Plant-based eating and weight management

Disease burden of obesity

According to the World Health Organisation, 39% of adults aged 18 years and over were overweight of which 13% were classified as obese in 2016.(1) Additionally, the prevalence of childhood and adolescent overweight and obesity increased hugely from 4% to over 18% between 1975 and 2016.

This is a significant challenge for public health because the risk of non-communicable diseases such as cardiovascular disease, musculoskeletal disorders and some cancers is increased amongst individuals with higher BMI. The 2019 Global Burden of Disease study highlights high BMI as one of the key risk factors for future health which is increasing at over 1% per year.(2)

As an example, during the COVID-19 pandemic, obese patients were more likely to experience severe symptoms and had a greater chance of mortality compared with non-obese patients.(3)

Why plant-based diets for weight management?

Diet plays a major role in weight management. In particular, an energy imbalance between calorie intake from food and calorie output from physical activity.

Plant-based diets have been shown to be supportive for healthy weight management and weight loss. The nutritional characteristics of plant-based diets are thought to be responsible for the lower prevalence of obesity, lower rates of coronary heart disease, hypertension and diabetes observed in people whose diets consist of mainly plant-based foods.

Plant-based foods, in the main, have a lower energy density, are typically low in saturated fat, high in heart-healthy unsaturated fat and fibre. These components are associated with lower body weight and less weight gain over time.

Evidence from observational studies

Observational cohort studies have shown that individuals who follow a more plant-based diet tend to have a lower body weight and BMI compared to those with higher intakes of animal products.

The Seventh-day Adventist Study-2, included 22,434 men and 38,469 women, approximately half of whom were omnivores and half vegetarians.(4) Participants were categorised as vegan (2,731), lacto-ovo-vegetarian (20,408), pesco-vegetarian (5,617), semi-vegetarian (flexitarian) (3,386) or non-vegetarian (28,761). There was a clear inverse correlation between degree of plant-based foods in the diet and BMI score. BMI scores progressively increase from the lowest BMI in the vegan group to the highest BMI in the non-vegetarian group.

Researchers from Harvard University also analysed data from three large cohorts: the Nurses’ Health Study 1 and 2 and the Health Professionals’ Follow-Up Study (total of 126,982 adult participants).(5) Over a 20-year follow up period, the researchers observed that healthy plant-based diets resulted in 0.68kg less weight gain over 4-year intervals. This study is interesting as it highlighted the influence of the quality of plant foods consumed. A diet high in refined plant-based foods such as fruit juices, sugar-sweetened beverages and refined grains actually led to weight gain over time.

Evidence from clinical trials

Evidence from clinical trials has demonstrated the effectiveness of whole-food plant-based diets for healthy weight loss. A 2016 meta-analysis of randomised controlled studies confirmed that a greater weight loss was achieved following a vegetarian diet compared to a non-vegetarian diet and that the greatest loss was achieved with an energy restricted vegan diet.(6)

For example:

1. A 16-week randomised clinical trial, tested the effect of a low-fat plant-based diet on body composition.(7) Overweight participants (n=75) were randomly assigned to either a plant-based diet or an omnivorous control diet. The plant-based diet was superior in terms of improving body weight (average decrease of 6.5kg), fat mass (average fat loss of 4.5kg) as well as insulin resistance markers. The authors concluded that a plant-based diet is an effective strategy in the treatment of obesity.

2. Similar results were observed in the GEICO study, an 18-week dietary intervention using a low-fat plant-based diet in a corporate setting.(8) Overweight participants, some with diabetes type II diagnosis, were randomised to either a low-fat plant-based diet or a control diet. Following the intervention, participants following the plant-based diet lost on average 2.9kg, reduced their LDL cholesterol by 8.1mg/dL and HbA1c by 0.6%. These were significantly greater improvements than in the control group (body mass -0.06kg, LDL cholesterol -0.9mg/dL and HbA1c –0.08%)

3. The BROAD study, was a longer-term randomised control trial comparing a plant-based dietary intervention with standard care in 49 participants.(9) At 6-month follow up, participants following the plant-based diet lost on average 10.6kg more weight and dropped 3.9 more BMI points compared to the control group. Notably in this study, participants were not asked to limit their food intake and ate ad libitum, highlighting that plant-based diets allow for satiation and weight control without hunger and deprivation.

Soya versus meat-based weight loss diets

High-animal protein diets have often been cited as being effective for weight loss, however they may also have negative health impacts, particularly on gut health and serum lipid profiles, as well as not being environmentally sustainable.

Prof. Johnstone, Rowett Institute of Nutrition and Health, University of Aberdeen, explored the role of plant-based soya protein versus animal protein in weight loss interventions.(10) Her team of researchers carried out a randomised crossover trial to assess the hunger and appetite responses in 20 healthy, obese men eating a soya-based high-protein weight loss diet compared to a meat based high-protein weight loss diet.

Over a two-week period, subjects lost similar amounts of weight (-2.41kg soya vs. -2.27 kg meat) and both high-protein weight loss diets reduced appetite in participants. Blood biomarkers improved with weight loss for both diets but there was a greater reduction in total and low-density lipoprotein cholesterol with the soya-based diet.

The researchers concluded that a plant-based high-protein diet is an effective healthier alternative to meat-based high-protein weight loss diets, achieving the desired results without the negative health effects.

This research was supported by an Alpro Foundation research grant.

Conclusions

In summary, plant-based diets offer a sustainable and effective solution to weight management and obesity prevention. Adopting a diet based on healthful plant foods supports weight management over time and is an effective strategy for sustainable weight loss.

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