By Robby Berman on December 11, 2023 — Fact checked by Kelsey Costa, MS, RDN
- A new study supports a vegan diet as a means of losing weight.
- In the study, the researchers compared two groups eating vegan (plant-based) diets and omnivorous (plant and animal sources) diets.
- The researchers also observed that the quality of a plant-based diet matters for weight loss and health in general.
A new study finds that following a plant-based diet is more associated with weight loss for overweight adults than a standard omnivorous diet. It also concludes that the quality of such a diet makes a difference when it comes to how much weight is lost.
Participants who followed a vegan diet lost an average of 5.9 kilograms (13 pounds), irrespective of diet quality.
The study employed a trio of commonly used indices. These allowed researchers to separately assess a healthy plant-based diet (hPDI), an unhealthy plant-based diet (uPDI), and a plant-based diet in general (PDI).
This was a secondary analysis of a randomized clinical trial investigating the effects of a low-fat vegan diet on body weight, insulin sensitivity, postprandial metabolism, and intramyocellular and hepatocellular lipid levels in overweight adults.
244 overweight adults took part in the trial. The researchers assigned 122 individuals to a vegan-diet group and the remaining 122 to a control group, who were asked to continue eating their normal omnivorous fare. In the end, 223 people completed the 16-week trial.
Participants submitted three-day dietary records to the researchers at baseline and at week 16, and were left to their own choice of foods. They did not receive nutritional guidance.
Participants’ records were re-analyzed, with all plant-based foods receiving positive scores in the PDI index. Only healthy plant-based foods were awarded positive scores in the hPDI index, and only unhealthy foods scored positively in the uPDI index.
As the trial progressed, the numbers of participants belonging to all three plant-based indices rose, and these scores closely aligned with weight loss.
The PDI demonstrated the strongest negative correlation with body weight (r = -0.40).
Higher hPDI scores correlated with lower body weight at a rate of -0.37, while a modest correlation of -0.21 was observed between high uPDI scores and lower body weight.
The study was published in the European Journal of Clinical Nutrition.
“This study confirms some things that we already know,” said Sarah Herrington of Alchemy Training & Nutrition, who was not involved in the study.
The study observed that whole grains and legumes were associated with weight loss in the vegan group, while meat [in the control group], vegetable oils, and sweets were linked to weight gain.
“The study’s strengths: randomized design, accounting for seasonal effects, and the identification of specific food components associated with weight loss,” according to Nutra Nourish’s Menka Gupta, who was also not involved in the study.
Michelle Routhenstein, registered dietitian nutritionist and preventive cardiology dietitian at EntirelyNourished.com, was also not involved in the research, said “This is not ‘new’ news, given that any diet can facilitate weight loss when a caloric deficit is present.”
“Even though vegan diets may aid in weight loss, it’s important to note that unhealthy variations in these diets can still contribute negatively to metabolic syndrome and cardiometabolic health.”
— Michelle Routhenstein
Previous research generally supports the study’s findings. A 2022 study found that a healthy plant-based diet is associated with a reduction in the risk of obesity, a lower risk of coronary heart disease, and of colorectal cancer.
Avery Zenker of EverFlex Personal Training, who was also not involved in the study, said the study makes an important point about diet quality.
“Not all plant-based diets are created equal, and the quality of the food choices within the diet plays a significant role in health outcomes,” said Zenker.
Participants in both the vegan and control groups consumed less energy during the study, although only the vegan group lost weight.
Herrington suggested that some people in the vegan group may have “reduced their caloric intake due to simply being unsure of what to eat, rather than due to the quality of their food items.”
“One finding that stuck out to me was the fact that most of the participants’ weight loss was from body fat and pro-inflammatory visceral fat, not muscle,” said Stephanie Wells of Thyme to Go Vegan, who was also not involved in the study.
“Maintaining muscle mass when trying to lose weight in the absence of a resistance training program and a high-protein diet is difficult, so it was encouraging to see favorable changes in body composition in this study,” said Wells.
“The healthful plant-based index, defines a nourishing plant-based diet as centered around whole foods such as nuts, legumes, fruits, vegetables, whole grains, tea, coffee, and vegetable oils,” said Routhenstein.
“Vegetables are rich in prebiotic fiber, which supports the health of the microbiome and digestive system. They also contain various phytochemicals like indoles, found in broccoli and Brussels sprouts, which contain benefits that range from protecting cellular DNA to deactivating carcinogens we take in from our environment,” said Herrington.
Herrington also noted that fruits are “a powerful source of antioxidants, and these phytochemicals, with compounds such as ellagic acid, can lower the risk of certain cancers. Legumes, nuts, and seeds contain amino acids, and whole grains contain B vitamins and trace minerals such as manganese and copper,” she added.
Gupta pointed out, “Unhealthy plant-based foods can include highly processed products, like vegan burgers or sugar-sweetened snacks, which often contain added sugars and unhealthy fats like trans fats or over-saturated fats.”
In fact, she said, “High-fructose and low-sugar versions of these foods may have fewer nutrients than unprocessed whole-plant foods, and some may contain artificial additives, preservatives and flavorings.”
The veteran vegan’s “dirty little secret”: Many junk foods are vegan.
“There are plenty of foods that are ‘plant-based’ but are not necessarily the most healthful. Oreos, for example, as well as potato chips, are technically plant-based,” said Herrington.
Commercial oils are also a concern.
“These types of vegetable oils are high in omega-6 fatty acids, and while these are essential for normal inflammatory immune responses, when we consume them in excess, we can put our body in an overly pro-inflammatory state,” said Herrington.
Additionally, excessive sugar is an ingredient in low-quality plant-based foods.
“It’s important to note for this specific study,” cautioned Wells, “we can’t conclude that the plant-based diet was more effective for weight loss than the omnivorous control group.”
“We can’t compare [the vegan and control] groups directly since the diets were not matched for calorie content but the study did suggest that plant-based diets of varying nutrient density can be effective for people looking to lose weight,” said Wells.
“One study rarely can confirm causation,” Zenker said while Gupta added that the study acknowledges limitations “such as self-reported diet records and the volunteer nature of participants.”
