The largest study ever to compare the obesity rates of those eating plant-based diets was published in North America. Meat eaters topped the charts with an average body mass index (BMI) of 28.8—close to being obese. Flexitarians (people who ate meat more on a weekly basis rather than daily) did better at a BMI of 27.3 but were still overweight. With a BMI of 26.3, pesco-vegetarians (people who avoid all meat except fish) did better still. Even U.S. vegetarians tend to be marginally overweight, coming in at 25.7. The only dietary group found to be of ideal weight were those eating strictly plant-based (the “vegans”), whose BMI averaged 23.6.

People who had once eaten vegetarian diets but then started to consume meat at least once a week were found in one study to experience a 146 percent increase in odds of heart disease, a 152 percent increase in stroke, a 166 percent increase in diabetes, and a 231 percent increase in odds for weight gain.

But vegetarians may suffer high rates of chronic disease if they eat a lot of processed foods. Take India, for example, where rates of diabetes, heart disease, obesity, and stroke have increased far faster than might have been expected given its relatively small increase in per-capita meat consumption. This has been blamed in part on the apparent shift from brown rice to white and substitution of other refined carbohydrates, packaged snacks, and fast-food products for India’s traditional staples of lentils, fruits, vegetables, whole grains, nuts, and seeds.

The dividing line between health-promoting and disease-promoting foods may be less plant- versus animal-sourced foods and more whole plant foods versus most everything else.

A dietary quality index was developed that simply reflects the percentage of calories people derive from nutrient-rich, unprocessed plant foods on a scale of 0 to 100. The higher the score, the more body fat may be lost over time and the lower the risk may be of abdominal obesity, high blood pressure, high cholesterol, and high triglycerides. The standard American diet was found to rate 11 out of 100. According to U.S. Department of Agriculture estimates, 32 percent of our calories come from animal foods, 57 percent from processed plant foods, and only 11 percent from whole grains, beans, fruits, vegetables, and nuts. That means on a scale of one to ten, the American diet would rate about a one.