Graphical Abstract: Simplified schematic representation of the effects of the UK Biobank diets on cardiovascular disease mediated by the main nutrients of each diet (yellow boxes) influencing metabolic and vascular physiology pathways (orange boxes). Red wine can be an ingredient of all four diets, and is included because of its known salutary effects on cardiovascular health. Arrow colours for the different effects ascertained in the study: green, beneficial; orange, neutral; red, harmful. For simplicity, red meat and poultry share nutrients and a harmful effect on cardiovascular disease. The health effects of a vegetarian diet may vary depending on the predominant type of carbohydrate, beneficial if complex or harmful if simple (sugars), when all positive metabolic effects may be reversed. COH, carbohydrate; HDL-C, high-density lipoprotein cholesterol.
Humans have been evolutionarily adapted to be omnivores, i.e. to obtain calories from both plant and animal food sources. However, presently, many people overconsume animal products, among which processed meats, rich in saturated fats, salt, and chemical additives, are particularly harmful. Animals as sources of meat for human consumption are usually raised in poor conditions, as many are stuck in crammed quarters, fed unnatural foods, and often treated with antibiotics and/or hormones.[1] This, together with the high content of saturated fatty acids and cholesterol in meat, predisposes meat eaters to a wide range of chronic diseases such as cardiovascular disease (CVD), diabetes, and cancer.[2] Last, but not least, it should be underlined that raising lamb, beef, and pork for the production of red meat has the greatest environmental impact, from greenhouse gas emissions to water usage, compared with the production of most other foods. Thus, focusing our dietary pattern on a greater consumption of plant-based than animal-based foods will help reduce or prevent chronic non-communicable diseases and lead to longer, healthier lives, but will also have a positive impact on planetary health.[3]
In this issue of the European Heart Journal, Petermann-Rocha et al.[4] analysed the diet of 422 791 participants (55.4% women) from the UK Biobank and divided them into four groups: vegetarians, fish eaters, fish and poultry eaters, and meat eaters to ascertain associations with cardiovascular outcomes after a mean follow-up of 8.5 years. All groups were also consumers of milk and cheese. The main findings were that fish eaters had a reduced risk of CVD, ischaemic heart disease, myocardial infarction, stroke, and heart failure by 7, 21, 30, 21, and 22%, respectively, compared with meat eaters. Vegetarians only reduced the risk of CVD incidence by 9%, also compared with meat eaters, while the risk of adverse cardiovascular outcomes of fish and poultry eaters was not different from that of meat eaters. No associations were identified between diets and CVD mortality. The beneficial associations of diet types with cardiovascular outcomes were strongest in men, as already described for vegetarian diets.[5] Surprisingly, the number of vegan participants was extraordinarily low (0.01%), and they were excluded from the analysis. Another limitation of this study is that, reflecting the dietary habits of contemporary Western societies, nearly 95% of the study subjects were meat eaters, the other three groups comprising together only 5.3% of the cohort. As acknowledged by the authors, a further limitation is that a single dietary record was obtained at baseline, hence changes in diet during follow-up could be a source of bias. Also, food records were incomplete for a significant proportion of participants. Still, the large size of the study allows a fair level of evidence to be obtained.
In recent years, restricted dietary patterns with abstinence from meat and animal products, mainly vegetarian and vegan diets, have become very popular. Since almost all epidemiologists and nutrition researchers agree that unhealthy diets are one of the main risk factors for CVD and other non-communicable diseases, a debate is ongoing as to whether we should exclude all animal products from our diet, not only meat and meat products, but also fish, cheese, and other dairy products, and move to a straight vegan diet.
Compared with meat eaters, vegans and vegetarians have more favourable CVD risk factors, including lower body mass index (BMI), blood pressure, and plasma LDL-cholesterol.[6] In contrast, since animal products are almost the sole dietary sources of vitamins B12 and D, deficiencies of these vitamins may cause neurocognitive disorders, anaemia, and immunodeficiency, in addition to increased risk of bone fractures, sarcopenia, and depressive symptoms in vegetarians and vegans. In fact, veganism (abstention from all animal-based foods) may result in deficiencies of several nutrients such as vitamin B12, iron, zinc, high-quality protein, omega-3 fatty acids, calcium, and vitamin D.[7] Thus, veganism, although it is a highly sustainable dietary pattern, does not seem the best option for overall health. Why little cardiovascular benefit was associated with the vegetarian diet in the UK Biobank study[4] might be due to the small numbers of vegetarians and ensuing low power to detect associations. The quality of the plant foods in the diet might also be relevant, as there are diets abundant in healthful (whole grains, fruits/vegetables, nuts/legumes, oils, tea/coffee) and unhealthful (juices/sweetened beverages, refined grains, potatoes/fries, sweets) plant-based foods with divergent effects on CAD risk.[8] Indeed the vegetarians in the UK Biobank consumed a fair amount of crisps, pizza, and sugary drinks, which could help explain in part the observed weak associations with cardiovascular outcomes.
A meta-analysis of five prospective studies evaluated long-term coronary artery disease (CAD) mortality rates among vegetarian and non-vegetarians cohorts from Western countries. Compared with meat eaters, CAD mortality was reduced by 34% in pesco-vegetarians and lacto-ovo-vegetarians, 26% in vegans, and 20% in occasional meat eaters.[9] In the Adventist Health Study 2, a higher reduction in all-cause mortality, CVD mortality, and mortality from other causes was observed in pesco-vegetarians compared with vegans, lacto-ovo-vegetarians, and semi-vegetarians.[10] In contrast, in the EPIC-Oxford study, which included 48 188 participants followed for a mean of 18 years, CAD incidence was also significantly lower in vegetarians and pesco-vegetarians compared with meat eaters, but unexpectedly vegetarians had significantly higher rates of total stroke and haemorrhagic stroke, which was not observed in pesco-vegetarians.[2]
Graphical abstract shows the possible effects and mechanisms of the four dietary patterns analysed by Petermann-Rocha et al.[4] These findings concur with abundant data in the literature showing that fish and other seafood are an essential component of a cardioprotective diet. The salutary effects of plant-based diets are firmly established, since many cohort and intervention studies have demonstrated the association of consumption of vegetables, fruits, nuts, seeds, legumes, whole-grain, extra-virgin olive oil, and fermented dairy products with beneficial health outcomes. While being animal products, fish and other seafood are rich sources of omega-3 fatty acids, B vitamins, zinc, iodine, selenium, calcium, and magnesium.[2] Furthermore, seafood provides high-quality protein, which is both satiating and helpful for the building and maintenance of bone and muscle mass. As recently reviewed,[2] results from epidemiological studies support the cardioprotective effects of fish (and omega-3 fatty acids) consumed with the usual diet. Also, two meta-analyses comprising 106 237 mother–offspring pairs and 25 960 children reported that fish/seafood consumption (>100 oz/week) was associated with dose-dependent benefits in neurocognitive development, despite possible associated increases in mercury exposure. Despite these results, the recommendation should be to choose low-mercury fish such as sardines, herring, salmon, trout, and anchovies, as well as scallops, shrimp, lobster, oyster, and clams, that all are rich in omega-3 fatty acids and low in mercury.[2]
Dairy products, which were an integral part of all types of diets examined in the UK Biobank report,[4] are also important dietary sources of proteins, in addition to non-sodium minerals (calcium), probiotics, and vitamin D. Fermented low-fat dairies, such as yogurt, kefir, and soft cheeses, are preferable, whereas butter and hard cheese are discouraged due to their high content in saturated fats and salt. Eggs (not mentioned in the UK Biobank report) are also beneficial as sources of essential amino acids, minerals (selenium, phosphorus, iodine, and zinc), vitamins (A, B2, B3, B12, and D), and carotenoids (lutein and zeaxanthin). The recommendation is to consume no more than five egg yolks/week (egg whites have no limits).[6]
Wine is another important component of a healthy diet and, consumed in moderation with meals, has been associated with lower rates of all-cause mortality and CVD mortality, as well as incidence of several chronic diseases (CVD, myocardial infarction, heart failure, diabetes, and cognitive decline).[11] However, due to its ethanol content, it is not usually included in vegetarian diets. Beverages of choice are water (still, carbonated, or flavoured, but not sweetened), coffee, and tea. Of note, alcoholic drinks were consumed to a similar extent in the four diet groups of the UK Biobank participants (including vegetarians).[4]
Finally, we should beware of the environment. The best diets to preserve the future of the planet, i.e. those with the highest sustainability, appear to be the Mediterranean, flexitarian (occasional red meat eaters), vegetarian, and vegan diets, since all are plant-forward or plant-based diets, and hence have much lower environmental impact than animal-based diets. However, this does not mean that animal protein should disappear from our menus, rather that it is important to focus on high-quality protein from animals raised ethically and reduce the high meat consumption in our usual diet. We should remember that modest amounts of wholesome animal-based foods such as fish and fermented dairy products continue to play an essential role in an ideal dietary pattern, even better if they are consumed within the context of the Mediterranean diet.[12]
In summary, diets should be chosen according to their effects on human health and also on the environment (planetary health). Mediterranean, flexitarian, and pesco-vegetarian diets, all rich in seafood, seem to accomplish these two premises and are all beneficial for health. While more high-quality nutritional studies evaluating both aspects, health and environmental effects, are clearly warranted; in the meantime, we can choose any of these three dietary patterns, with seafood no less often than three times per week, to further our cardiovascular and general health.
Eur Heart J. 2021;42(12):1144-1146. © 2021 Oxford University Press
Copyright 2007 European Society of Cardiology. Published by Oxford University Press. All rights reserved.
