There seem to be a lot of people who are highly confident that veg*n diets are either very good for human health or very bad for it. Because studies about the health effects of veg*nism often contradict each other, and show different conclusions based on the thing being measured (bone mineral density, BMI, lifespan, etc.), it’s possible to collect a bunch of studies that agree with your view, and then present these as though there is unanimous scientific agreement that veg*nism is awesome/terrible for health.
Nutrition research is often a confusing mess of studies that contradict each other, because nutrition is complicated and hard to run good studies on. RCTs are often infeasible for studying long-term dietary changes. (“Thanks for signing up for our study! Please refrain from eating animal products for the next decade.”) Systematic reviews aren’t perfect by any means, but can help weed out poorer studies and aggregate the results from what’s left. When I first became a vegetarian a few years ago I put together a list of systematic reviews on the topic. Below is an updated list. (Disclaimer: As noted above, nutrition science is hard to do well. A lot of the reviews use observational studies. Apply appropriate amounts of salt.)
Type 2 Diabetes
Meta analysis on 3 RCTs and 1 crossover controlled trial found that a vegetarian diet helped type 2 diabetes management. The vegetarian diet decreased HbA1C, weight, total cholesterol, and LDL cholesterol significantly more than the control diet. No significant difference in fasting glucose were found.
Meta-analysis on 6 RCTs. The vegetarian diets had significant benefits when measuring HbA1c and HDL. Found no significant effect on fasting plasma glucose, cholesterol, triglycerides, LDL, and weight. The effectiveness of vegetarian diets were related to their fiber content.
Consumption of vegetarian diets is associated with a significant reduction in HbA1c and a non-significant reduction in fasting blood glucose concentration.
Veg*n diets decrease the risk of developing type 2 diabetes, maybe.
Bone Mineral Density
Veg*n diets are associated with a decrease in bone mineral density, but the effect size is very small. Vegans should be more concerned about bone mineral density than lactoovovegetarians. The effect was only significant for women.
Vitamin and Mineral Deficiencies
Dietary zinc intakes and serum zinc concentrations are lower in veg*ns.
Veg*ns are at increased risk for vitamin B12 deficiencies.
The research is pretty crappy, but as far as we can tell veg*n diets seem safe for pregnant women, as long as attention is paid to vitamin and mineral intake.
Colorectal Cancer/Colorectal Adenomas
“High-frequency red meat intake is an important factor increasing the risk of colorectal cancer development.”
The relationship between meat consumption and colorectal cancers is positive, but might be confounded.
“These results indicate an elevated risk of colorectal adenomas with intake of red and processed meat, but further prospective studies are warranted.”
Also see first study under “Other Cancers” section.
“More studies are needed, particularly large population-based case-control and cohort studies to confirm some of these findings and for those sites investigated by too few studies to date. However, for some of the most investigated sites, including breast, lung, esophagus, pancreas and stomach, it seems likely that reducing meat consumption will be an important goal in decreasing cancer risk. For colorectal cancer it seems very likely that reducing meat consumption will decrease the risk.”
Higher consumption of red meat and processed meat is associated with higher risk of esophageal cancer, while higher consumption of fish intake is associated with lower risk.
People who follow vegetarian diets have lower blood pressure.
Found no evidence that plasma HDL levels differ in vegetarians and non-vegetarians.
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Conclusions: Someone that is certain that veg*n diets are awesome/terrible for health is overconfident. The research is messy, but it seems likely that veg*n diets have some benefits for things like type 2 diabetes management and decreasing cancer risk. On the flip side, veg*n diets likely increase the risk of vitamin and mineral deficiencies in people who aren’t careful.