Many studies compare dietary patterns in terms of their nutrient profile and healthfulness but also with respect to their effect on body weight. It’s been shown time and again that vegan diets lead to healthy body weight even without portion restriction and they are the most effective in long-term weight management. Being overweight or obese is associated with hyperlipidemia (increased fat levels in the blood), hypertension, diabetes, cardiovascular disease, certain cancers and all-cause mortality so a healthy weight is a key factor in overall good health.
Berkow & Barnard (2006) reviewed all available studies on vegetarian diets and body weight and found that the majority of them reported a significantly lower body weight (four to 20 per cent) in vegetarians compared to non-vegetarians.
A large study of women and their dietary patterns (Newby et al., 2005) reported that the prevalence of being overweight or obese among vegetarians is significantly lower than among omnivores. They highlighted that, in general, women eating plant-based diets have a much lower risk of becoming overweight or obese in the long term with vegans being at the lowest risk. The study also confirmed that vegans have the highest intake of fruit and vegetables and healthy carbohydrates (wholegrains, pulses and fibre).
The reason vegan diets work so well in terms of healthy weight maintenance is that they are lower in fat, have a better fat intake profile (less saturated and more essential unsaturated fats), lead to a higher intake of fibre and nutrient-rich foods and lower calorie intake (Berkow & Barnard, 2006; Huang et al., 2015). People consuming low-fat carbohydrate-rich diets can eat more food in weight compared to others because these foods have lower energy density.
It’s been well documented that vegans tend to have healthy weight, are at the least risk of being overweight but also do not tend to be underweight. When Tonstad et al. (2009) examined the relationship between different types of diet and body weight, they found that vegans were the only group who had a healthy BMI (body mass index – a ratio of body height and weight), whilst vegetarians, pesco-vegetarians and omnivores were all in the overweight category. The same conclusion was also reached in a later study of dietary patterns, health and cancer incidence (Tantamango-Bartley et al., 2013).
To explore how a change of diet affects people who previously didn’t follow a plant-based diet, Turner McGrievy et al. (2015) compared the effects of five different diets over the period of six months on participants’ body weight. They assigned volunteers who were all overweight to one of the five groups – vegan, vegetarian, pesco-vegetarian, semi-vegetarian and omnivore. All were advised to eat a low-fat diet based on foods with low glycemic index (foods with low proportion of fast absorbing sugars) but there was no restriction on energy intake recommended to any of the groups in the study. Participants were free to eat until they were satisfied and physical activity did not differ between the groups. After six months, the vegan group participants lost on average 7.5 per cent of body weight, vegetarians 6.3, pesco- and semi-vegetarians lost 3.2 and omnivores 3.1 per cent. Based on the final analysis of the participants’ diets, the vegan group had the lowest intake of fat in general, and saturated fat specifically, and the highest intake of carbohydrates and fibre. Also their cholesterol levels dropped significantly more than in any other group.
Huang et al. (2015) reviewed other intervention studies where people were prescribed a diet change and reached a similar conclusion as the study above. Individuals assigned to the vegetarian diet groups lost significantly more weight than those assigned to the non-vegetarian diet. A more detailed analysis revealed that vegan groups lost more weight than vegetarian ones. The study authors also remarked on the healthfulness of plant-based diets and their higher nutritional quality compared to other diets.
To investigate what a low-fat vegan diet can achieve in people who are overweight and/or have diabetes, Mishra et al. (2013) enlisted people on a trial that involved workplace cafeterias offering low-fat vegan options. The participants were instructed to eat a wholesome vegan diet low in fat for 18 weeks with no other changes to their daily life. Again, there were no portion or energy intake restrictions.
Here's their sample menu:
At the end of the 18 week study, the volunteers lost on average 4.3 kg, their cholesterol levels decreased dramatically and their blood sugar control improved. No significant differences were achieved by the control group (people who didn’t change their diet or lifestyle but fulfilled the same criteria as the intervention group).
All these studies clearly show that a low-fat vegan diet can help achieve and maintain healthy weight without portion restriction and in the long-term. Many experts also recommend plant-based diets as a healthy, nutrient dense approach to weight management and prevention of obesity for adults and children alike (Farmer et al., 2011; Sabaté & Wien, 2010).