A vegan diet is generally healthy, low in cholesterol and protective of heart disease, but its followers must take vitamin B12 supplements or risk a condition that causes permanent numbness in their hands and feet, experts say.

Most people get their vitamin B12 from milk, but the plant-based substitutes do not have high enough levels to protect adults and children from peripheral neuropathy, which is irreversible.

Young festival-goers on a vegan diet may be at particular risk. “Kids these days inhale laughing gas,” said Tom Sanders, professor emeritus of nutrition and dietetics at King’s College London. “That can actively cause vitamin B12 deficiency. There is a danger of young people going vegan, not having B12 and it could tip the balance to them getting a serious neuropathy.”

It could easily be remedied by the manufacturers of plant-based milks, he said. “Levels should be higher in plant milks than they are at the moment. If they were three times higher, there wouldn’t be a problem.”

Internet claims that vegans do not need extra B12 were not evidence-based, he said. “I’m concerned that many people think it is a myth,” said Sanders. Gorillas eat a vegan diet, but B12 is produced in the colon and “they probably don’t wash their hands”, he said, so end up ingesting it. The Jains in India eat a vegan diet, but, he said, “all the Jain doctors I know have B12 injections”.

Studies have shown that B12 levels in vegans are about a third of what is needed. Other aspects of the vegan diet are healthy but, said Sanders, “B12 is the one thing we are concerned about”. There is particular worry about children. In one case, a baby who was breastfed by a mother who was B12 deficient ended up with neuropathy.

Sanders and Tim Key, professor of epidemiology and deputy director of the Cancer Epidemiology unit at Oxford University, have both researched the health of those who eat a vegan diet for many years. Key is himself a vegan – and takes vitamin B12 tablets regularly.

Key said there was limited data available. The Epic study based at Oxford follows about 2,000 people. Together with a study in California, there is data on about 10,000 people in total. “It’s not enough. We don’t have precise estimates of long term health in vegans,” said Key.

But what they have been able to observe so far is that people eating vegan diets tend not to be overweight – if anything, some are very thin and may have problems in older age when weight loss becomes an issue. They have low rates of type 2 diabetes and diverticular disease of the colon. They have low cholesterol and low rates of heart disease and they may have lower cancer rates.

However, bone fractures are about 30% higher as a result of lower bone density. “That might have something to do with calcium and possibly even B12,” said Key. They may also have higher rates of hemorrhagic stroke.

A vegan diet that is high in fruit and vegetables and pulses is likely to be healthy, but vegans can still overdo the biscuits, confectionery, cakes, chips and beer, said the scientists, and some meat substitutes, such as vegan sausages, are high in salt.

So far, there is no evidence that vegans live longer. “There is no significant difference in total mortality between vegans and meat eaters,” said Key.