Research has found people who cut meat out of their diet – such as vegetarians or vegans – are more likely to become depressed.
The study by the University of Bristol found that meat eaters are generally happier.
It is thought to be because of a lack of vitamin B12 in vegetarian or vegan diets – which is essential for producing brain chemicals that influence mood.
Indeed, the nutrient can be found in rich amounts in red meat, shellfish and liver.
In the study, the researchers looked at 10,000 men, 350 of whom were full-time vegans and vegetarians.
They found those who consumed no meat had a significantly higher average depression score compared with those who did.
The researchers discovered just over 50 per cent of vegans and 7 per cent of vegetarians were deficient in vitamin B12.
Deficiency in the key nutrient has also been linked to vision loss, muscle problems and heart palpitations.
According to the NHS, adults need 1.5mcg a day, and it is not naturally present in grains, fruit and vegetables.
In addition to a lack of vitamin B12, they believe it may also be due to an excess of omega-6 fatty acids in their bodies.
This is because vegans can often be reliant on nuts in their diet.
In the past too much omega-6 fatty acids have been linked to mental health issues.
However, a meat-free diet has been linked by research to a number of long-term health benefits.
In the past, red meat has been associated with increased risk of heart disease.
Processed meat is considered to be the worst for you.
Research by Harvard University found that people who eat the most processed meats have a higher overall risk of death.