Is it true that 1/3 of the world is lactose intolerant?
It is true that a significant proportion of the population is lactose intolerant because the gene that helps in digesting lactose is typically not expressed in adults. Most people from Scandinavian countries are lactose tolerant because milk and milk products were the most abundant source of food there and they evolved to have a tolerance towards lactose due to a mutation on their gene. It is simple to understand why this is so. If you look at other mammals, you will observe that the calves or younger animals drink milk, but once they grow into adults, they stop drinking milk. In other words, milk is necessary for children but not necessarily in adults. Human beings are the only animals to drink milk even in adulthood because they have domesticated farm animals.
Now on to the reason why people think it is not as common as science says it is. Most people have mild forms of lactose intolerance. Because they do not express the gene that aids in digestion of lactose, the lactose intolerance can lead to symptoms like flatulence - a person with flatulence is not going to think of lactose intolerance as the reason for his symptom. Only in severe cases does it lead to symptoms like diarrhea and abdominal cramps/bloating.
Lactose Intolerance: Why do so few Asians consume milk and its products?
I am going to challenge the premise a little bit here. While dairy is not nearly as big a part of most Asian food cultures as it is in parts of Europe, I think it's far from true to say that "Asians don't really drink milk and milk products."
To name one newsworthy example: remember the big hubbub over melamine-tainted powdered milk in China? Without millions of people feeding milk to their kids, it wouldn't have been a news story. Mainland Chinese people want powdered milk so badly that they go to Hong Kong to fill their suitcases with it (a practice widespread enough to cause Hong Kong to place limits on the amount of milk powder that can be carried out of the country).
Yogurt is a staple of several styles of Indian food (both on its own and as an ingredient in cooking) and is also popular elsewhere in Asia, including China.
Go to Hong Kong or Taiwan and you'll find milk tea being sold on practically every city block and being consumed by large numbers of locals.
In Mongolia, milk products including cheeses are some of the core elements of the traditional diet, though they use milk from a variety of animals, not just cows. Even the local candies in Mongolia are milk-based.
Practically every convenience store in Japan sells cheesecake, and milk coffee is ubiquitously stocked in the country's drink vending machines.
So while lactose intolerance is indeed a real thing, it isn't enough to completely remove milk from Asia's cookbooks and menus.
Can one lose lactose intolerance as an adult?
I want to say the answer is yes through anecdotal evidence (stories of friend's parent's sibling, etc who was born lactose intolerant and became tolerant when they became older), and through personal experience, although I think my case is rather unusual and may not "count".
I was lactose intolerant for 6 months in college. I believe it's because I had too much dairy within a short period of time, and therefore somehow "overused" my storage of lactase enzymes. So for 6 months I avoided dairy, got my lattes with soy, and drank lactose-free milk; and then, almost as suddenly as it began, I was able to drink milk/consume dairy products without any problem.
That said, I still limit my daily dairy consumption and stick to lactose-free milk. (I also prefer the taste of lactose-free milk, so I really don't mind).
 "Too much dairy" and "short period of time" is highly relative. In my case, "too much" was a regular diet of cereal for breakfast, and a bagel with cream cheese and a yogurt for lunch; "short period of time" was over the course of a week, every day. This probably seems like nothing to the average person.
Is it common to become lactose intolerant as an adult?
That depends on what you mean by "common" and "adult".
There's a difference between "lactose intolerance" and "lactase non-persistance". They are related, but are not necessarily the same thing.
Lactase, the enzyme that breaks down lactose, is highly expressed in human babies, but lactase transcription is down-regulated (to varying degrees) during weaning. However, a genetic mutation has developed in humans, and about half the population worldwide continue to produce lactase later in life.
Not surprisingly, lack of lactase in your system can lead to symptoms of lactose intolerance.
However, the causes of lactose intolerance is a bit more complicated. There are adults who are lactase-persistent but lactose intolerant, and vice-versa. In addition to lactase gene, lactose intolerance also depends on composition of gut flora (which can be affected by diet, antibiotics, etc) and health of the digestive system (stomach bugs can damage the intestines and cause lactose intolerance). Consequently, people can have varying degrees of lactose-intolerance.
So, is it common? It's estimated that more than 60% of people worldwide have reduced ability to digest lactose after infancy (i.e. their lactase gene gets downregulated after infancy). But how many of those people develop true lactose intolerance and how many of them developed it after adulthood is much harder to quantify.
Are there any home tests to find out if one is Lactose intolerant?
The DIY lactose intolerance test we did at our hospital:
drink 4 glasses (˜1 liter) of whatever kind of real (not soy) milk you like in quick succession, if nothing happens afterwards, you don't have it, on the other hand if within 1/2 to 2 hours, sometimes up to 6 hours afterwards you have bloating, flatulence, hear lots of rumbling belly noises, and have non bloody watery loose stools, you are lactose intolerant.
Hydrogen breath test after ingesting 25 grams of lactose with hydrogen levels peaking because of hydrogen being formed in the large intestines because of the undigested lactose reaching the large intestines and getting fermented there by bacteria there.
old fashioned lactose tolerance test measuring if the blood glucose levels rise after ingesting 50 grams of lactose as a sign lactose was split into its components glucose and galactose.
Genetic tests, the validated ones being lactase activity persistence in adults is associated with two polymorphisms: C/T 13910 and G/A 22018 located in MCM6 gene.
Can someone who has lactose intolerance eat Greek yogurt?
It depends on how the "Greek yoghurt" is made.
Traditionally, milk is fermented (which removes most, if not all of the lactose) to make yoghurt. Then the yoghurt is drained to make it thicker.
Some companies cut corners, though, and instead of making the effort and spending the time to drain the yoghurt, add dried milk powder to thicken it. The dried milk powder is usually not lactose-free, so lactose is being added to the yoghurt. This could be problematic to one who is lactose intolerant.
What are some good yogurts that are safe to eat for someone who is lactose intolerant?
Yoghurt really shouldn't present a problem for most who are lactose intolerant.
The bacteria that ferment the yoghurt are taking over the role that lactase plays in lactose tolerant people, that is, consuming most of the lactase. Even lactose intolerant people can generally consume 1-5g of it without problems.
Any yoghurt should be okay, at least in moderate quantities. If it's still presenting a problem for you, then you should be reducing the size of your portion.
If no amount is digestible, then you should look into the possibility that you're allergic to lactose, not just intolerant of it.
Are most Chinese people really lactose intolerant?
The answer is probably the result of two factors:
Cattle and dairy products are rare in most parts of China, because of the resources needed to nourish a cow. Cattle were occasionally used as beasts of burden. Beef itself used to be a relative luxury on menus. Meat production is on the increase, and we might expect that dairying may follow.
Because of the absence of dairying in most parts of China, the genetic mutation for lactose TOLERANCE is rare among Chinese.