- There is some evidence that high consumption of plant foods can be associated with better health but not necessary with increased life expectancy, as observed in studies in vegetarians (Crit Rev Food Sci Nutr.).
- Comparison of plant food intake and life expectancy by country makes no sense without considering other factors, such as genetics, medical care availability, etc. India has the highest percent of vegetarians (31%) and one of the lowest meat intakes (4.4 kg/person/year), but their life expectancy is only 69.4 years.
Is the advice to eat a lot of fruit, vegetables and grains sound?
In general, plant foods can be helpful, because:
1) Plant foods contain dietary fiber, which contributes to bowel regularity.
Effect of dietary fiber on constipation: A meta analysis (World Journal of Gastroenterology, 2012):
In summary, our meta-analysis demonstrated that dietary fiber can
obviously increase stool frequency in patients with constipation.
2) Plant foods are, in general, less energy dense than animal foods, so they can be more satiating and thus help to maintain healthy body weight.
A plant-based diet for overweight and obesity prevention and treatment (Journal of Geriatric Cardiology, 2017):
In summary, individuals consuming PBDs tend to have lower BMI than
those consuming non-PBDs. The adoption of PBDs also appears effective
for weight loss. (PBDs = plant-based diets ; BMI = body mass index = kg/height in m2)
3) Vegetarian/vegan diets have been associated with lower risk of heart disease and cancer.
Vegetarian, vegan diets and multiple health outcomes: A systematic review with meta-analysis of observational studies (Critical Reviews in Food Science and Nutrition, 2017):
With regard to prospective cohort studies, the analysis showed a
significant reduced risk of incidence and/or mortality from ischemic
heart disease (RR 0.75; 95% CI, 0.68 to 0.82) and incidence of total
cancer (RR 0.92; 95% CI 0.87 to 0.98) but not of total cardiovascular
and cerebrovascular diseases, all-cause mortality and mortality from
4) Plant foods can be beneficial for health, but it's not clear why.
The effects of plant-based diets on the body and the brain: a systematic review (Translational Psychiatry, 2019):
Based on this systematic review of randomized clinical trials, there
is an overall robust support for beneficial effects of a plant-based
diet on metabolic measures in health and disease. However, the
evidence for cognitive and mental effects of a plant-based diet is
still inconclusive. Also, it is not clear whether putative effects are
due to the diet per se, certain nutrients of the diet (or the
avoidance of certain animal-based nutrients) or other factors
associated with vegetarian/vegan diets.
BUT, there is no reliable evidence to advise how much plant foods one needs to consume to have health benefits from them. The 2015–2020 Dietary Guidelines for Americans recommends that people needing 2,000 calories per day include 2 cups of fruit and 2.5 cups of vegetables per day (USDA.gov), but this amount is not clearly based on any evidence.
Are there any harms side effects of high plant food intake?
This depends on the person and his/her underlying health disorders.
- In general, high intake of plant foods high in soluble fiber (barley, oats, rye, legumes, apples, prunes, pears, bananas) can result in excessive abdominal bloating and flatulence.
- In individuals with fructose malabsortion, plant foods high in fructose (apples, pears, onions, agave...) can result in bloating and diarrhea.
- In individuals with hereditary fructose intolerance, even minute amounts of fructose or sucrose (in fruits, nuts and many vegetables) can cause severe kidney and liver problems or even death.
- In individuals with oxalate kidney stones, avoidance of foods high in oxalates (spinach, rhubarb, nuts, legumes...) can decrease the risk of new stones.
Plant food intake and life expectancy by country
Comparison of plant food intake (concluding from meat intake, which is roughly inverse to plant food intake) and life expectancy by country is not helpful because other factors, such as genetic and environmental factors, availability of medical care, economical status, etc. can override the effect of high plant food intake on life expectancy in a negative or positive way. For example:
- Bangladesh: meat: 4 kg...exp: 72.3 y.
- India: meat: 4.4 kg...exp: 69.4 y.
- Japan: meat: 45.9 kg...exp: 84.5 y.
- China: meat: 58.2 kg...exp: 76.7 y.
- Brazil: meat: 85.3 kg...exp: 71.4 y.
- Germany: meat: 88.1 kg...exp: 81.2 y.
- United States: meat: 120.2 kg...exp: 78.9 y.
- (meat = kg of meat available to a person/year, exp = life expectancy)