On the occasion of International Vegetarian Week, we reviewed the evidence supporting plant-based diets, assessed whether they’re all created equal and teased out health benefits that can be derived from specific plant-based foods.

Are plant-based diets linked to good health?

Although plant-based diets are generally considered healthy because of increased intakes of fibre, vitamins, antioxidants, minerals and protective phenolics (more on this later), vegetarianism or veganism per se is neither necessary nor necessarily sufficient for a good diet.

For example, french fries and soda are vegetarian, as are other potentially harmful foods, such as refined grains and starches. Thus vegetarian eating is not a guarantee of health, whereas a nonvegetarian diet can be rich in healthful foods.

A “good” diet should primarily be characterised by the healthful foods that are included, not simply specific ones to be avoided. However, it is possible for a vegetarian or vegan eater to be healthy and thrive as long as nutrient needs are met.

To this end, it’s important to remember that we all metabolise nutrients differently and each of us operate optimally through different diet strategies. Genetic differences also determine how well we convert precursor nutrients into the nutrients we actually need.

Definitions of plant-based eating and research implications

Several types of mostly plant-based or vegetarian diets are consumed around the world, including pesco-vegetarians (who consume fish); lacto-ovo-vegetarians (who consume milk and eggs); and strict vegans (who consume no animal products).

Each variant has potentially different health effects and something to be mindful of when reviewing plant-based diet studies is that different researchers use different definitions of a plant-based diet.

This makes it difficult to interpret results and compare studies, in addition to the fact that these are usually very highly controlled, and in very specific populations at specific doses for a specific duration.

In addition to this, vegetarians are often generally more health conscious, so other lifestyle characteristics, in addition to diet, could contribute to lower rates of disease and longevity benefits.

Health effects of vegetarian vs nonvegetarian diets

In several observational studies, vegetarians experienced improved health outcomes compared with nonvegetarians, especially when it comes to heart health. A vegetarian diet has been linked with lower risks of coronary heart disease (CHD), stroke, ischaemic heart disease and all-cause mortality.

Among randomised controlled trials (RCTs) conducted on vegetarian diets, small trials have found significant reductions in blood pressure (BP) with vegetarian diets versus typical Western diets.

Two trials found no differences between lacto-vegetarian or vegan diets versus standard diets in terms of improving weight loss, BP, blood lipids, or insulin resistance.

Other research studies that contributed to our understanding of the benefits or harms of plant-based diets have looked at a set of possibly healthful foods that these different ways of eating generally all have in common.

These include minimally processed, plant-derived foods such as fruits, nonstarchy vegetables, legumes, nuts and seeds, as well as plant or vegetable oils like extra virgin olive oil.

The evidence for a plant-based diet

The majority of long-lived populations adopted diets containing these foods. Traditional Japanese diets, particularly from Okinawa, that emphasise their consumption in combination with soy products, fish, seaweed and green leafy vegetables have been linked to the lowest rates of cardiovascular disease (CVD) worldwide.

More recently, a large study from 18 countries, across seven geographic regions (PURE) has found that even relatively moderate intakes of fruit, vegetables and legumes may lower a person’s risk of CVD and all-cause mortality.

Several lines of evidence suggest that having at least two of these common plant-based foods in the diet is generally associated with both better cardiovascular and metabolic health, lowering the incidence of CHDor stroke and benefiting type 2 diabetes.

In RCTs, mediterranean-style diets higher in these health-promoting plant-based foods exert benefits through the improvement of a range of biomarkers and disease risk factors, including blood sugars, insulin levels, blood pressure and cholesterol levels (lowering triglycerides while increasing HDL cholesterol).

A number of study reviews show that the consumption of green leafy vegetables such as spinash and kale (at one serving a day=one cup of raw leafy vegetables), legumes and nuts/seeds (at four servings per week) appear particularly beneficial in type 2 diabetes, while average to low intakes of other vegetables and fruits have a more neutral effect.

Furthermore, a 2014 meta-analysis of 17 studies involving over 350,000 participants found that higher total dietary fibre and fruit fibre intakes were consistently associated with a significantly lower risk of type 2 diabetes, as opposed to lower intakes.

Protective compounds in plant foods

Researchers are still investigating what makes plant-based foods healthful. However, based on available evidence, a lot of the most nutrient-dense foods in vegetarian diets (for eg, berries or nuts) contain important phytonutrients and phytochemicals.

These very potent phytochemicals and phytonutrients, found in high amounts in plant foods that have bitter and astringent types of flavours as well as darker coloured grapes, constitute a mechanism of plant survival for protection against predators in the wild.

Many foods with growing evidence for heart health benefits, for eg, berries, nuts, extra virgin oil, are rich in phytochemicals called polyphenols (phenolic compounds).

These polyphenols, which are strong antioxidants, include flavonols (eg, in broccoli and various fruits); flavones (in parsley, celery); flavanones (in citrus fruits); flavanols such as catechins and procyanidins (in cocoa, apples, grapes, red wine, tea); anthocyanidins and resveratrol (in coloured berries) and isoflavones (in soy).

For instance, extra virgin olive oil contains oleocanthal, a phenolic that has similar anti-inflammatory properties to certain NSAIDs drugs by targeting the same COX enzyme that metabolises arachidonic acid to initiate inflammation.

In the PREDIMED randomised trial, participants receiving extra virgin olive oil and dietary advice to consume a mediterranean diet rich in other polyphenols experienced a 30 per cent lower risk of stroke, myocardial infarction or death, in comparison with those that didn’t.

Phytonutrients found in green leafy vegetables called nitrates have also been found to benefit heart health as they increase dilation in blood vessels, among other things. Nitrates have a fairly short half-life so it’s a good idea to spread the intake of vegetables that are higher in nitrates (eg, arugula, lettuce, carrots, celery) throughout the day in order to get the benefit stretched out for longer.

Are benefits of plant-based eating dose-dependent?

There is an ongoing debate about how many servings of fruits, vegetables and legumes one should aim to consume everyday to get all these benefits.

The most recent position on this in the aforementioned PURE analysis was that optimal health benefits can be achieved with a more modest level of consumption than suggested by previous research.

While recommended daily intake of these foods ranged from 400 to 800 grams per day, the PURE study indicates that an intake of three to four servings per day (equivalent to 375-500 grams per day) could be just as beneficial.

Are there risks to a vegan diet?

A recent review of evidence, published in the Journal of the International Society of Sports Nutrition, brought to light some nutritional inadequacies that can arise from consuming a vegan diet.

Although this meta-analysis was focusing primarily on studies conducted among people with physical activity, there are some useful insights that come out of it when it comes to the challenges that need to be accounted for when designing a “nutritious” vegan diet.

It suggests that vegans can run the risk of deficiencies in a number of vitally important micronutrients and some macronutrients. These nutrient inadequacies include the sufficiency of energy and protein, getting adequate amounts of vitamin B12, iro, zinc, calcium, iodine and vitamin D and the lack of the long-chain n-3 fatty acids EPA and DHA in most plant-based sources.

EPA and DHA are crucial for brain and cardiovascular function. We primarily get them from eating fish and there are no sources of those in a plant-based diet except algae, which few people consume.

ALA, an omega 3 plant fat in walnuts and flaxseeds can be converted into the long chain EPA and DHA that we need, but it has been estimated that only about 5 to 10 per cent gets converted, so one would have to take a lot of it to make up for the lack of bioavailable forms of EPA and DHA.

In other words, vegan diets can put someone at risk of eating little overall, too little protein along with deficiencies in B12, iro, zinc, calcium, iodine Vit D and EFA intake, although making good choices and supplementation might of course make it better.

Your key takeaways

In sum, there are many health benefits attributable to plant-based foods and most studies of vegetarian or mediterranean diets rich in them show a trend towards lower risk of cardiovascular disease and higher metabolic health.

However, not all plant-based diets are created equal and one must ensure that the diet meets nutrient needs and that consumption levels of fruit, vegetables and legumes are in the ballpark range of those suggested in the newest PURE data.

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