You can see now why lifestyle is so important in causing (and potentially preventing) type 2 diabetes. During the same time that our lifestyle has changed, type 2 diabetes has transformed from a disease that was barely recognized to one that affects 8 percent of the entire adult population, 13 percent of those older than forty, and 20 percent of those older than sixty-five. Being obese substantially increases the risk of developing diabetes, but even being a little bit overweight increases the chances of developing diabetes.
Similarly, our sedentary lifestyle, independent of obesity, increases the risk of developing type 2 diabetes. Inactivity leads to insulin resistance and prediabetes, both of which lead to type 2 diabetes. Some of the pernicious effects of inactivity are secondary to obesity, but inactivity also makes our muscles less sensitive to the effects of insulin.
In addition to advancing age, being overweight or obese, and inactivity, inheritance plays a large role in diabetes. Although all of the specific genes that underlie obesity, the metabolic syndrome, and diabetes have not been identified, it is clear that the risk of developing these conditions and diseases is inherited. But how can we explain the widespread inheritance of diseases, the effects of which are so devastating? How would Darwin explain the natural selection process that leads to diabetes? The widespread inheritance of risk for obesity, diabetes, and the insulin resistance that underlies much of type 2 diabetes does make sense according to a theory called the "thrifty gene hypothesis." Until just a few centuries ago, human beings lived in peril of famine. During times of famine, "thrifty" genes that decreased energy expenditure were highly advantageous to human survival: because the energy people took in was small, and because of difficulty in finding food, burning less energy protected people against starvation.
However, according to the thrifty gene hypothesis, genes that protect us during famine make us vulnerable when food is plen tiful. An adaptive response has become maladaptive. Put another way, genes that protected our primate ancestors for nearly a million years have made our species vulnerable in the past few hundred years because, for most of us, food has become readily available and the need to do hard physical work has been reduced.
The consequences of this maladaptive response are profound. Obesity, diabetes, and metabolic syndrome—and the cardiovascular disease that accompanies them—represent the major public health problems not only in the Western world but increasingly in Asia, Africa, and South America. The inherited propensity to develop type 2 diabetes emerges when unhealthy lifestyle changes and obesity come into play. If a person with a genetic risk for type 2 diabetes doesn't become obese, the disease may never develop.
We cannot change our "thrifty" genes, but we can change our lifestyle. Moreover, there is now hard data—most notably from the multicenter Diabetes Prevention Program study—showing that lifestyle changes really work. You don't need to eat only certain types of foods or train for a marathon. Very simple changes that nearly anyone can make are clinically proven to protect against the metabolic syndrome, prediabetes, and diabetes. The rest of this book will tell you what those changes are and show you how to incorporate them into your life.
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