Diet Science: Support for Plant-Based Eating
A recent study support the idea that a plant-based, low-fat diet has significant health benefits. Note that I didn’t say “is the healthiest for everyone” or “is the best wait to achieve any and all goals”. As I enjoy doing from time to time (#nerd), I’ll walk you through the study and weigh in on the findings. This study (in JAMA, no less) has demonstrated some of the short-term metabolic effects of a low-fat vegan diet. There are a lot of them! So, let’s read the study:
Who was studied? 244 people (that’s not a lot– but remember this isn’t a population-based study like lots of big nutrition studies; it’s a clinical trial) in the Washington, DC area, between the ages of 25 and 75, with a BMI between 28 and 40. The final study sample was 87% female (that’s noteworthy!), mean age was 54, and 48% were white. These are pretty broad inclusion criteria– that increases the external validity of the study (applicability to other people/settings).
Who was NOT studied? Smokers, people with diabetes, people already eating vegan, normal weight people, extremely obese people, very young and very old people.
What was the design? Randomized controlled trial. Obviously, this study wasn’t double blind– people knew which group they were in– but researchers who measured outcomes were blinded to participant assignment. RCTs are expensive and challenging especially for lifestyle factors like nutrition.
What was the intervention? Half the participants did nothing different (control group); the other half were instructed to follow a low-fat vegan diet. The intervention diet (approximately 75% of energy from carbohydrates, 15% protein, and 10% fat) consisted of vegetables, grains, legumes, and fruits without animal products or added fats. They got classes, cooking demonstrations, and written instructions; they didn’t get meals. Everyone was asked to keep their exercise and medications the same, and to limit alcohol to 1 daily for women or 2 for men.
What were the outcomes measured? body weight, body composition, insulin resistance, postprandial metabolism, and intramyocellular and hepatocellular lipid levels (these last two in a subset of 44 participants only); all of these metrics were assessed using well-validated measures.
What was the timeline? 16 weeks. So not long-term. But probably long enough to assess real-world adherence and definitely long enough to measure the primary outcomes. This is totally different from an epidemiological study that might have many years worth of data– but an RCT like this can show causation much, much better.
Sources of bias? It’s possible that people in the control group changed their habits because they knew they were in the study– this would likely cause an underestimation of effect, though. Researchers also relied on people accurately reporting what they ate, which, though practical, is not necessarily highly accurate. And this study measured specific outcomes related to weight, body composition, and metabolism– so effects on other hormones, mood, performance, or other variables are not known.
What are the key findings? The intervention (the low-fat vegan diet instructions) caused significant reduction in bodyweight and fat mass and improvement in blood sugar and metabolism. These findings were both statistically significant and clinically meaningful (e.g., the intervention group lost an average of 6.4 kg; the control group lost an average of 0.5 kg). One mechanism appears to be reduced lipid concentration in the liver and muscle cells, leading to reduced insulin resistance (insulin resistance is a feature of diabetes).
Bottom line? Good study! Relevant participants, sound measurements, appropriate statistical analysis, adequate timeframe. This study showed that a low-fat vegan diet can improve some important health markers in overweight adults in a real-world setting. Good to know! Helpful! This increases my confidence that, for a patient who wanted to do it, a low-fat vegan diet is likely to improve their metabolic health.
