The spectrum of vegetarian eating runs from the most restrictive, vegan, to lacto-ovo (eating eggs and dairy-based foods but no animals). All plant-based eaters generally consume fruits, vegetables, nuts, seeds, legumes and grains.
Research rundown: Comparatively, the evidence on vegetarian eating runs long and deep, with a host of health benefits including weight control and disease prevention.
Among the largest and longest research projects are the Adventist Health Studies, conducted over the past 40 years. The largest study in the series was conducted among nearly 100,000 Seventh-day Adventists. The researchers, based at Loma Linda University in California, use periodic questionnaires to gather data about participants' health status, disease risk factors, eating and lifestyle habits and more. Results show that Adventist vegetarians have a lower risk of overweight, insulin resistance, metabolic syndrome, prediabetes and Type 2 diabetes than non-vegetarians in this population. And the closer people abide by a vegan diet, vs. the least-restrictive lacto-ovo vegetarian plan, the healthier they are.
The Adventist Health Study and several other large observational studies have drawn a link between reduced red-meat and processed-meat consumption and lower chronic disease risk. Turner-McGrievy points out that the more people choose plant-based foods over animal-based foods, the more they can cut down on saturated fat and cholesterol. That's a healthy move.
But it's not just plants that keep vegetarians healthier. Studies show they also tend to practice other healthful behaviors such as not smoking, being physically active, watching less television and getting sufficient sleep. That might turn out to be true for long-term paleo followers, but we don't have the research yet.
Nutrition pitfalls: The more food groups a vegetarian eats, the easier it will be to meet his or her nutrition needs. Nutrients to keep an eye on: vitamins B12 and D, omega-3 fats, iron and zinc.