Vegetables: some love 'em, some hate 'em. But there's no denying the important role fresh produce plays in a healthy diet. And no, consuming endless potato chips apparently doesn't count.
We were all told as children to finish our veggies - well, told and then bribed with ice cream - to make sure we grew up to be healthy humans. The Ministry of Health recommends consuming at least five servings of vegetables and two servings of fruit each day to ensure we're getting enough dietary fibre, vitamins, minerals and antioxidants - vegetables are an important source of many nutrients, including potassium, folate, vitamin A and vitamin C. The ministry also stresses the importance of variety as well as quantity.
While a diet rich in both vegetables and fruits can help to reduce blood pressure and lower the risk of stroke, eye conditions and digestive problems, the humble food group has also frequently been touted as reducing the risk of heart disease - which according to new research, is actually not the case.
A team of UK scientists have found no evidence that vegetables provide protection against heart disease, according to the new findings of a large-scale study, published in the journal Frontiers in Nutrition.
Researchers analysed long-term data from almost 400,000 people regarding the amount of cooked and uncooked vegetables they were consuming. While the original results indicated that veggies were beneficial for the heart, the team found there was near to no evidence of a protective effect after adjusting the data for confounding factors.
The results of the large-scale 'UK Biobank' study - conducted by researchers from the Nuffield Department of Population Health at the University of Oxford, the Chinese University of Hong Kong, and the University of Bristol - concluded there is little or no evidence that consuming a higher amount of cooked or uncooked vegetables will affect the risk of developing cardiovascular disease (CVD).
"That the consumption of vegetables might lower the risk of CVD might at first sight seem plausible, as their ingredients such as carotenoids and alpha-tocopherol have properties that could protect against CVD. But so far, the evidence from previous studies for an overall effect of vegetable consumption on CVD has been inconsistent," the researchers said.
Previous studies that indicated vegetables do offer protection against the potentially fatal disease may not have been sufficiently adjusted to account for confounding socio-economic and lifestyle factors, the new analysis suggests.
"When known socio-economic and lifestyle confounding factors are corrected for, the small apparent positive effect that remains could likely also be explained away by further confounders."
Professor Naomi Allen, UK Biobank's chief scientist and co-author on the study, said the BioBank has allowed researchers to reliably assess the association of vegetable intake with the risk of subsequent CVD.
"The UK Biobank is a large-scale prospective study on how genetics and environment contribute to the development of the most common and life-threatening diseases. Here we make use of the UK Biobank's large sample size, long-term follow-up, and detailed information on social and lifestyle factors," Prof Allen said.
The UK Biobank follows the health of half-a-million adults in the UK by linking to their healthcare records. Upon their enrollment in 2006-2010, the volunteers were interviewed about their diet, lifestyle, medical and reproductive history, and other factors.
The researchers used the enrollment responses of 399,586 participants (of whom 4.5 percent went on to develop CVD) as well as questions about their daily average consumption of uncooked versus cooked vegetables. They analysed the association with the risk of hospitalisation or death from myocardial infarction, stroke, or major CVD, controlling for a wide range of possible confounding factors - including socio-economic status, physical activity, and other dietary considerations.
The average daily intake of total vegetables, raw vegetables and cooked vegetables was 5.0, 2.3 and 2.8 heaped tablespoons per person. The risk of dying from CVD was about 15 percent lower for those with the highest vegetable intake compared to the lowest intake. However, this apparent effect was substantially weakened when possible socio-economic, nutritional, and health-and-medicine-related factors were taken into account.
Controlling for these factors reduced the predicted power of vegetable intake on CVD by over 80 percent.
"Our large study did not find evidence for a protective effect of vegetable intake on the occurrence of CVD. Instead, our analyses show that the seemingly protective effect of vegetable intake against CVD risk is very likely to be accounted for by bias from residual confounding factors, related to differences in socioeconomic situation and lifestyle," said Dr Qi Feng, a researcher at the Nuffield Department of Population Health at the University of Oxford and the study’s lead author.
The researchers suggest that future studies should further assess whether particular types of vegetables or their method of preparation might affect the risk of CVD.
With all that said, the new findings don't let you off the hook - not by any means. A sufficient intake of vegetables is still important for maintaining a balanced diet and avoiding a wide range of diseases.
"However, eating a balanced diet and maintaining a healthy weight remains an important part of maintaining good health and reducing risk of major diseases, including some cancers. It is widely recommended that at least five portions of a variety of fruits and vegetables should be eaten every day," author Dr Ben Lacey, an Associate Professor in the department at the University of Oxford, concluded.