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Do Veggies Protect Your Heart? Recent Low Quality Evidence Questions Vegetables’ Value

Posted: 2022-03-10
Do Veggies Protect Your Heart? Recent Low Quality Evidence Questions Vegetables’ Value

What’s happening?

A recent study observed that vegetable consumption was only minimally protective for incident cardiovascular disease (CVD) (1). Media headlines subsequently reported that “eating veggies won't protect your heart” (2). The PEN Team thought an analysis of the study was needed to determine if any changes to current recommendations to “eat lots of vegetables” are required.
 

What the article found

The British cohort study, which had the strength of a very large sample size (n=399,586 adult participants) and 12 years of follow up, observed that vegetable consumption was only minimally predictive of CVD incidence (i.e. hospitalization or death from myocardial infarction or stroke) (1). The study observed that the highest consumers of total, cooked and raw vegetable intake had a 10% lower CVD incidence compared to the lowest vegetable intakes (HR 0.74, 95%CI 0.69 to 0.80; HR 0.77, 95%CI 0.71 to 0.84; HR 0.79, 95%CI 0.74 to 0.84, respectively). However, once the researchers adjusted for socioeconomic and lifestyle factors (including physical activity, BMI, hypertension, smoking and other nutrients), any protective effect of vegetable intake became even less important (HR 0.90, 95%CI 0.83 to 0.97 for total vegetables; HR 0.89, 95%CI 0.83 to 0.95 for cooked vegetables) and nonstatistically significant for raw vegetables (1.00, 0.91 to 1.09). These findings might leave us wondering whether this study will impact nutrition recommendations in the many countries that currently encourage high vegetable consumption? 
 

Now what?

There are a few reasons why this recent study should not be used to change nutrition recommendations that encourage higher vegetable consumption. These reasons include: 
  1. The study’s findings (1) are not supported by the body of existing evidence demonstrating the health-promoting and protective cardiovascular effects of higher vegetable consumption (3-6). Specifically, the study’s findings are in contrast to other studies' findings that support better cardiovascular as well as other health benefits from eating more vegetables: 
    • Randomized controlled trial evidence supports that to prevent or help to reduce hypertension, individuals are recommended to consume a healthy diet pattern with a limited sodium intake that emphasizes vegetables, fruit, whole grains, low fat dairy products, protein from plant sources, with moderate intakes of lean, unprocessed meat, poultry, fish and unsaturated fats (3). 
    • A systematic review of randomized trials addressed the question of whether diets with increased fruit and vegetable consumption (FVC) can decrease blood pressure in people with BMIs ≥25 (4). The estimated effect of FVC interventions on systolic blood pressure (SBP) was 2.16 mmHg (P< 0.001) and -0.55 mmHg (P=0.39) for diastolic blood pressure (DBP). Moderator analyses showed greater drops in SBP from DASH diet interventions and studies that measured intake with food diaries. In higher quality studies that used concealed allocation, the decreases in SBP were lower. Greater increased FVC led to greater decreases in blood pressure.
    • Evidence from an umbrella review of observational studies supports that higher vegetable consumption probably helps to protect individuals from CVD mortality as well as other disease outcomes including colorectal cancer, stroke, all-cause mortality and CVD mortality (5). 
    • Moderate quality evidence supports that high intakes of total dietary fibre (of at least 25-29 g/day) from whole grains, vegetables, fruit and legumes supports both primary and secondary prevention of CVD (6). This evidence is based on systematic reviews of observational cohort studies.
  2. The study is one observational study (1), which may not have measured vegetable intakes accurately, causing measurement errors (1). The study participants reported very low vegetable intakes compared to the recommended intakes as the average reported daily consumption was only 2.3 and 2.8 tablespoons/day for raw and cooked vegetables, respectively. It is possible that the study participants had difficulty  visualizing their vegetable intakes as their number of “heaping tablespoons”, which the questionnaire asked them to estimate for their vegetable intakes.
  3. The study adjusted for important variables that could influence the cardiovascular disease risk in their participants (1), which is an important methodology for observational studies. A problem when adjusting for lifestyle variables is that these variables are all interrelated (7), which when adjusting can affect the estimates of the association with CVD. For example, people who practice heart healthy lifestyles, including getting regular exercise and avoiding smoking, are likely to eat more vegetables. When highly related variables are adjusted for in the analysis of observational studies, it is not possible to accurately estimate the effect of the primary variable (i.e. vegetable consumption) and vegetable consumption may appear to be less important than it actually is.
 
So, don’t throw away the “eat your vegetables” refrain just yet! This is only one study and health advice should not be changed on the basis of one study. Dietitians can be confident that the recommendation to include plenty of vegetables (and fruit) for heart healthy eating is still valid.  
 
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References

  1. Feng Q, Kim JH, Omiyale,  Bešević j, Conroy M, May M, et al. Raw and cooked vegetable consumption and risk of cardiovascular disease: a study of 400,000 adults in UK biobank. Front Nutr. 2022 Feb; 9:831470. doi: 10.3389/fnut.2022.831470. Available from: https://www.frontiersin.org/articles/10.3389/fnut.2022.831470/full 
  2. LaMatte S. Eating veggies won't protect your heart, study says, but critics disagree. CTV News. 2022 Feb. Available from: https://www.ctvnews.ca/lifestyle/eating-veggies-won-t-protect-your-heart-study-says-but-critics-disagree-1.5789645 
  3. Dietitians of Canada. Does diet have a role in preventing hypertension? In Practice-based Evidence in Nutrition® [PEN]. 2020-09-01. Available from: https://www.pennutrition.com/KnowledgePathway.aspx?kpid=674&pqcatid=146&pqid=668. Access only by subscription. Click Sign Up on PEN login page.
  4. Arnotti K, Bamber MD, Brewer V. Dietary interventions and blood pressure in overweight or obese individuals: a systematic review and meta-analysis. Clin Nutr. 2021 Jun; S0261-5614(21)00290-9. doi: 10.1016/j.clnu.2021.06.003. Abstract available from: https://pubmed.ncbi.nlm.nih.gov/35031150
  5. Angelino D, Godos J, Ghelfi F, Tieri M, Titta L, Lafranconi A, et al. Fruit and vegetable consumption and health outcomes: an umbrella review of observational studies. Int J Food Sci Nutr. 2019 Sep;70(6):652-67. Abstract available from: https://pubmed.ncbi.nlm.nih.gov/30764679/ 
  6. Dietitians of Canada. Are diets higher in total dietary fibre (including whole grains, cereals, vegetables, fruit and legume/pulse fibre) recommended for primary or secondary cardiovascular disease (CVD) prevention? In Practice-based Evidence in Nutrition® [PEN]. 2019-12-10. Available from: https://www.pennutrition.com/KnowledgePathway.aspx?kpid=2671&pqcatid=146&pqid=28050. Access only by subscription. Click Sign Up on PEN login page.
  7. Patel CJ, Manrai AK. Development of exposome correlation globes to map out environment-wide associations. Pac Symp Biocomput. 2015;20:231-42. Abstract available from: https://pubmed.ncbi.nlm.nih.gov/25592584/ 
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