A sufficient intake of vegetables is important for maintaining a balanced diet and avoiding a wide range of diseases. But might a diet rich in vegetables also lower the risk of cardiovascular disease (CVD)?
Unfortunately, researchers from the Nuffield Department of Population Health at the University of Oxford, the Chinese University of Hong Kong, and the University of Bristol found no evidence for this.
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.
Now, new results from a powerful, large-scale new study in Frontiers in Nutrition shows that a higher consumption of cooked or uncooked vegetables is unlikely to affect the risk of CVD. They also explain how confounding factors might have explained previous spurious, positive findings.
“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, to assess reliably the association of vegetable intake with the risk of subsequent CVD,” said Prof Naomi Allen, UK Biobank’s chief scientist and co-author on the study.
The UK Biobank, follows the health half a million adults in the UK by linking to their healthcare records. Upon their enrollment in 2006-2010, these volunteers were interviewed about their diet, lifestyle, medical and reproductive history, and other factors.
The researchers used the responses at enrollment of 399,586 participants (of whom 4.5% went on to develop CVD) to questions about their daily average consumption of uncooked versus cooked vegetables.
They analyzed the association with the risk of hospitalization or death from myocardial infarction, stroke, or major CVD. They controlled for a wide range of possible confounding factors, including socio-economic status, physical activity, and other dietary factors.
Crucially, the researchers also assessed the potential role of ‘residual confounding’, that is, whether unknown additional factors or inaccurate measurement of known factors might lead to a spurious statistical association between CVD risk and vegetable consumption.
The mean 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% lower for those with the highest intake compared to the lowest vegetable intake. However, this apparent effect was substantially weakened when possible socio-economic, nutritional, and health- and medicine-related confounding factors were taken into account.
Controlling for these factors reduced the predictive statistical power of vegetable intake on CVD by over 80%, suggesting that more precise measures of these confounders would have completed explained any residual effect of vegetable intake.
Dr. Qi Feng, a researcher at the Nuffield Department of Population Health at the University of Oxford, and the study’s lead author, said: “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.”
Feng et al. suggest that future studies should further assess whether particular types of vegetables or their method of preparation might affect the risk of CVD.
Last author Dr. Ben Lacey, Associate Professor in the department at the University of Oxford, concluded: “This is an important study with implications for understanding the dietary causes of CVD and the burden of CVD normally attributed to low vegetable intake. 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.”
educing the burden of cardiovascular disease (CVD) is a top public health priority in the UK and worldwide . A poor diet is a major contributor to morbidity and premature mortality, especially CVD [2, 3], in part by promoting excess weight, but also by raising total cholesterol and low-density lipoproteins concentrations (LDL) and increasing the risk of diabetes and hypertension . Traditionally, the vast majority of epidemiological studies investigating diet and health associations have usually focused on single nutrients and this evidence is reflected in current dietary recommendations [4,5,6].
These emphasize the importance of achieving and maintaining a healthy weight, reductions in saturated fatty acids (SFAs) and free sugars [7, 8], and increases in dietary fiber . High dietary energy density and free sugars are associated with increased risk of weight gain  which can further increase CVD and mortality risk [8, 9], while SFAs increase total blood cholesterol and LDL [10, 11]. However, other recent meta-analyses and observational studies have not found evidence for a beneficial effect of reducing SFA intake on CVD and total mortality [12, 13], or found protective effects against stroke . Dietary fiber may lower the risk of CVD, through improved glucose control and lower serum cholesterol concentration .
However, despite years of public health efforts, population dietary change has been slow [1, 16]. This may reflect in part the difficulties of translating present dietary recommendations into food-based public health advice , and some existing recommendations are not universally echoed across countries .
The public have frequently been confused by apparently conflicting messages, for example about the importance of reducing saturated fat or free sugars , without recognizing that these nutrients frequently co-exist in foods and that the consequence may be a diet that is high in both saturated fats and free sugars and they may have synergistic effects on health. Dietary guidelines which focus on foods rather than individual nutrient recommendations could help avoid confusion and avoid inadvertent increases in one nutrient of concern at the expense of another. Despite the inclusion of some food-based recommendations in recent dietary guidelines (especially regarding fruits, vegetables, dairy), nutrient-based advice still remains the most common, often co-existing with food-based guidance, as seen in the latest release of the Dietary Guidelines for Americans 2020–2025 .
Increasingly, researchers have sought to characterize complex dietary patterns using either a priori (based on adherence to a specific patterns, e.g., Mediterranean diet, or a score which reflects overall dietary quality)  or a posteriori (based on the observed dietary intake using empirical methods such as factor analysis or principal component analysis (PCA)) [20, 21].
Reduced rank regression (RRR) is a data-dimension reduction technique that aims to identify the combination of food groups that explain the maximum amount of variation in a set of response variables (e.g., nutrients) hypothesized to be on the causal pathway between diet and health outcomes. This approach can test a priori hypotheses of the pathophysiology of disease . To our knowledge, only six longitudinal cohort studies have examined overall CVD risk and/or all-cause mortality using RRR, but all included smaller populations and none was focused in the UK (Additional file 1: Table S1). This population-specificity is important given that dietary patterns can vary substantially even when nutrient intakes are broadly similar, owing to cultural differences in food preference.
Using data from the UK Biobank study, we aimed to identify food-based dietary patterns explaining the variability in known dietary risk factors which operate through excess energy intake, such as energy density, free sugars, saturated fat, and low fiber intakes, and to investigate their association with total and fatal cardiovascular disease (CVD) and all-cause mortality.
In this sample of middle-aged British adults, two principal dietary patterns explained 43% and 20% of the variance in specific nutrients, namely energy density, saturated fat, free sugars, and fiber, which are hypothesized to be on the pathway between the associations of food groups and CVD and all-cause mortality through their contribution to excess energy intake. In the primary pattern, greater consumption of chocolate and confectionery, butter, refined bread, and table sugar and preserves together with low intakes of fresh fruit, vegetables, and wholegrain foods was significantly associated with increased CVD and all-cause mortality.
A second pattern was related to higher intakes of free sugars, predominately from sugar sweetened beverages, fruit juice, chocolate and confectionary, and table sugar and preserves but low in butter and higher fat cheese. The association of this dietary pattern with incident CVD and all-cause mortality was non-linear, with only evidence of increased risk for those with the highest dietary pattern z-scores. Exploratory analyses suggested the association observed with dietary pattern 1 was potentially mediated by excess weight.
RRR has not been widely used to identify dietary patterns and their associations with CVD risks. The first dietary pattern largely confirms previous studies reporting associations with a priori “Western” dietary patterns and the benefits of “Mediterranean” diets, and with a large body of data reporting the associations between individual food groups or nutrients and disease outcomes from prospective cohort studies in the USA and Europe [9, 11, 33, 34].
It is notable that people in the dietary pattern quintile with the lowest risk had mean intakes of energy from SFA of 9.7%, very close to the national and international recommendations, and free sugars accounted for 8.8% of total energy, below the World Health Organization (WHO) guidelines , though this level still exceeded the more stringent UK recommendations .
The second dietary pattern is more unusual and is characterized by higher intakes of sugar-sweetened beverages, fruit juice, and table sugar and preserves, together with lower intakes of high fat cheese and butter. This dietary pattern is striking because people in the highest quintile, with very high free sugars intake, otherwise followed other healthy behaviors, with higher physical activity, lower alcohol intake, and were less likely to smoke, and their intake of SFA met the recommended levels.
People in the highest quintile for this dietary pattern had increased risks for CVD and all-cause mortality and consumed on average, 17.3% of dietary energy from free sugars, more than three times the UK dietary guideline, but only 10% SFA, which is the recommended level. While some previous research has shown that higher consumption of SSBs and other added sugars are associated with a higher risk of CVD [9, 37,38,39,40] and all-cause mortality , recent reviews of the evidence by the WHO , and by the UK Scientific Advisory Committee on Nutrition  did not identify a specific link between sugar intakes and total mortality.
reference link :https://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-021-01958-x
More information: Qi Feng et al, Raw and Cooked Vegetable Consumption and Risk of Cardiovascular Disease: A Study of 400,000 Adults in UK Biobank, Frontiers in Nutrition (2022). DOI: 10.3389/fnut.2022.831470