Plant-based diets for maintaining a healthy weight
By Dr Shireen Kassam, Director of Plant-Based Health Professionals UK
Plant-based diets are a great way to maintain a healthy weight, and in doing so, can lead to significant reductions in the risk of chronic conditions associated with carrying excess body fat, particularly when this fat accumulates in body organs (visceral fat).
Overweight and obesity are at epidemic proportions with 1 in 8 people globally living with obesity. In the UK, around 65% of adults are either overweight or living with obesity, with rates higher in those living in deprived areas and in non-White ethnicities. Overweight and obesity also affects around 22-25% of children.
Living with overweight and obesity significantly increases the risk of common chronic conditions including cardiovascular diseases, type 2 diabetes, certain cancers, fatty liver diseases and arthritis, whilst also lowering life expectancy and adversely impacting quality of life.
Definition of overweight and obesity
The definition of obesity up till now has mainly been based on the body mass index (BMI). BMI is calculated as weight (kg) divided by [height (m)2] with normal considered as 18.5-24.9. The best health outcomes are associated with a BMI of 20-22. People from Asian, Middle Eastern, Black African and African–Caribbean backgrounds are prone to central adiposity, resulting in cardiometabolic conditions occurring at lower BMI levels, so a normal BMI is considered up to 22.9.
BMI is a useful tool for assessing overall health of populations, but on an individual level has been criticised as a flawed method of determining body fatness and hence health status because the measure does not distinguish between fat and muscle mass. To overcome these issues, two recent pieces of guidance – The Lancet Commission on obesity and the UK’s NICE guidance – recommend the use of anthropometric measurements, such as waist to height ratio, or waist circumference, alongside BMI.
Evidence supporting plant-based diets for weight management
The causes of overweight/obesity are numerous and multifactorial and the magnitude of the problem should not be underestimated. However, dietary behaviours are important for prevention and treatment. In general, diets high in meat, especially red meat, and ultra-processed foods, whilst low in fruit and vegetables are associated with an increased risk of overweight and obesity. In contrast, healthy plant-based diets are a very effective way of maintaining a healthy weight and losing weight if needed.
Observational studies have consistently shown that people eating diets centred on fruit, vegetables, whole grains, beans, nuts and seeds tend to have a healthier body weight. The greatest advantage is seen in people following a 100% plant-based or vegan diet. For example, in both the EPIC-Oxford study and the Adventist Health Study-2, vegans had the lowest BMI when compared to vegetarians, pescatarians and meat-eaters.
Combined data from the Nurses’ Health Studies and the Health Professionals Follow-up study show that consumption of certain foods is associated with either weight gain or weight loss over time. Regular consumption of processed foods (sweets and desserts, fries, butter, refined grains), sugary beverages, processed and unprocessed meat was associated with weight gain. Regular consumption of vegetables, fruits, whole grains, yogurt and nuts was associated with weight loss.
A more recent analysis of these study cohorts examined associations of changes in intake of overall, healthful, and unhealthful – with weight change tracked over 4-year intervals spanning more than 20 years. Adherence to an overall plant-based diet was associated with significantly less weight gain over each 4-year period, with greater benefit seen for those adhering to a healthful plant-based diet. However, an unhealthful plant-based diet (emphasising refined grains, potato/fries, sweets, sugar-sweetened drinks/juices) was associated with significantly more weight gain over the follow-up period. Similarly, in a subgroup analysis of the Nurse’s Health Study, a diet high in healthy plant-based foods was associated with less weight gain around the menopause.
Randomised studies on dietary patterns and weight management
The PCRM conducted a randomised study of a whole food plant-based (WFPB) or healthy vegan diet and compared it to the National Cholesterol Education Program recommended diet (control group). Sixty-four overweight women were included.
After 14 weeks, the mean weight loss in the WFPB diet group was 5.8kg, which was significantly greater than in the control group (weight loss 3.8kg).
Another randomised study from PCRM included 244 participants with a BMI of 25–40 who were randomised to follow either a low-fat, WFPB/vegan diet or to make no dietary changes.
After 16 weeks, the results showed that the vegan group lowered their body weight by 6.4 kg and reduced liver and muscle fat by 34% and 10%, respectively, which correlated with improvements in insulin resistance. They also increased their after-meal metabolism by 18.7%. There were no significant changes in the measured parameters in the control group.
The BROAD study was a randomised trial using a low-fat WFPB diet in a community setting for people with obesity, ischaemic heart disease or type 2 diabetes. The intervention led to a significant and sustained reduction in weight and BMI at all time points when compared to the control group with improvements sustained for the 12-month follow-up.
What is notable about all three studies mentioned is that food was not provided to participants, there was no requirement to restrict calorie intake or reduce portion size and exercise was not part of the intervention. This is especially important because many diet programs for weight loss focus on restricting calories and portion sizes. When focusing on whole-food, plant-based eating, the diet is not restrictive, but rather has an abundance of plant foods and meals, making it an enjoyable and sustainable way of eating.
A 2025 meta-analysis of randomised studies shows clear benefits for a WFPB/vegan diet for weight reduction.
How do plant-based diets help maintain a healthy weight?
1) Reduced energy density: Plant foods are lower in energy density, whilst still being high in healthy nutrients, when compared to animal-sourced foods. Most people eat the same volume of food everyday — around 1.8kg or 4lbs. So, on a plant-based diet you can generally eat to fullness whilst consuming less calories compared to other diet patterns.
2) Increased fibre intake: The increased fibre consumption on a plant-based diet is more satiating, giving the feeling of fullness with less calorie intake. Fibre also supports a healthier gut microbiome, which in turn has been associated with a healthier body weight. Fibre does not provide calories and is only present in plant foods.
3) Reduced fat intake: The lower fat intake, particularly saturated fat, can be helpful. This is because fat has nine calories per gram but carbohydrates and protein only four calories per gram.
4) Increased postprandial (post-meal) energy expenditure: Daily total energy expenditure is as follows; resting energy expenditure — 60–75% of total, physical activity — 15->30% of total, thermic (effect of food) — 10% of total. Physical activity is the energy expenditure we have most control over and the ability to change.
However, there are differences in the energy expenditure after eating, based on the diet composition. Studies have shown that a low fat, WFPB diet results in greater postprandial energy expenditure. This is because the fat content of a meal influences the action of mitochondria in the cells. Mitochondria in cells convert fuel to energy. A high fat meal adversely affects mitochondrial function resulting in reduced energy expenditure.
5) Modulation of appetite hormones: A number of hormones control our feelings of hunger and satiety. The hypothalamus, in the brain, is the control centre of hunger and satiety and hormones are produced in the hypothalamus and in the gastrointestinal tract.
Gastrointestinal hormones are involved in regulation of glucose metabolism, energy regulation, satiety, and weight management. Consumption of food triggers secretion of incretin hormones from the gastrointestinal tract, namely glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP), which enhance insulin secretion and help maintain glucose control. The satiety hormones GLP-1, peptide YY (PYY), pancreatic polypeptide (PP) and amylin control appetite and energy regulation. The release of these satiety hormones can depend on meal composition and differs between states of normal and abnormal glucose control.
A study conducted by PRCM involved a randomised crossover design and examined the effects of two energy- and macronutrient-matched meals – a processed-meat and cheese meal and a vegan meal with tofu – on gastrointestinal appetite hormones and satiety in 20 men with type 2 diabetes, 20 men with obesity, and 20 healthy men. The results showed that the vegan meal led to a greater increase in the levels of gut satiety hormones, which led to greater satiety in the participants.
Areas of continued debate
Low-carbohydrate diets
There are continued debates within the scientific community as to whether a low-carbohydrate diet (however that may be defined) is more effective than a low-fat diet. However, diet quality matters more, with different macronutrient combinations leading to similar weight loss, if the overall diet emphasises healthy foods.
There are long-term adverse effects of a low-carbohydrate diet, especially when carbohydrates are replaced by animal-sources of protein and fat. This includes elevated LDL-cholesterol, increased risk of cancers and premature death.
Genetics predict response to different diets
Another common narrative is that body weight and response to different diets is dependent on genetic factors. Both the low-carbohydrate diet and genetic theory were put to the test in the DIETFITS study, which compared weight loss on a low-fat diet with a low-carbohydrate diet. After 12 months, there was no significant difference in weight change between a healthy low-fat diet versus a healthy low-carbohydrate diet. In addition, this study examined different genes that had previously been suggested to affect response to fat and carbohydrate. However, these genes did not predict weight loss or associate with the effectiveness of either diet pattern.
GLP-1 medications and dietary strategies
GLP-1 weight loss medications have significantly changed the management of obesity. These medications are hugely effective, more so than diet and lifestyle interventions. However, it must be remembered that individuals on these medications require appropriate nutrition and lifestyle support. This is to counter the effect of reduced appetite and food intake, which leads to nutritional deficiencies, especially if diet quality is not addressed. In addition, loss of lean muscle mass can be a problem and requires adequate strength building exercise along with sufficient protein intake (1-1.5g/kg/day). This is also an opportunity to embed healthy, sustainable, lifestyle habits, because when the medications are discontinued, most of the weight is regained within a year.
Summary
- Fruit, vegetables, whole grains, legumes and nuts are key foods associated with a healthier body weight.
- There is robust evidence to support a plant-based diet, including healthy vegan/whole food plant-based diets, for preventing and treating overweight and obesity.
- When using a WFPB diet for weight loss, calorie counting and portion control is generally not required making this way of eating more sustainable.
- Meat and ultra-processed foods are associated with higher body weight.
- Any shift towards eating more plant-based foods, whilst reducing meat intake and ultra-processed foods, is going to have a positive impact on body weight if weight loss is desired.
- Plant-based diets can support people on GLP-1 medications maintain adequate protein and nutrient intakes
Resources
10 tips for weight management on a plant-based diet
Energy density – the key to successful weight loss
Better results, starting now. Your nutrition roadmap for GLP-1 therapy – Proveg International
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