Is the therapeutic use of a vegetarian diet effective for bringing about improvements in weight status in adults?

Short-term (less than one year) Improvements Conclusion Statement

The therapeutic use of a vegetarian diet is an effective way to bring about short-term improvements in weight status in adults, as part of a multi-component weight management program.

Longer-term (more than one year) Improvements Conclusion Statement

The therapeutic use of a vegetarian diet as part of a multi-component weight-management program is an effective way to bring about longer-term (more than one year) improvements in weight status among adults who are compliant.

Vegetarian Diets Compared to Other Therapeutic Diets for Treating Overweight or Obesity Conclusion Statement

Compared to omnivorous diets that could be used to treat obesity, findings are mixed as to whether therapeutic vegetarian diets are more or less effective for improving weight status in adults.

Evidence Summary

Therapeutic Use of Vegetarian Diets

Observational research has often shown an association between vegetarian dietary patterns and various health benefits (Sabate, 2003). However, in part because of variations within vegetarian diets, the findings are sometimes blurred or inconsistent (see Types of Vegetarian Diets).

In order to control for the wide variation in types of vegetarian diets and practices, we limited our evidence analysis to clinical trials in which vegetarian dietary patterns were controlled and in which vegetarian diets were used as a therapeutic intervention.

Using Managed Vegetarian Diets to Treat Overweight and Obesity

We identified seventeen intervention studies in which a vegetarian diet was used therapeutically.

• In eight of these studies overweight or obesity was the target disease
• In nine studies the target disease was something other than obesity (e.g., cholesterol control, diabetes management, etc,). However, because these studies involved the therapeutic use of a vegetarian diet and because they also reported weight outcomes, they were included in the analysis.

We examined this research in order to answer three questions:

1. Is the therapeutic use of a vegetarian diet effective for bringing about short-term (less than one year) improvements in weight status?
2. Is the therapeutic use of a vegetarian dietary pattern effective for bringing about longer-term (more than one year) improvements in weight status?
3. How does the therapeutic use of a vegetarian diet compare to omnivorous therapeutic diets for treating overweight or obesity?

Short-Term Improvements in Weight Status (Less than One Year)

The following studies reported statistically significant weight loss or changes in body composition at the end of treatment using a vegetarian diet:

• Obesity Focused Interventions
  • In all eight studies where obesity or overweight was the target disease, using a vegetarian diet resulted in statistically significant short term (less than one year) improvement in weight status. Seven of the studies were of positive quality () and six were calorie deficit.
  • Burke LE, Hudson AG et al, 2007 (calorie restricted)
  • Burke LE, Styn MA et al, 2006 (calorie restricted)
  • Dansinger ML, Gleason JA et al, 2005
  • Hakala P and Karvetti RL, 1989 (calorie restricted)
  • Karlsson J, Hallgren P et al, 1994 (compliers only) (calorie restricted)
  • Mahon AK, Flynn MG et al, 2007 (change relative to controls) (calorie restricted)
  • Marniemi J, Seppänen A et al, 1990 (neutral quality, calorie restricted)
  • Turner-McGrievy GM, Barnard ND et al, 2007 .

All of the above interventions for weight management included the vegetarian diet as a part of a multi-component program which included such things as group support classes, individual counseling and exercise recommendations.

Studies Focused on Other Diseases

We included research on the therapeutic use of vegetarian diets for other diseases, such as diabetes or cholesterol management, when the study reported weight outcomes. We were interested in whether a medically managed vegetarian diet could bring about improvements in weight status even when they were not designed specifically to bring about weight loss.

All but one of the nine studies reported a statistically significant improvement in subjects' weight status, even when obesity was not the disease being treated (none of the diets were calorie restricted). The one study that did not show an improvement in weight status was a very short, negative-quality () clinical trial (four weeks) that compared the effects of two types of vegetarian diets on cholesterol control (Kaartinen K, Lammi K et al, 2000).

• Barnard, Cohen et al, 2006
• Barnard ND, Scialli AR et al, 2000
• Bhumisawasdi J, Vanna O et al, 2006
• Daubenmier JJ and Weidner G, 2007
• de Mello VD and Zelmanovitz T, 2006
• Jenkins DJ, Kendall CW et al, 2003 (Metabolism)
• Kaartinen K, Lammi K et al, 2000
• Kestin M, Rouse IL et al, 1989 .

Conclusion

All but one study that met the inclusion criteria showed statistically significant improvement in weight status at post-treatment. The one study that did not find a statistically significant difference was not focused on treating overweight or obesity, was very short (four week intervention) and compared two vegetarian diets.

The therapeutic use of a vegetarian diet is an effective way to bring about short-term improvements in weight status as part of a multi-component weight-management program.

Longer-Term Improvements in Weight Status (More than One Year)

A major concern in the treatment of overweight and obesity is the maintenance of improvements in weight status. Is a medically-managed vegetarian dietary pattern effective for bringing about longer-term (more than one year) improvements in weight status?

All five studies that reported on weight status at one year or more from baseline show that improvements in weight status were maintained. Four of the studies reported follow-up measurements (that is, measurements some time after the treatment phase had stopped). All of these studies reported a maintenance of weight status.

• Hakala P and Karvetti RL, 1989 (treatment, 10 weeks; follow-up measure, one year)
• Turner-McGrievy GM, Barnard ND et al, 2007 (treatment, 14 weeks; follow-up measure, one year)
• Burke LE, Hudson AG et al, 2007 (treatment, one year; follow-up measure, 18 months)
• Marniemi J, Seppänen A et al, 1990 (treatment, one year; last measure, one year)
• Dansinger ML, Gleason JA et al, 2005 (treatment, one year; last measure, one year).

All of the studies included the vegetarian diet as part of a multi-component support program which included such things as educational support, group meetings and exercise recommendations. The ADA Adult Weight Management Guideline recommends that weight management programs include diet, exercise and behavioral therapy components since combination therapy is more successful than using any one intervention alone. See Adult Weight Management (AWM) Comprehensive Weight Management Program.

Conclusion

The therapeutic use of a vegetarian diet as part of a multi-component program is an effective way to bring about longer-term (more than one year) improvements in weight status among those who are compliant.

Comparison of Vegetarian Diets and Other Diets

How do medically managed vegetarian diets compare to omnivorous therapeutic diets for treating overweight or obesity?

In order to answer this question we focused on research that compared two types of diet interventions: A therapeutic vegetarian diet and a therapeutic omnivorous diet. We did not compare therapeutic vegetarian diets to control or “usual” diets (that is, diets where there was no therapeutic intervention). Our question was concerned with how vegetarian diets used to treat obesity compared to other diets used to treat obesity.

• Two studies reported that vegetarian diets performed better than comparison diets: Turner-McGrievy GM, Barnard ND et al, 2007 (at years one and two) de Mello VD and Zelmanovitz T, 2006 (BMI)
• Eight studies reported no significant difference between interventions using a vegetarian dietary pattern and those using an omnivorous dietary pattern:
  • Barnard, Cohen et al, 2006
  • Burke LE, Hudson AG et al, 2007
  • Burke LE, Styn MA et al, 2006
  • Dansinger ML, Gleason JA et al, 2005
  • Hakala P and Karvetti RL, 1989
  • Karlsson J, Hallgren P et al, 1994
  • Kestin M, Rouse IL et al, 1989
  • Mahon AK, Flynn MG et al, 2007 (vegetable vs. beef).
• Two studies reported that a vegetarian diet performed worse than one of the study comparison diets. Mahon AK, Flynn MG et al, 2007 (vegetarian diet vs. a therapeutic diet that included chicken) and Marniemi J, Seppänen A et al, 1990 (the authors state that mixed calorie deficit diet performed better (-14kg) than vegetarian calorie deficit diet (-9.2kg) at 12 months (P<0.001).

Conclusion

Compared to omnivorous diets that could be used to treat obesity, findings are mixed as to whether therapeutic vegetarian diets are more or less effective for improving weight status.

Overview Table Key

Indicates a statistically significant decrease from baseline or lower measure in a comparison
Indicates that there is no statistically significant change from baseline or difference in a comparison
Indicates a statistically significant increase from baseline or higher measure in a comparison.