Don’t get me wrong. I think that the Human Genome Project was a great idea. However, I don’t think that genetic studies are going to help us unravel our main causes of death and disability in the United States. That’s because they are largely the result of our food, not our genes. You can tell this from the simple fact that the rates of these diseases go up and down, depending on how a population eats.
The rate of truly genetic diseases (such as hemophilia or cystic fibrosis) stays remarkably stable in a population from one generation to the next. The only way it could increase is if there were a sudden wave of immigration of people carrying the gene (in which case, the disease would mostly be in the immigrants and their children) or if the people with the gene suddenly became much more fertile (which is unlikely).
When you see the rate of a disease go up and down dramatically within the space of 10 years, you know for sure that the cause of that change is environmental, not genetic. The classic illustration is the sharp decline in heart disease in Norway during World War II. After the Nazis invaded Norway (and stole a lot of their farm animals), the Norwegian population had to shift to a low-fat, plant-based diet. As a result, the diseases of affluence (heart disease, type 2 diabetes, multiple sclerosis) became much less common. The rates of those diseases went right back up again after the war ended and people went back to their old ways of eating.
Another clue to whether a disease is genetic or environmental comes from its geographic distribution. If a disease is most common in an area with small gene pool, it’s likely to be genetic. Examples include the hereditary deafness that was once common on Martha’s Vineyard and Tay-Sachs disease in Eastern European Jews and French Canadians. These are recessive gene diseases. You can get the disease only if you got the same defective gene from both parents, which is most likely to happen if they are related to each other.
On the other hand, if a disease is most common in an area whose population came from a wide variety of immigrants, then you can bet your bottom dollar that the underlying cause for the high rate of the disease is environmental, not genetic. The classic example of this is Parkinson disease, which is most common in Buenos Aires, Argentina. Although there may be some genes that predispose a person to Parkinson disease, especially the early-onset form, the fact that the world’s highest rate is in a population made up of recent immigration from many different places tells me that the major risk factor is something in the environment. Considering that the people of Buenos Aires also eat more beef than anyone else in the world, I’d be willing to bet that the environmental factor is red meat consumption.
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