, a meta-analysis of randomized controlled trials shows.
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“To the best of our knowledge, this meta-analysis is the first that generates evidence from randomized controlled trials to assess the association of vegetarian diets with outcomes in people affected by cardiovascular diseases,” report the authors. The study was published online in JAMA Network Open.
“The greatest improvements in hemoglobin A1c and low-density lipoprotein cholesterol (LDL-C) were observed in individuals with type 2 diabetes and people at high risk of cardiovascular disease, highlighting the potential protective and synergistic effects of vegetarian diets for the primary prevention of cardiovascular disease,” they say.
Poor diet is well-established as increasing the morbidity and mortality associated with cardiovascular disease; however, although data has linked vegetarian diets to cardiovascular disease prevention in the general population, research on the effectiveness of such diets in people at high risk of cardiovascular disease is lacking.
“To the best of our knowledge, no meta-analysis of randomized controlled trials has been conducted to investigate the association of vegetarian diets with outcomes among people with CVD – indeed, research here has primarily focused on observational studies,” writes Tian Wang, RD, and colleagues at the University of Sydney.
Greater decreases in LDL-C, A1c, and body weight with vegetarian diets
For the meta-analysis, researchers identified 20 randomized controlled trials involving vegetarian diets that included 1,878 adults with or at a high risk of cardiovascular disease and included measurements of LDL-C, A1c, or systolic blood pressure.
The studies were conducted in the United States, Asia, Europe, and New Zealand between 1990 and 2021. Sample sizes ranged from 12 to 291 participants.
The mean range age of participants was 28-64 years. Studies included patients with cardiovascular disease (four studies), diabetes (seven studies), and those with at least two cardiovascular risk factors (nine studies).
The mean duration of the dietary intervention was 25.4 weeks (range 2-24 months). The most commonly prescribed diets were vegan (plant-based foods only), lacto-ovo-vegetarian (excluded meat, poultry, seafood, and dairy products, but allowed eggs), and lacto-vegetarian (same as previous but allowed dairy products).
Overall, those who consumed a vegetarian diet for an average of 6 months, versus comparison diets, had significantly greater decreases in LDL-C (6.6 mg/dL beyond the reduction achieved with standard therapy); A1c (0.24%); and body weight (3.4 kg), but the reduction in systolic blood pressure (0.1 mmHg) was not significantly greater.
Assessment of the overall certainty of evidence evaluated using the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) tool showed a moderate level of evidence for reductions in LDL-C and A1c with the vegetarian diet.
Lacto-ovo vegetarian diets were associated with the greatest reduction in LDL-C (14.1 mg/dL); however, four out of the five trials restricted energy intake.
Of note, vegetarian diets were most effective for achieving glycemic control among people with type 2 diabetes and leading to improvements in weight among those at high risk of cardiovascular disease as well as those with type 2 diabetes.
The effects “suggest that vegetarian diets might have a synergistic [or at least nonantagonistic] use in potentiating the effects of optimal drug therapy in the prevention and treatment of a range of cardiometabolic diseases,” the authors write.
Although previous studies have shown similar improvements associated with a vegetarian diet, most studies did not stratify populations based on disease status, type of vegetarian diet, or comparison diet, the authors note.
The lack of improvement in systolic blood pressure is consistent with previous meta-analyses of vegetarian diets in general and suggests that salt intake may be the more important factor for those measures.
“[The meta-analysis] suggests that diet quality plays a major role in lowering blood pressure independent of animal food consumption, as the DASH [Dietary Approaches to Stop Hypertension] ... trial demonstrated,” the authors note.