Just Plants? Impact of a Vegetarian Diet on Mortality

Case History

A 50-year-old woman presents for a comprehensive preventive medical exam. Her history includes hyperlipidemia, hypertension, and impaired fasting glucose. She has tried several diet strategies in the past, including Weight Watchers, the Atkins diet, and most recently, the Paleo diet. Her grandfather died of a myocardial infarction at the age of 67, and her grandmother had postmenopausal breast cancer and a stroke. She is the primary meal planner for her family of five. She asks, “Will switching to a vegetarian or vegan diet be the healthiest choice?”

You reply:

A. Vegetarian diets are not supported by the Academy of Nutrition and Dietetics.

Discussion

The optimal diet for promotion of health and longevity with prevention of disease is, at best, controversial among experts and confusing to women. Scientists and clinicians search for evidence and then debate best recommendations for inclusions, exclusions, and ratios of macro- and micronutrients. All the while, the food industry and other self-proclaimed dietary gurus preach their own interpretations and agendas through the media and publications. With abundant food choices, insufficient education, and lack of agreement among the experts, women struggle to determine which diet best supports overall health for themselves and their families.

The vegetarian diet has had many proponents over time. Beginning in the 1980s, T. Colin Campbell, PhD, led the China-Cornell-Oxford Project, one of the most comprehensive scientific nutritional efforts to date. The authors of that 27-year-long observational study endeavored to link the nutritional patterns of 6,500 families in rural China with disease and published the findings in the well-known book The China Study. ¹ A higher intake of animal-based food correlated with higher cholesterol ² and breast cancer risk. ³ Higher intake of plant-based food appeared to protect against cancer, heart disease, diabetes, osteoporosis, multiple sclerosis, and Alzheimer's disease. ¹ These findings further spread to the public through the documentary Forks over Knives, released in 2011. ⁴

Although, the China Study is probably the most publicly known scientific evidence supporting vegetarian diets, additional evidence also exists. In a collaborative analysis of five prospective cohort studies including more than 76,000 men and women, vegetarians had a 24% reduction in mortality from ischemic heart disease (death rate ratio 0.76, 95% confidence interval [CI] 0.62–0.94). ⁵ However, there were no significant differences between vegetarians and nonvegetarians in mortality from other causes.

If vegetarian diets are healthy, is eating meat unhealthy? A 2012 review of two large prospective cohort studies correlated red-meat intake with higher all-cause mortality. ⁶ The data included 37,698 men from the Health Professionals Follow-up Study and 83,644 women from the Nurses' Health Study. The investigation found that for each serving-per-day increase of unprocessed and processed red meat, the pooled hazard ratio (HR) for total mortality was 1.13 (95% CI, 1.07–1.20) and 1.20 (95% CI 1.15–1.24), respectively. That same year, another systematic review and meta-analysis of cardiovascular disease mortality and cancer incidence in seven observational studies including nearly 125,000 participants did not find a statistically significant decrease in all-cause mortality among those following a vegetarian diet, with a pooled relative risk (RR) 0.91 (95% CI, 0.66–1.16). ⁷ It did, however, find a significant decrease in mortality from ischemic heart disease, RR 0.71 (95% CI, 0.56–0.87) and decrease in cancer incidence, RR 0.82 (95% CI, 0.67–0.97) in vegetarians.

One difficulty in evidence for vegetarian diets is that their definition is not necessarily consistent. One meta-analysis defined the vegetarian diet as one allowing meat or fish to be eaten less than once weekly. A strict vegan, on the other hand, would completely avoid animal products. ⁷ Other variations include pescovegetarians, lactovegetarians, and ovovegetarians, who all avoid meat but include fish, dairy, or eggs, respectively. The term plant-based diet encompasses all of them—emphasizing nutrient-dense plant foods, such as vegetables, fruits, legumes, and nuts while minimizing processed foods, oils, and animal foods (including dairy and eggs). ⁸

Several large trials have demonstrated benefit from vegetarian diets and increased vegetable and fruit intake. A large prospective cohort study of about 73,000 Adventists in North America showed that vegetarian diets were associated with reduction in all-cause mortality compared to their nonvegetarian counterparts, with HR 0.88 (95% CI, 0.80–0.97), with the most benefit in pescovegetarians, HR 0.81 (95% CI, 0.69–0.94). ⁹ In Sweden, another prospective cohort study, with almost 72,000 subjects, found dose-response of up to a 3-year shorter survival, with percentile difference (PD) −37 month (95% CI: −8, −16 month) and 53% higher mortality rate with HR 1.53 (95% CI: 1.19–1.99) in those consuming no fruits and vegetables compared with those eating five servings of fruits and vegetables per day. ¹⁰

The effects of a vegetarian diet do not appear to consistently differ between men and women. The HR for increased consumption of total red meat and processed red meat in women was 1.11 (1.08–1.13) and 1.21 (1.15–1.27), respectively, indicating no difference from men. ⁶ The Adventist Health Study HR on all-cause and cardiovascular mortality was insignificant for women compared to men; death from cancer was not significantly changed for either. ⁹

A caution to women considering a strict vegan diet is its effects on bone health. In the European Prospective Investigation into Cancer and Nutrition (EPIC)-Oxford study, meat eaters and lacto-ovo vegetarians have a similar risk of fracture; however, vegans have a 30% higher risk of bone fracture. ¹¹

The current dietary recommendations from the American College of Cardiology (ACC) and American Heart Association (AHA) in the 2013 ACC/AHA Lifestyle Management Guidelines in Circulation continue to recommend including low-fat dairy products, poultry, and fish but limiting red meats. ¹² However, the Academy of Nutrition and Dietetics (previously the American Dietetic Association) has publicly supported the vegetarian diet, stating, “Appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases” (p.1266). ¹³

Answer: The Correct Answer is C

The precise effects of animal-based food on disease and health are far from incontrovertible. Although the scientific evidence for health benefits from vegetarian diets is mixed, it does appear that a plant-based diet is beneficial. In addition, a correlation between a vegetarian diet and a lower rate of death from cardiovascular disease and incidence of cancer has been demonstrated.

The exclusion of meat from the diet has not been associated with lower all-cause mortality. Yet red meat, especially processed, is linked to an increase in total, cardiovascular diseas and cancer mortality.

The Academy of Nutrition and Dietetics has a published position in favor of vegetarian diets as being healthful. A vegan diet may need monitoring for specific nutritional deficiencies, particularly protein, vitamin B12, vitamin D, ω-3 fatty acids, calcium, iron, iodine, and zinc. However, strictly avoiding all animal-based products may not be necessary to reap the demonstrated benefits. A plant-based diet can easily provide adequate nutrition through legumes, whole grains, and vegetables and should be encouraged as a healthful option for women who express interest.