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Increasing numbers of us are opting for a plant-based diet, for reasons ranging from health issues to concerns about animal welfare and climate change. But the evidence on the long-term health impacts of vegetarian and vegan diets can at times seem conflicting. The internet is awash with news stories on the subject, with some claiming that plant-based diets bring manifold benefits, whilst others state that they increase the risk of certain conditions. It can be difficult to know who to trust.

Here at NDPH, our approach in health research is to carry out studies that involve many thousands of different people, to work out the overall trends that apply to the general population. Several of our research groups study how different diets may affect the risk of developing specific health conditions, such as heart disease and cancer. Typically, these studies involve large cohorts of participants who provide information on basic health measures (such as weight and blood pressure) and general dietary habits. Over a follow-up period of many years, the participants’ health records are updated if, for instance, they develop a disease, have an infection or experience a fracture. This allows researchers to work out the probability of an individual developing a particular disease based on their clinical and dietary characteristics at the start of the study.

Although some general trends may be slightly different for certain subpopulations (such as different ethnic groups) or different regions of the world, because the numbers of participants in these studies are so large, the results are much more reliable than those from smaller studies using fewer individuals.

NDPH researchers have investigated vegetarian and vegan diets, as well as differences in meat intake, using data from the Oxford Vegetarian study; European Prospective Investigation into Cancer and Nutrition (EPIC); UK Biobank and the China Kadoorie Biobank. As outlined below, the overall evidence indicates that limiting consumption of red meat (beef, pork and lamb) and processed meat (such as sausages, burgers and ham) may protect against various diseases. Nevertheless, there are some concerns about totally plant-based diets that strict followers need to be aware of.

Coronary heart disease, heart attack and stroke

Cardiovascular diseases remain the world’s greatest killer, causing approximately 17.9 million deaths globally each year. NDPH research has found strong evidence that vegetarian and vegan diets have a protective effect against coronary heart disease (CHD). Data from 2820 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 1000 people over 10 years.  Similarly, a study on the Pan-European EPIC Cohort concluded that CHD risk is positively associated with an individual’s consumption of red and processed meat. For the 400,000 study participants, each 100g per day increase in intake raised an individual’s risk by 19%.

This may be caused by vegetarians and vegans having lower blood plasma 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. In the Pan-European EPIC study, consumption of red and processed meat was positively associated with LDL cholesterol concentration and systolic blood pressure.

The evidence on stroke risk remains inconclusive. Analyses in the Oxford-EPIC cohort 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 3 more cases of total stroke per 1000 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. The risk of stroke in the pescatarian (fish-eating) participants, however, appeared no higher than for meat eaters.

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 plant-based meat substitutes (such as plant-based burgers and sausages) can be particularly high in salt. To minimise salt intake, vegetarians and vegans should use whole-foods as protein sources, such as lentils, beans and nuts, and like omnivores, avoid foods high in added salt.  

Cancer

The strongest association found so far between diet and cancer risk is for bowel cancer (also known as colorectal cancer), which kills over 16,000 people in the UK every year. Using data from the UK Biobank, NDPH researchers found that people who ate red and processed meat at least seven times a week were 40% more likely to get bowel cancer compared with those who ate meat once a week or less. Of the half a million participants in the study, 3 in 1,000 people who ate meat every day developed bowel cancer compared with 2 in 1,000 of those who ate meat no more than once a week. In a follow-up analysis, the researchers concluded that even those who ate moderate amounts of red meat (76g a day, in line with UK Government recommendations), still had a 20% higher chance of developing bowel cancer than those who only ate on average 21g a day.

The reasons for this increased risk are not fully understood, but they may involve naturally-occurring chemicals in red meat (such as heterocyclic amines) and preservatives found in processed meats (for instance, sodium nitrite). These can cause N-nitroso compounds to form, which can induce mutations in intestinal cells. The process of cooking meat can also result in the formation of chemicals called polycyclic aromatic hydrocarbons and heterocyclic amines, which are thought to be carcinogenic.

Nevertheless, there is little evidence that meat consumption increases the risk of other common cancers. A study led by NDPH on the UK Biobank found no robust associations between meat intake and 20 common cancers (other than bowel cancer), including prostate, breast, stomach and lung cancer.

Diabetes

Globally, around 422 million people have diabetes, and 1.6 million deaths are directly attributed to diabetes each year. Diabetes incidence is inextricably linked to obesity rates, since a high BMI is the most important risk factor. Various studies have confirmed that vegetarians and vegans generally have a lower BMI than otherwise comparable non-vegetarians. Whilst this may be due to vegetarians and vegans having increased awareness of healthful behaviours, it has been suggested that vegetarian/vegan diets may promote weight loss because they typically contain more fibre and lower levels of fat. NDPH research indicates that reduced obesity among vegetarians and vegans translates into lower levels of diabetes. In the Oxford-EPIC cohort, for instance, low meat-eaters and vegetarians were 37% less likely to develop diabetes compared with regular meat eaters, and fish eaters 53% less likely. These differences were significantly reduced after accounting for BMI, indicating that lower body mass in these groups was a key contributor.

In high-income countries, there is also some evidence suggesting a possible link between regular red meat consumption and diabetes. Along with colleagues from the China Kadoorie Biobank collaborative group, NDPH researchers investigated whether this is also the case in China. Here, red meat consumption has traditionally been low but is now 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. Potential mechanisms for this include the fact that red meat contains higher levels of cholesterol, saturated fatty acids and trans fatty acids. In addition, there is evidence that excess heme iron from red meat may reduce insulin sensitivity. Those who consume more red meat may be more affluent (particularly in low-income regions), which could be linked to other factors that increase diabetes risk.

The burden of meat consumption on healthcare systems

Based on these apparent associations between red/processed meat and bowel cancer, CHD and diabetes, NDPH researchers made estimates that quantified the cost that excess meat consumption places on healthcare systems. They estimated that, in 2020, perhaps 2.4 million deaths worldwide and US $285 billion in healthcare costs might 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. Ultimately, by encouraging reduced consumption of red and processed meat, they estimated that such a ‘health levy’ could prevent more than 220,000 deaths and save over $40 billion in US healthcare costs every year.

Other diseases

The costs of meat-heavy diets could be even higher, since evidence is emerging that vegetarian/vegan diets may protect against other diseases and conditions. An NDPH analysis of the EPIC-Oxford study found that the risk of diverticular disease was 31% lower in vegetarians and 72% lower in vegans, compared with meat eaters: this remained significant even when adjusted for fibre consumption (which protects against diverticular disease). In addition, vegetarians and vegans had a lower risk of eye cataract compared with meat eaters, with the risk progressively decreasing with smaller meat intakes. Vegetarians also had a 31% lower risk of kidney stones compared with those who ate a diet high in meat.

However, completely excluding meat, or indeed all animal products, can potentially increase the risk of some other diseases or health conditions:

Vitamin and mineral deficiencies

Meat, particularly red meat, is rich in heme iron, a well-absorbed source of iron, lack of which can cause anaemia (a lack of red blood cells to transport oxygen to the body’s tissues). Plant-based sources of iron are less easily absorbed by the body compared with heme iron; in addition, phytate and tannin compounds present in plants can inhibit non-heme iron absorption. NDPH 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 for white British women, vegetarians and low meat eaters were more likely to have anaemia (for instance, 12.8% of vegetarians compared with 8.7% of regular meat eaters in premenopausal women), but there was no significant difference for white British men. For British Indians, vegetarian men and postmenopausal women were more likely to have anaemia compared with meat eaters, but there was no statistically significant difference for premenopausal women.

Studies elsewhere have found that vegan diets can be deficient in other micronutrients, including vitamin D, iodine, selenium, riboflavin and vitamin B12. The latter can be particularly problematic, since it does not occur in plants, therefore vegans must rely on taking vitamin B12 supplements to acquire enough.

Broken bones

Vegans may have an increased risk of fractures, since dairy products such as milk and cheese are major dietary sources of calcium. An NDPH study on the EPIC-Oxford cohort found that vegans had a higher risk of fractures: roughly 20 more cases per 1000 people over a 10-year period, compared with those who ate meat. This increased risk was particularly strong for leg and vertebrae fractures, but the greatest difference was for hip fractures, which were also significantly more common in vegetarians and pescatarians. Compared with omnivores, the risk of hip fracture was 26% higher in fish eaters, 25% higher in vegetarians and 2.3 times higher in vegans. These increased risks, however, were significantly 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, neither too fat or too thin, and could benefit from taking calcium supplements.

A healthy diet for a healthy planet?

NDPH researchers are involved in ongoing work to integrate the health impacts of different diets with their environmental impacts. They have found that meat-free diets are associated with considerably lower greenhouse gas (GHG) emissions, besides less pollution and land and water use. Furthermore, these studies have demonstrated that current levels of global meat consumption are unsustainable if we are to remain within planetary boundaries and avoid irreversible climate change and biodiversity loss. Global adoption of plant-based ‘flexitarian' diets could reduce GHG emissions from food systems by more than half, and lower other environmental impacts, such as fertilizer application and the use of cropland and freshwater, by a tenth to a quarter. NDPH researchers also contributed to the landmark report ‘Food in the Anthropocene’, produced by the EAT-Lancet Commission on Healthy Diets From Sustainable Food Systems. This sought to establish what a daily diet optimised for both human and planetary health would look like. The study concluded that such a diet would see approximately 35% of calories originating from whole grains and tubers; protein sources provided mainly from plants; 500g per day of fruit and vegetables; and red meat limited to an average of 14g a day.

Taste

Even with this wealth of health and environmental information, for many of us the main question driving our meal choices is simply – does it taste good? Plant-based diets are often portrayed as repetitive, bland and predominantly based on nut roasts. But this ignores the incredible variety of ingredients, cultures and cuisines that vegetarian and vegan diets can embrace. At NDPH’s Old Road Campus, this can be seen every day (in non-pandemic circumstances) at the Tulip Tree Café, which became the University of Oxford’s first meat-free café in March 2020. The menu includes a full English vegetarian breakfast, daily soups such as miso noodle broth, and main courses that include vegan spiced pulled oats and seitan cottage pie.

In conclusion

Vegetarian and vegan diets can bring many benefits to people and the planet, but the evidence suggests that those who may be at risk from certain conditions such as osteoporosis or anaemia should pay particular attention to their nutrient intake. Diet is a personal choice, and individuals should take into account the specific benefits and risks for them, ensuring they maintain adequate levels of macro- and micronutrients, besides a healthy weight.

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