Depending on your tolerance and preferences, dairy may play a role in your diet. All humans are born with the ability to digest dairy (aka their mother's milk), but only those that evolved the ability to produce the lactase enzyme into adulthood (about 1/3 of the world) are able to digest dairy without issue as adults. Dairy consumption was such an important part of survival in some areas of the ancient world, that most people in Europe and some in Africa and the Middle East quickly evolved the beneficial lactose tolerance mutation. From an evolutionary standpoint, if people had this mutated lactose tolerance gene before the domestication of cattle, sheep, and goats, it wouldn't have done them much good. They wouldn't have had a regular source of milk as hunter gatherer adults, unless they went around stealing the breast milk of nursing mothers, compromising the nourishment and health of their offspring. Any tribes who tested this practice of stealing milk from nursing mothers likely did not pass on their genes since there were no well-nourished babies to do so. But once we had domesticated cattle, we had a regular source of milk. So those with the mutated lactose tolerance gene could survive during harder times, famines or droughts, while those without the mutation were left to starve. As a result, this beneficial mutation spread quickly.

Like eggs, which contain all the nutrients needed to turn a single cell into an entire chicken, or seeds that contain all the nutrients needed to grow an entire plant, cow's milk contains all the nutrients needed to grow a cow. Keep in mind that humans aren't cows, so while whole cow's milk is very nutritious, it contains too little iron, retinol, vitamin E, vitamin C, and vitamin D for human babies. But that doesn't mean that dairy products from cow's milk can't be a healthy part of a human diet, even those that are lactose intolerant. Just as fructose and sucrose are the sugars of fruit, and glucose is the sugar of root vegetables and grains, lactose is the sugar of milk. It's lactose in particular that causes those with lactose intolerance to have issues digesting milk. People with lactose intolerance don't have the enzyme (lactase) to breakdown lactose into a digestible substance, so it ends up being unabsorbed until it reaches our gut, where certain gut bacteria start breaking it down to some extent. Our gut bacteria convert lactose into byproducts, which contain gas. That's what gives some people a lot of gas when they eat dairy, especially milk, which is high in sugar, and therefore lactose. But some dairy products contain very little, if any, lactose. Remember lactose is simply the type of sugar found in dairy, so for dairy products with little to no sugar, there is little to no lactose.

For the lactose intolerant (two thirds of the world) the easiest way to determine if a dairy product is going to make you feel bloated is to look at how much sugar it contains. Unfortunately, we can't rely on the nutrition facts panel on the back of dairy products for this. For example, an ounce of cream cheese, which is about the amount spread on a bagel, contains about 1 gram of sugar/lactose, definitely enough to upset the stomachs of many; however, the serving size on the back of cream cheese is 1 Tablespoon (about a third of an ounce). So while that 1 Tablespoon contains 0.3 grams of sugar/lactose, food manufacturers are allowed to "round down" and state 0 grams of sugar per serving on the nutrition facts. To make your life easier (if you're lactose intolerant), here's a handy list of the lactose contained in one serving of certain dairy products [4]:

Milk Powder: 62 g
Evaporated Milk: 25 g
Milk: 12-13 g
Buttermilk: 9 g
Cottage cheese: 3 g
Sherbet: 2 g
American cheese: 1 g
Cream cheese: 1 g
Sour cream: 0.7 g
Whipped cream: <0.5 g
Cream: <0.2 g
Bleu Cheese: <0.2 g
Cheddar, Swiss, or Mozzarella cheese: <0.1 g
Butter: 0 g
Ghee (clarified butter): 0 g

If you're lactose intolerant, you can still use butter and ghee as cooking fats, and would probably be fine with a little bit of a hard aged cheese, like cheddar. You might even be okay with heavy whipping cream and sour cream, depending on your gut population. The above mentioned microorganisms in your gut that eat your undigested lactose and produce gas as a byproduct are not the only gut microbes with a craving for lactose. Other bacteria called "lactic acid bacteria" eat the lactose as well, but produce lactic acid as a byproduct instead of gas, which doesn't produce the unpleasant symptoms of lactose intolerance. For those who have built up a healthy population of lactic acid bacteria in their guts, even if they're lactose intolerant, they may be able to consume dairy regularly with no issues.

Why are some dairy products lower in lactose than others? Usually, it's because of fermentation or simple separation. In the case of separation, when cream is skimmed off the top of whole milk, the lactose remains in the sugary milk, and very little remains in the fatty cream. In cheeses, milk is fermented by allowing bacteria to "eat" the natural sugar (lactose) present in milk and convert it to the byproduct lactic acid. Cheesemakers add the enzyme rennet, which acts with the lactic acid to curdle the milk and eventually turn it into cheese, nearly sugar/lactose-free.

Why go to all this trouble to consume dairy? For one, it's quite delicious, and outside of dubious cravings for refined carbohydrates, sugar, and highly processed foods, our bodies are excellent at sending us signals of what to eat, via our taste buds. Just 2 ounces of cheddar cheese contains 9% of your daily nutrient requirements and a cup of cottage cheese contains 12%, in addition to gut-healthy bacteria. If you drink milk, try to buy whole milk from 100% grass-fed pasture-raised cows. Bonus points for finding raw milk, with all of its beneficial bacteria and enzymes still in place. But limit your consumption of milk as it does contain quite a lot of sugar (12-13 grams per cup, or about 25% of the recommended amount of sugar you have in the entire day), and stick with lower-lactose dairy products like cheese, yogurt, and butter.