Children die every day in africa due to malnutrition of rice diets. I assume you dont mean these or indian rice and bean diets, but western avocado diets that are magically much better, because you eat avocados from south america, and perhaps algae from japan and cocoa beans from north africa etc.
In western civilization, when you start feeling ill you have the nice option of buying a steak or going to the doctor, which is why 89% of vegans are not vegans after 5 years.
When you look at vegan societies like 500million indian vegetarians you see that their average IQ is 80, diabetes twice more than Sweden, and they have a lot of problems in general.
In USA beef consumption has halved since 1970s. They used to eat 40kg beef per person on average in a year and nowadays 20kg. You know what has happened with their health? Coronary heart disease has become 5x bigger. Because the problem is not in meat but in sugar/junk food, which has skyrocketed since 70s, and the actual junk food has become very processed compared to what it was in the 70s.
Simple pubmed search would yield you lots of results like:
lack of omega 3, fatty acids, b12 in ==> smaller brains:
vegans have 30% lower creatine levels:
Vegans poorer bone density, low vitamin d:
The adoption of the vegetarian diet tends to follow the onset of mental disorders:
Populations that habitually consume vegetarian diets have low zinc intakes:
"atmospheric depositions and marketing systems of vegetables play a significant role in elevating the levels of heavy metals in vegetables having potential health hazards to consumers of locally produced foodstuffs."
"The regular consumption of vegetables grown in sites can cause detrimental effects to the human population. The authors strongly recommend that consumption of tuberous and leafy vegetables from these sites should be restricted and continuous monitoring of vegetables from these sites should be done."
Im not even trying here, there is just so much research on it that you dont read on your vegan propaganda forums about