Can vegetarian and vegan diets save your health and the planet?

Increasing numbers of us are opting to eat less or no meat, for reasons ranging from health to concerns about animal welfare and climate change, but evidence on the long-term health and environmental impacts of meat-free diets can be confusing. The internet is awash with claims that plant-based diets bring manifold benefits, whilst others suggest that they increase the risk of certain conditions. It can be difficult to know what information to trust.

Here at Oxford Population Health, our researchers have carried out studies involving many thousands of people to understand how different diets may affect the risk of developing specific health conditions. They have also looked at how various diets impact the planet.

Does a vegetarian diet lower the risk of heart attack and stroke?

Cardiovascular diseases remain the world’s biggest killer, causing approximately 20 million deaths globally each year. Oxford Population Health research has found strong evidence that vegetarian and vegan diets have a protective effect against coronary heart disease (CHD). Data from 2,820 cases of CHD in the EPIC-Oxford cohort, for instance, indicated that fish eaters and vegetarians had 13% and 22% lower rates of CHD than meat eaters, respectively. This difference was equivalent to 10 fewer cases of CHD in vegetarians than in meat eaters per 1,000 people over 10 years. Similarly, a study of nearly 410,000 people from nine European countries concluded that CHD risk is associated with an individual’s consumption of red and processed meat. Each 100g per day increase in meat consumption raised an individual’s risk by 19%.

This may be caused by vegetarians and vegans having lower levels of low-density lipoprotein (LDL) cholesterol due to reduced consumption of saturated fatty acids, and lower prevalence of hypertension (high blood pressure) – both of which are risk factors for CHD.

The evidence on stroke risk remains inconclusive. Analyses in the Oxford component of the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford) showed that vegetarians had a 20% higher risk of stroke than meat eaters, mainly due to an increase in risk of haemorrhagic stroke (bleeding into the brain), and equivalent to three more cases of stroke per 1,000 people over 10 years. It is not known why this is the case, but one factor may be that low LDL cholesterol increases the risk for haemorrhagic stroke.

It is worth bearing in mind that vegan and vegetarian diets are not necessarily lower in two of the greatest dietary determinants of CHD: alcohol and salt. Some studies have found that processed meat substitutes (such as plant-based burgers and sausages) can be higher in salt and sugar levels than their meat-based counterparts. To minimise salt intake, vegetarians and vegans should rely on wholefoods such as lentils, beans and nuts for protein, and avoid foods high in added salt.

How does diet affect cancer risk?

The largest ever study of vegetarian diets and cancer risk involving data from 1.8 million people across three continents, led by researchers at Oxford Population Health, found that meat-free diets were associated with lower risks of breast, prostate, kidney and pancreatic cancers, and multiple myeloma, but a higher risk of squamous cell carcinoma of the oesophagus (the most common form of oesophageal cancer). Vegans were found to have a higher risk of colorectal (bowel) cancer, but the numbers of vegans in the study was small, limiting the certainty of some findings.

The researchers found no statistical difference in the risk for 11 other types of cancer when comparing meat eaters, poultry eaters (who do not eat red or processed meat), pescatarians, vegetarians, and vegans.

In an earlier study, using data from the UK Biobank, our researchers found that low meat-eaters were at a 2% lower risk of developing any type of cancer compared to regular meat-eaters. For pescatarians, the risk was 10% less and for vegetarians it was 14% less. The risk of post-menopausal breast cancer was 18% less for vegetarians than regular meat-eaters.

A study drawing on data from more than 542,000 UK women in the Million Women Study found that foods rich in calcium, such as milk and yoghurt, were associated with a lower risk of bowel cancer. Alcohol and red and processed meat were associated with an increased risk of bowel cancer.

What about other diseases?

Various studies have confirmed that vegetarians and vegans generally have a lower body mass index (BMI) than meat-eaters, which may translate into lower levels of obesity and diabetes. In the EPIC-Oxford cohort, for instance, low meat-eaters and vegetarians were 37% less likely to develop diabetes compared with regular meat eaters, while pescatarians were 53% less likely.

In high-income countries, there is also some evidence suggesting a link between regular red meat consumption and diabetes. Along with colleagues from the China Kadoorie Biobank collaborative group, Oxford Population Health researchers investigated whether this is also the case in China where red meat consumption has traditionally been low but is rising rapidly, along with the rate of diabetes. This study found that each 50g/day increase in red meat intake was associated with a 11% higher risk of diabetes.

Based on the associations between red/processed meat and bowel cancer, CHD and diabetes, Oxford Population Health researchers made estimates that quantified the cost that excess meat consumption places on healthcare systems. They estimated that, in 2020, 2.4 million deaths worldwide and US$285 billion in healthcare costs could have been caused by excess red and processed meat consumption. The researchers then quantified the extent to which red/processed meat would need to be taxed in 149 world regions to offset these increased healthcare costs. In high-income countries, they concluded that red meat would need to be 20% more expensive than current levels, and processed meat would have to more than double in price to cover the healthcare costs associated with their consumption.

What are the health risks of a meat-free diet?

Completely excluding meat, or all animal products, can result in vitamin and mineral deficiencies that increase the risk of some health conditions. Meat, particularly red meat, is rich in heme iron, a well-absorbed source of iron, the lack of which can cause anaemia (a lack of red blood cells to transport oxygen to the body’s tissues). Oxford Population Health researchers investigated the prevalence of anaemia in the UK Biobank cohort, with separate analyses for white British participants and British Indian participants, since a particularly high proportion of the latter were vegetarians (24.6% compared with 1.7% in the overall cohort). This found that white British women, vegetarians and low-meat eaters were more likely to have anaemia, but there was no significant difference for white British men.

Studies elsewhere have found that vegan diets can be deficient in other micronutrients, including vitamin D, iodine, selenium, potassium and vitamin B12. The Feeding the Future (FEED) study compared eating habits across dietary groups (including vegans, vegetarians, flexitarians, pescatarians and meat-eaters) and concluded that vitamin D levels were low across all groups, not just vegans, although plant-based diets were low in some additional micronutrients, such as iodine and selenium.

Vegans may also have an increased risk of fractures, since dairy products are major dietary sources of calcium. An Oxford Population Health study on the EPIC-Oxford cohort found roughly 20 more cases of fractures per 1,000 people on a vegan diet over a 10-year period, compared with those who ate meat. The greatest difference was for hip fractures, which were significantly more common in vegans. These increased risks, however, were reduced if individuals had a healthy BMI and a good calcium intake. This suggests that vegans should take particular care to maintain an ideal weight, and could benefit from taking calcium supplements.

Is a vegetarian diet better for the planet?

Oxford Population Health researchers have looked at the environmental impacts of different diets. Examining indicators such as greenhouse gas (GHG) emissions, water use and impacts on biodiversity, they found that low meat-eaters have around 30% less negative impacts on the environment than high meat-eaters. Another study demonstrated that current levels of global meat consumption are unsustainable if we are to avoid irreversible climate change and biodiversity loss. Global adoption of ‘flexitarian' diets involving moderate, occasional consumption of meat, could reduce GHG emissions resulting from food systems by more than half.

Oxford Population Health researchers also contributed to the landmark EAT-Lancet report ‘Food in the Anthropocene’, which sought to establish what a daily diet optimised for both human and planetary health would look like. The study concluded that moving to a sustainable food production system that operates within planetary boundaries would require global consumption of red meat and sugar to decrease by half and consumption of nuts, fruits, vegetables and legumes to double.

Studies used by OXFORD POPULATION HEALTH to investigate the health impacts of vegetarian and vegan diets

The EPIC-Oxford Study

This is a UK cohort of 65,000 men and women living in the UK, many of whom are vegetarian, with most recruited between 1993 and 1999. EPIC-Oxford is part of the European Prospective Investigation into Cancer and Nutrition (EPIC) study, which is one of the largest cohort studies in the world, and is coordinated by the World Health Organization. The aim of EPIC is to understand how diet, nutritional status, lifestyle and environmental factors influence the incidence of cancer and other chronic diseases. It involves more than half a million participants recruited across 10 European countries between 1992 and 1999 and followed for about 20 years.

UK Biobank

This is a large-scale biomedical database and research resource containing genetic, lifestyle and health information from half a million UK individuals. Participants aged between 40-69 years were recruited between 2006 and 2010, and provided detailed information about their lifestyle and physical measures. Blood, urine and saliva samples were also collected and stored for future analysis.

China Kadoorie Biobank

A long-term prospective study involving more than half a million participants recruited from 10 diverse areas across China, including both rural and urban regions. The participants were recruited between 2004 and 2008, with data collected by questionnaire and physical measurements, and with blood samples taken for long-term storage. All the participants are now being closely monitored for death and other health-related outcomes through linkage with established registries and health insurance claim databases.