Scientific Update

A Review of Recent Scientific Papers Related to Vegetarianism

Lower Risk of Type 2 Diabetes Seen in Vegans

Type 2 diabetes, also known as adult-onset diabetes, is the most common type of diabetes. It affects at least 10 percent of adults in the United States. This kind of diabetes is responsible for more than 70,000 deaths each year. Risk factors for type 2 diabetes include family history, low activity level, and excess body weight. A recent large study strongly suggests that vegetarians, especially vegans, are less likely to develop type 2 diabetes. Researchers from Loma Linda University studied more than 60,000 Seventh-day Adventists who were categorized as vegan, lacto-ovo vegetarian, fish-eaters, low meat-eaters (red meat and poultry less than once a week), and non-vegetarians. Vegans had the lowest body mass index (BMI), and non-vegetarians had the highest BMI, with the other groups having intermediate BMIs. Since excess body weight is a risk factor for type 2 diabetes, it was not surprising that vegans were at lower risk. Type 2 diabetes was seen in 2.9 percent of vegans, 3.2 percent of lacto-ovo vegetarians, 4.8 percent of fish-eaters, 6.1 percent of low meat-eaters, and 7.6 percent of non-vegetarians. Even vegans who were obese had a lower rate of type 2 diabetes than obese non-vegetarians. When the data were adjusted for many factors, including BMI, vegans and lacto-ovo vegetarians were still nearly half as likely to develop type 2 diabetes compared to non-vegetarians.

Tonstad S, Butler T, Yan R, Fraser GE. 2009. Type of vegetarian diet, bodyweight, and prevalence of type 2 diabetes. Diabetes Care 32:791-96.

Vegan Buddhist Nuns in Vietnam

In Mahayana Buddhism (a form of Buddhism practiced in Vietnam, China, and Japan), nuns follow a vegan or lacto-vegetarian diet. These women, who have been vegan or nearly vegan for many years, offer an opportunity to find out more about the effect of a vegan or near-vegan diet on health. Researchers compared the bone density of 105 Buddhist nuns to that of 105 non-vegetarian women in the same community. The nuns had followed their diet for an average of 33 years and had markedly lower intakes of calcium and protein compared to the non-vegetarian women (375 versus 683 milligrams calcium; 35.4 versus 62.6 grams of protein). Despite their lower calcium intake, the nuns’ bone density was similar to that of the non-vegetarian women, and both groups had a similar prevalence of osteoporosis. Calcium intakes of both groups were considerably below the recommendations used in the United States. While both the nuns and the nonvegetarian women had a similar prevalence of osteoporosis, we have to wonder if less osteoporosis would have occurred if both groups had a higher intake of calcium. We also don’t know if there are other differences between the two groups that accounted for similar bone densities, despite the nuns having markedly lower calcium intakes. Perhaps the nuns had more sun exposure and higher vitamin D levels, or they may have had higher intakes of other nutrients important for bone health, like potassium and vitamin K. Other studies have found adequate protein is important for bone health ( 2008.pdf) and that vegans have a lower risk of fracturing a bone if their calcium intake is at least 525 milligrams a day ( vj2007issue4.pdf).

Ho-Pham LT, Nguyen PLT, Le TTT, et al. Veganism, bone density, and body composition: a study in Buddhist nuns. Osteoporos Int 2009 Apr 7 [Epub ahead of print].

Vegetarians and Eating Disorders?

A recent study examined whether self-described current or former vegetarians were at higher risk of developing eating disorders. This study looked at more than 2,500 males and females ages 15 to 23 years. Of the 108 subjects who identified themselves as ‘vegetarian,’ most consumed dairy products and eggs, close to half ate fish, and a quarter ate chicken. Thus, only about a quarter to a half of the so-called current vegetarians were truly vegetarian. Therefore, we are actually looking at approximately half meat-eaters or more and about half vegetarians in the 'current vegetarian' group.

Two hundred sixty-eight subjects said they had been vegetarian for more than a month at some point in the past . We do not know how many former self-described vegetarians ate chicken and fish when they were 'vegetarian.' Both current and former 'vegetarians' were more likely to binge eat than subjects who were never 'vegetarian.' Former ‘vegetarian’ teens were more likely to use diet pills, vomiting, and other extreme weight control measures than non-vegetarians; former 'vegetarian' young adults were more likely to use extreme weight control measures than non-vegetarians or current 'vegetarians.'

What does all of this mean? This study adds to the body of evidence suggesting that some teens and young adults choose a vegetarian or partial vegetarian diet as a way to camouflage an existing eating disorder, as another way to restrict food intake. That could help to explain why former vegetarians were more likely to use extreme weight control measures—they had apparently given up on a vegetarian diet and were trying other ways to lose weight. Of course, most teens who choose to be vegetarian do not have eating disorders. This study also shows that it is important to carefully define the term ‘vegetarian’ since the study’s results made it appear that current and former vegetarians were at increased risk for eating disorders when, indeed, many of these people were meat-eaters. Maybe the best conclusion that we can draw from this study is that, when someone with an eating disorder says they are vegetarian or were vegetarian in the past, it’s important for health care professionals to assess if they actually are vegetarian and what their true needs are for a treatment plan.

Robinson-O'Brien R, Perry CL, Wall MM, et al. 2009. Adolescent and young adult vegetarianism: better dietary intake and weight outcomes but increased risk of disordered eating behaviors. J Am Diet Assoc 109:648-55.

Diet and Male Fertility

There have been some reports of declines in male fertility due to factors like exposure to polychlorinated biphenyls (PCBs) and other environmental contaminants and to pesticides. Diet has also been thought to play a role in male fertility, possibly even prior to birth. One study found that sons of high beef consumers had a 24 percent lower sperm concentration than men whose mothers ate less beef during their pregnancy. Spanish researchers examined 61 men, 31 of whom had normal semen and 30 who had poor semen quality. Men with poor semen quality had higher intakes of yogurt, meat, and potatoes, while the other group had higher intakes of raw or cooked vegetables, some fruits, and skim milk. The researchers theorize that meat and yogurt are higher in fat and that toxic substances are more likely to accumulate in fat-rich foods. The antioxidants found in vegetables and fruits may have protected the men in the group with normal semen from harmful substances. These results suggest that a plant-based diet may be associated with improved semen quality.

Mendiola J, Torres-Cantero AM, Moreno-Grau JM, et al. 2009. Food intake and its relationship with semen quality: a case-control study. LFertil Steril 91:812-18.

Soy and Prostate Cancer

Prostate cancer is the most common cancer, other than skin cancers, in American men. Approximately one man in six will be diagnosed with prostate cancer. Prostate cancer is the second leading cause of cancer death in men in the U.S. Prostate cancer rates vary, with Asian countries having lower rates than the U.S. and Europe. One difference between Asian countries and the U.S. is the higher soy consumption in Asian countries. Could this partially explain the different rates of prostate cancer? A number of studies have examined the possible connection between soy and prostate cancer. A recent meta-analysis combined the results of 14 studies on soy consumption and prostate cancer. Consumption of soy foods was associated with approximately a 25 percent reduction in risk of prostate cancer when men eating the most soy foods were compared to men eating the least. Tofu and soymilk consumption were associated with around a 30 percent reduction in risk, while fermented soy foods (like miso and tempeh) were not associated with a reduction in risk. Some studies have reported an increased risk of prostate cancer with dairy product consumption. Perhaps replacing dairy products with soy products could help to reduce risk of prostate cancer.

Yan L, Spitznagel EL. 2009. Soy consumption and prostate cancer risk in men: a revisit of a metaanalysis. Am J Clin Nutr 89:1155-63.