Huge Meta-Analysis Shows Plant-Based Diets Are Full of Health Benefits

People opt to eat a vegetarian or vegan diet for all sorts of reasons. Often ethical, moral, religious or environmental considerations are key factors. But a new, massive analysis hammers home that there are also plenty of selfish reasons to avoid eating meat and animal products. Plant-based diets are correlated with a slew of health benefits, from reduced cancer and heart disease risk to lower inflammation, according to a large “umbrella” review study published May 15 in the journal PLOS One.

Review studies assess the results from multiple primary studies at once. Meta-analyses are a type of review that synthesizes results from more than one past study in a statistically rigorous way. Umbrella reviews take all of that one step further: grouping and assessing past reviews and meta-anaylses into a single sweeping study to cover as much of the available research as possible. They’re a particularly rigorous scientific approach.

The new work looked at 49 reviews spanning more than 20 years of diet and health research. It only included studies of vegetarians (including those who eat eggs and dairy) and vegans (those who abstain from all animal-based products) — not pescatarians or those who otherwise reduce or restrict their meat consumption. The scientists also focused their analysis on studies of generally healthy people, and excluded research on populations with diagnosed illnesses or special dietary requirements (like professional athletes). The resulting umbrella review indicates some strong trends across the field.

Across past review studies, they found that both vegetarians and vegans have significantly better metabolic health and are less likely to develop certain types of cancer and heart disease. Specifically, the researchers noted that total cholesterol levels, LDL (i.e. “bad”) cholesterol levels, glycemic control, lipid profile, blood pressure, and body weight were all significantly healthier among the non-meat-eaters in multiple past studies. Further, C-reactive protein, a measure of systemic inflammation, was significantly lower among vegetarians and vegans. Plus, vegetarians and vegans had significantly lower risk of heart disease caused by blocked blood vessels and a lower risk of mortality from cardiovascular disease, according to the new research. Finally, the researchers found a lower risk of gastrointestinal and prostate cancers among the vegetarians and vegans, compared with omnivores, and a lower risk of mortality from those diseases.

The review suggests multiple possible contributing factors for all of those benefits. For one, the authors note that veg-heads engage in healthier lifestyles overall, exercising more and smoking less. Essential, health-promoting nutrients like fiber and complex carbohydrates are abundant in plant-based foods. Some studies also suggest that plant foods include protective bioactive compounds that actively reduce cancer risk like vitamins and antioxidants. Other research has indicated that avoiding meat also reduces consumption of unhealthy fats, sugars, and processed foods that we know are detrimental. And some of the findings could come down to weight. Since vegetarians and vegans generally had lower body mass index, that could have knock-on health benefits specific to reduced body fat. Regardless of the exact reasons, the upsides are clear.

Though, there are a few caveats. The umbrella analysis didn’t find significant evidence of every examined health benefit. For example, avoiding meat and animal products doesn’t seem to reduce the risk of gestational diabetes or hypertension during pregnancy. And some outcomes are still up for debate, such as the impact of vegetarianism or veganism on pancreatic cancer or HDL (AKA “good”) cholesterol.

Plus, the review notes there are confounding factors that could be influencing the results. The data reviewed was heterogeneous, varying in type and quality– so some of the studies might be stronger reflections of real-world trends than others. And people in different parts of the world may experience different outcomes from adopting a meat-free diet due to regional dietary variation and food availability, so observed effects may differ by location, which the authors didn’t control for in their analysis.

Crucially, herbivore diets can vary in makeup and healthfulness, just as much as omnivore diets can. “Even vegetarian or vegan diets that emphasize consumption of unhealthy plant foods, such as fruit juices, refined grains, potato chips, and even sodas might have detrimental effects on the body,” said study co-author Davide Gori, a biomedical scientist at the University of Bologna in Italy, to Everyday Health. And there are some well-established risks of a vegan diet, like the potential for developing B-12 deficiency and other nutritional shortfalls.

Because of these risks and limitations, the study authors don’t offer a blanket recommendation that everyone suddenly stop eating meat. “Specific patient needs should be considered,” and more research is needed, before suggesting vegetarian and vegan diets “on a large scale,” the authors write.

Yet altogether, the new review is compelling evidence that going meat-free can contribute to a longer and healthier life. In that way, vegetarians and vegans may be killing two birds with one stone, metaphorically speaking: saving animals and preserving their own well-being at the same time.