Do veg*ans have better health because of their diet or lifestyle?

Question

Several statistics show veg*ans have better health ^1,2,3; however one could argue that this is due to several factors other than their diet, e.g. they smoke less, drink less alcohol, do more physical activity, do meditation or yoga, etc. So how can we say that health benefits of veg*ns come from the diet and not from these behaviours that are more related to lifestyle?

In medical terms, are there any publications that have researched the reasons for better health of veg*ns controlling the confounding factors?

NOTE: On this website there is another question that looks similar. However one is asking which are the health benefits, while the other is focusing on the control of confounding factors. In other words, the first question might include correlations between veg* diets and health benefits, while the second is only focusing on causal relations (check also this). It's way more specific by the statistical, epidemiologic and pathophysiological point of view.

Answer 1

The argument that veg*s enjoy better health because of the confounding factors (reduced smoking habit, reduced alcohol consumption, more physical activity, etc.) could be valid for cohort studies, but there are actually several clinical trials (RCT and even cross-over) that show that - although there are some health benefits derived from these behaviours - a part of the health improvements are due to the diet alone.

Here are some examples:

In Barnard 2009 (RCT), 99 patients with type 2 diabetes were randomly assigned to a low-fat vegan diet (n = 49) or a diet following 2003 American Diabetes Association guidelines (conventional, n = 50) for 74 weeks. The low-fat vegan diet appeared to improve glycemia and plasma lipids more than did conventional diabetes diet recommendations.

In Barnard 2005 (RCT), 64 overweight, postmenopausal women were randomly assigned to a low-fat, vegan diet or a control diet based on National Cholesterol Education Program guidelines, without energy intake limits, and were asked to maintain exercise unchanged. The group on the low-fat vegan diet improved their body weight, insulinemia and glicemia more than the other.

In Kahleova 2011 (RCT), 74 patients with Type 2 diabetes were randomly assigned to either the experimental group (n = 37), which received a vegetarian diet, or the control group (n = 37), which received a conventional diabetic diet. Several improvements where higher in the first group than the second: BMI reduction, waist circumference, body weight, total and LDL cholesterol, homocisteine, glicemia, insulinemia, HbA1c.

In Siener 2003 (cross-over), 10 healthy male subjects ingested a self-selected meat-containing diet (SD) for two weeks, and three different standardized diets for a period of 5 days each. The Westerntype diet (WD) was representative of the usual dietary habits,whereas the balanced omnivorous diet (OD) and the ovo-lacto-vegetarian diet (VD) were calculated according to the requirements. The risk of uric acid crystallization was highest on the ingestion of diets SD and WD, due to the high urinary uric acid excretion and the acidic urinary pH. The relative supersaturation with uric acid declined significantly by 85% on the intake of diet OD, consequent to the decrease in uric acid excretion and concentration and the increase in urinary pH value. The ingestion of the vegetarian diet VD led to a further significant reduction in the risk of uric acid crystallization by 93% compared to diet WD.

In Cooper 1982 (cross-over), 15 free-living individuals were randomly assigned to 3-week periods on either the experimental diet or a control diet which closely approximated the usual intake in the U.S.A. Significant reductions in total cholesterol (12.5%), low density lipoprotein cholesterol (14.7%), and apo B (13.2%) were observed.

In Bunner 2014 (cross-over), 42 adult migraine sufferers were divided randomly into two groups. This 36-week crossover study included two treatments: dietary instruction and placebo supplement. Each treatment period was 16 weeks, with a 4-week washout between. During the diet period, a low-fat vegan diet was prescribed for 4 weeks, after which an elimination diet was used. The patients showed greater improvement during the diet period than placebo.