Nutrition & Metabolism volume 22, Article number: 21 (2025) Cite this article
Low-fat plant-based diets cause weight loss in clinical trials. However, many foods are highly processed, raising the question as to their effect on body weight. This secondary analysis assessed the associations between changes in processed food intake and weight loss in 244 overweight adults randomly assigned to a vegan (n = 122) or control group (n = 122) for 16 weeks. Three-day dietary records were analyzed using the NOVA system, which categorizes foods from 1 to 4, based on degree of processing. A repeated measure ANOVA, Pearson correlations, and a multivariate regression model were used for statistical analysis. The consumption of animal foods in categories 1–4 decreased in the vegan group, compared with the control group. Body weight decreased in the vegan group (treatment effect − 5.9 kg [95% CI -6.7 to -5.0]; Gxt, p < 0.001). Changes in consumption of animal foods in categories 1–4 were positively associated with changes in body weight: r = + 0.34; p < 0.001 for category 1; r = + 0.18; p = 0.008 for category 2; r = + 0.17; p = 0.01 for category 3; and r = + 0.22; p = 0.001 for category 4. In no NOVA category was the consumption of plant-based processed foods positively and significantly associated with weight gain. The top three independent predictors of weight loss were reduced intakes of processed, unprocessed or minimally processed, and ultra-processed animal foods. These findings suggest that replacing animal products with plant-based foods may be an effective weight-loss strategy, even when processed plant-based foods are included.
In clinical trials, low-fat, plant-based diets have been shown to significantly reduce body weight [1, 2], among other benefits. However, among the wide variety of available plant-derived foods are some that are heavily processed, raising the question as to whether they would impair weight loss. In the one carefully controlled randomized clinical trial published on the effects of processed foods on body weight, the consumption of ultra-processed foods, in the context of an omnivorous diet, appeared to contribute to weight gain [3]. However, not all ultra-processed foods may have the same effects on body weight. Further studies are necessary to assess the association of changes in processed food intake with body weight. Also, given that animal-based and plant-based processed foods may have very different health effects [4], it is also useful to consider whether this association may be modified by food origin (animal vs. plant-based). Using data from a randomized trial of the effect of a low-fat plant-based diet on body weight [5], this secondary analysis assessed whether the degree of processing influenced the effects of a plant-based diet on body weight. Our hypothesis was that animal and plant-based foods within the NOVA categories would affect body weight differently.
The overall study methods have been described earlier [5]. Briefly, this randomized trial was conducted between January 2017 and February 2019 in Washington, DC (trial protocol in Supplement 1). This study follows the Consolidated Standards of Reporting Trials (CONSORT) reporting guideline. The protocol was approved by the Chesapeake Institutional Review Board. All participants provided written informed consent. Overweight adults, 25 to 75 years old, with a body-mass index between 28 and 40 kg/m2, were enrolled. Exclusion criteria were diabetes, smoking, alcohol or drug abuse, pregnancy or lactation, and current use of a vegan diet.
Participants were randomly assigned to a vegan or control group in a 1:1 ratio (Suppl. Figure 1). The vegan group was asked to avoid animal products and minimize oils and received weekly classes to support adherence. The control group was asked to make no diet changes. No instructions on energy intake or diet quality were given to either group and no meals were provided.
Body weight was measured using a calibrated digital scale at baseline and week 16. A 3-day dietary record (two weekdays and one weekend day) was completed by each participant at baseline and week 16, and analyzed by a registered dietitian certified in the Nutrition Data System for Research [6], version 2016, with all the detailed ingredients, amounts, and brands of all foods consumed. All foods were categorized for degree of processing using the NOVA system [7], and by their origin (animal, plant-based, or mixed if they contained a combination of plant-based and animal ingredients, which was the case only for a few foods in category 4). NOVA category 1 is defined as unprocessed or minimally processed foods; category 2 includes processed ingredients, such as salt, sugar, oil, and butter; category 3 includes processed foods made by adding salt, sugar, preservatives, etc.; and category 4 is composed of ultra-processed foods, greatly modified by industrial techniques and processes. TZ-M, AJ, and GM categorized the food groups into the corresponding NOVA categories. LC served as an independent reviewer, blinded to group assignment, to check the accuracy of food categorization. All authors approved the final categorization (see Suppl. Table 1). The mean daily intake from the 3-day diet record was calculated for each participant and was used for the analysis. A repeated measure ANOVA was used by a statistician blinded to dietary interventions, with factors group (vegan or control), subject and time. Dietary intake in each NOVA category (in g/day) was the dependent variable. All results are presented as means with 95% confidence intervals (CI). Pearson correlations were performed for both groups combined between changes in body weight and changes in the NOVA categories where the changes in intake were significantly different between the groups. After Bonferroni correction, p-values less than 0.006 (0.05/9) were considered significant. To assess factors predictive of change in body weight independently of other food categories, a multivariable linear regression model was fit including as predictors those food categories that were significantly predictive of weight change in univariable assessments after Bonferroni adjustment, first unadjusted, and then also adjusted for changes in energy intake, and lastly, controlling for changes in energy intake, age, and physical activity. In addition, a ratio of ultra-processed to unprocessed and minimally processed foods (category 4/category 1), of animal and plant-based origin separately, has been calculated, and its association with changes in body mass index has been assessed, using Pearson correlations.
Of 3,115 people screened by telephone, 244 overweight adults met participation criteria and were randomly assigned to the vegan (n = 122) or control (n = 122) groups. The analysis included 223 (91.0%) completers. The vegan group decreased consumption of animal products and increased plant-based foods in all four NOVA categories (Table 1.)
The consumption of animal foods in category 1 decreased in the vegan group, compared with the control group; effect size: -141 g/day (95% CI -173 to -109); p < 0.001. Concomitantly, the intake of plant-based foods in category 1 increased in the vegan group, compared with the control group; effect size: +236 g/day (95% CI + 63 to + 410); p = 0.008. In categories 2–4, animal foods decreased in the vegan group, compared with the control group. The consumption of plant-based foods in categories 2–4 decreased in the vegan group, with no significant between-group difference (Table 1). Body weight decreased in the vegan group, compared with the control group (effect size − 5.9 kg [95% CI -6.7 to -5.0]; between-group p < 0.001).
Changes in consumption of animal foods in categories 1–4 were positively associated with changes in body weight: r = + 0.34; p < 0.001 for category 1; r = + 0.18; p = 0.008 for category 2; r = + 0.17; p = 0.01 for category 3; and r = + 0.22; p = 0.001 for category 4 (Table 2). The changes in food intake and body weight are listed in detail in Suppl. Table 2. Suppl. Table 3 lists the food groups included in the NOVA categories. The multivariable regression model showed that the top 3 predictors of weight loss were: (1) Reduced consumption of processed animal foods (each reduction of 50 g/day was independently associated with a 1 kg weight loss over 16 weeks of the study; p = 0.01; when adjusted for changes in energy intake, each reduction of 63 g/day was associated with a 1 kg weight loss; p = 0.048; in the fully adjusted model, this association was no longer significant). (2) Lower consumption of unprocessed or minimally processed animal foods (each reduction of 93 g/day was independently associated with a 1 kg weight loss; p < 0.001; after adjusting for changes in energy intake, each reduction of 105 g/day was associated with a 1 kg weight loss; p < 0.001; in the fully, adjusted model, each reduction of 101 g/day was associated with a 1 kg weight loss; p < 0.001); and (3) Reducing ultra-processed animal foods (each reduction of 120 g/day was associated with a 1 kg weight loss in 16 weeks; p = 0.001; after adjusting for changes in energy intake, each reduction of 158 g/day was associated with a 1 kg weight loss; p = 0.02; in the fully adjusted model, each reduction of 156 g/day was independently associated with a 1 kg weight loss; p = 0.02. There was no association between the ratios of category 4 to category 1, of animal or plant-based origin, with changes in body mass index (r = 0.03; p = 0.73; and r = 0.10; p = 0.13, respectively).
Although processed foods have been proposed as a cause of weight gain, this current 16-week randomized trial demonstrated that minimizing the consumption of animal foods and substituting low-fat plant-derived foods in their place, regardless of their level of processing, was associated with clinically significant weight loss. The top three independent predictors of weight loss were reduced intakes of processed, unprocessed or minimally processed, and ultra-processed animal foods. This effect appears to result from the reduced fat and energy intake, and increased fiber intake, and to increased postprandial energy expenditure (the thermic effect of food.) A vegan diet has been previously shown to promote weight loss in a 2-year randomized clinical trial, compared with a conventional low-fat diet [1], suggesting a long-term effectiveness of this dietary approach.
The strengths of the current trial include a randomized, parallel design, which accounted for seasonal effects by providing a control group that experienced the same seasonal fluctuations as the vegan group, allowing researchers to isolate the true impact of the dietary intervention being studied from any natural seasonal variations in the data. The low-fat vegan diet was ad libitum, with no energy limits, maximizing the satiating effects of fiber and the concomitant weight loss. The study also has limitations. The food consumption was based on self-reported diet records. The participants were volunteers and may not represent the general population, and the study findings cannot be extrapolated on other populations with different characteristics. Finally, this was a secondary analysis, and the research questions should be addressed in future studies specifically designed to answer them.
In conclusion, these findings suggest that replacing animal products with low-fat plant-based foods may be an effective weight-loss strategy in overweight adults, even when significantly processed plant-based foods are included. This does not exclude the possibility of other health benefits of favoring minimally processed foods.
No datasets were generated or analysed during the current study.
Dr. Kahleova had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
This work was funded by the Physicians Committee for Responsible Medicine.
The study was conducted in accordance with the Declaration of Helsinki, and the study protocol was approved by the Advarra Institutional Review Board, located in Columbia, MD, USA. The study was registered on ClinicalTrials.gov (NCT02939638). All participants gave informed, written consent.
Dr. Kahleova, Ms. Znayenko-Miller, Ms. Jayaraman, and Ms. Motoa, and Dr. Holubkov received compensation from the Physicians Committee for Responsible Medicine for their work on this study. Dr. Chiavaroli received funding from a Toronto 3D New Investigator Award. Dr. Barnard is an Adjunct Professor of Medicine at the George Washington University School of Medicine. He serves without compensation as president of the Physicians Committee for Responsible Medicine and Barnard Medical Center in Washington, DC, nonprofit organizations providing educational, research, and medical services related to nutrition. He writes books and articles and gives lectures related to nutrition and health and has received royalties and honoraria from these sources.
ClinicalTrials.gov number, NCT02939638.
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Kahleova, H., Znayenko-Miller, T., Jayaraman, A. et al. Vegan diet, processed foods, and body weight: a secondary analysis of a randomized clinical trial. Nutr Metab (Lond) 22, 21 (2025). https://doi.org/10.1186/s12986-025-00912-5
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DOI: https://doi.org/10.1186/s12986-025-00912-5
