate as a byproduct of metabolism, though only under certain conditions. Specifically, says Dr. Gary Thibodeau in his book "Anatomy and Physiology," you make lactate when your cells process sugars for energy in the absence of oxygen, such as when you're engaging in a hard sprint or power efforts during exercise.

Lactate in Food

Lactate can occur naturally in some foods, and in other cases, it's added. Fermented foods like sourdough bread and yogurt contain lactate naturally. This is because these foods are manufactured by allowing bacteria to convert a certain amount of the sugar in the dough or dairy into lactate, yielding a sour flavor and change in texture. Because lactate changes a food's acidity, it can make it hard for pathogenic bacteria to grow in the food. As such, lactate is sometimes added to foods as a preservative.

Lactose

Unlike lactate, lactose is a sugar. While it's possible for humans, other animals and bacteria to convert lactose to lactate through the process of fermentation, the chemicals have little in common. You take in lactose any time you consume milk or other dairy products. An enzyme called lactase, which occurs in your intestines, digests lactose into smaller molecules that you absorb into your bloodstream and can use for energy, say Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."

Lactose Intolerance

Some people don't produce enough lactase to digest lactose effectively. This is referred to as lactose intolerance. If you're lactose intolerant, consuming dairy causes you to experience a variety of uncomfortable gastrointestinal symptoms, including bloating and cramping. If you're lactose intolerant, you have to avoid lactose-containing foods. Alternately, you can take a lactase supplement when you consume dairy. You do not, however, need to avoid foods with natural or added lactate.

One case involves our ability to digest lactose, a sugar found in milk. An enzyme called lactase breaks down this sugar into the more easily absorbed sugars glucose and galactose. We are born with the ability to digest milk, of course, for that’s always been the main food of infants. But after we’re weaned, we gradually stop producing lactase. Eventually, many of us entirely lose our ability to digest lactose, becoming “lactose intolerant” and prone to diarrhea, bloating, and cramps after eating dairy products. The disappearance of lactase after weaning is probably the result of natural selection: our ancient ancestors had no source of milk after weaning, so why produce a costly enzyme when it’s not needed?

But in some human populations, individuals continue to produce lactase throughout adulthood, giving them a rich source of nutrition unavailable to others. It turns out that lactase persistence is found mainly in populations that were, or still are, “pastoralists”—that is, populations who raise cows. These include some European and Middle Eastern populations, as well as Africans such as Masai and Tutsi. Genetic analysis show that the persistence of lactase in these populations depends on a simple change in the DNA that regulates the enzyme, keeping it turned on beyond infancy. There are two alleles of the gene—the “tolerant” (on) and “intolerant” (off ) form—and they differ in only a single letter of their DNA code. The frequency of the tolerant allele correlates well with whether populations use cows: it’s high (50 to 90 percent) in pastoralist populations of Europe, the Middle East, and Africa, and very low (1 to 20 percent) in Asian and African populations that depend on agriculture rather than milk.

Archaeological evidence shows that humans began domesticating cows between 7,000 and 9,000 years ago in Sudan, and the practice spread into sub-Saharan Africa and Europe a few thousand years later. The nice part of this story is that we can, from DNA sequencing, determine when the “tolerant” allele arose by mutation. That time, between 3,000 and 8,000 years ago, fits remarkably well with the rise of pastoralism. What’s even nicer is that DNA extracted from 7,000-year-old European skeletons showed that they were lactose-intolerant, as we expect if they weren’t yet pastoral.

The evolution of lactose tolerance is another splendid example of gene-culture coevolution. A purely cultural change (the raising of cows, perhaps for meat) produced a new evolutionary opportunity: the ability to use those cows for milk. Given the sudden availability of a rich new source of food, ancestors possessing the tolerance gene must have had a substantial reproductive advantage over those carrying the intolerant gene. In fact, we can calculate this advantage by observing how fast the tolerance gene increased to the frequencies seen in modern populations. It turns out that tolerant individuals must have produced, on average, 4 to 10 percent more offspring than those who were intolerant. That is pretty strong selection.