New research suggests that vegan diets promote weight loss. There's a little bit more to the story, though.
If you want to lose weight, there seems to be only one way to do it: find a tolerable, nutritious diet that will allow you to cut your calorie consumption over the long term—and stick with it.  There's no magic macronutrient or food group everyone can avoid or over-consume to reach their weight-loss goals.
I was reminded of this after reading recent coverage of an unpublished meta-analysis showing that vegan diets may help type-2 diabetics lose weight and reduce their blood sugar levels. There's little information about the research available for now, though we can glean some of the specifics from press reports and comments from experts who have reviewed the study poster presented at the European Congress on Obesity.
Despite the favorable coverage the study has received, it seems unlikely that avoiding animal products or loading up on plants drove the weight loss observed in the meta-analysis. Participants in the reviewed studies likely ate less than they had previously, driving their weight loss. Simple as that.
What do we know?
The researchers analyzed the results of 11 clinical trials involving 796 participants. These studies compared the impacts of vegan and control diets on cardiometabolic risk factors including body weight, BMI, blood sugar, blood pressure, total cholesterol, HDL, LDL, and triglycerides.
In contrast to participants on some form of a Mediterranean diet, those who followed a plant-based program for three months lost a little over nine pounds. Compared to a true control group that ate “a normal diet without dietary changes,” the vegan group lost 16 pounds on average. The diet's impact on total cholesterol, LDL, and blood sugar was quite small, though.
The pro-vegan crowd was unsurprisingly impressed with the results. “The best way to lose weight for these patients, according to Danish researchers, is by following a vegan diet,” The Beet contributor Maxwell Rabb said of the meta-analysis. “The Danish researchers’ study will join a consistently growing body of research that link plant-based diets to weight loss and even improved diabetes symptoms.” The animal-rights activists at Physicians Committee for Responsible Medicine (PCRM) likewise reported that “Plant-Based Foods Aid Weight Loss and Improve Blood Sugar.”
Color me skeptical for several reasons, the most important of which come from the reviewers themselves. According to the paper's press release, all of the clinical trials had relatively small sample sizes, suggesting that their results may not be generalizable. Perhaps more importantly, the macronutrient composition of the vegan diets “varied substantially,” and “none of the studies prescribed a control diet that exactly matched the intervention diet in all other aspects except veganism.”
In other words, there's really no way to know exactly which, if any, aspects of a plant-based diet promote weight loss. It may be that individuals assigned to a vegan diet in these trials just ate fewer calories than participants in the control groups. Of course, anybody can create a calorie deficit whether they eat steaks or salads. Professor Ian Givens, director of the Institute of Food, Nutrition, and Health at the University of Reading, put it this way:
"...[A] key question is whether the findings of this study were due to the vegan diets or a reflection of a lower energy intake, in other words were the control diets isoenergetic … with the vegan diets. This seems not to have been the case as the authors note in their poster conclusion ‘Some of this effect may be contributed to differences in the macronutrient composition and energy intake in the vegan diets versus control diets.'”
People lose weight on weird diets
Does this mean vegan diets are useless? No, I'm sure they work quite well for some people. But the fact remains: almost any diet can work, even if it's really strange. For example, Duke University researcher Walter Kempner helped thousands of people lose weight over his career by putting them on a diet of white rice, fruit, fruit juice, refined table sugar, and, in some instances, vitamin supplements. Just consider this madness Kempner published in JAMA in 1974:
“A patient takes an average of 250 to 350 gm. of rice … All fruit juices and fruits are allowed, with the exception of nuts, dates, avocados and any dried or canned fruit or fruit derivatives to which substances other than white sugar have been added. Not more than one banana a day should be taken. White sugar and dextrose may be used … on an average a patient takes about 100 grams daily, but, if necessary, as much as 500 grams daily should be used. Tomato and vegetable juices are not allowed.”
If this regimen sounds psychotic to you, I empathize. Surely guzzling fruit juice and white rice can't possibly battle obesity—except that it did. In 1975, Kempner and three colleagues published these results in the Archives of Internal Medicine:
“One hundred six massively obese patients, who each lost at least 45 kg, were treated as outpatients with the rice/reduction diet, exercise, and motivational enhancement under daily supervision. Average weight loss was 63.9 kg [roughly 140 pounds].”
Aside from the fact that today's fat-acceptance gurus would find Kempner's language “problematic,” what can we learn from this paper? The prescribed diet was low-salt, low-fat, low-protein, mostly cholesterol free, and added up to less than 1,000 calories a day. Patients were also prescribed a daily exercise plan that pushed them “to the point of maximum tolerance in terms of subjective comfort.” They were also allowed to reincorporate vegetables and meats as time went on.
I'm in no way endorsing Kempner's approach, or any other dieting method for that matter. What's clear, however, is that advocates who promote specific weight-loss plans as generally effective for anybody and everybody aren't working from good data. The very unsexy conclusion we're left with is that calorie reduction drives weight loss. Eat animals or plants to your heart's content. It really doesn't matter—as long as you eat less.
 This isn't weight-loss advice. Consult a physician before you go on any diet.