 MEaT should be eaten only on feast days, to prevent obesity and climate change, according to a government adviser.  Tim Lang, professor of food policy at City University, London, said meat consumption was "out of control". He suggested we should go back to ancient traditions whereby meat was considered a treat and eaten only on feast days, such as Christmas.  "Let's go back to where culture has been for thousands of years, which is meat is an exception," Prof Lang said. "If you were growing meat yourself, it is an incredibly slow process and killing and eating an animal is a special day.  "at Christmas if we were well off we had beef. It was a big deal. We killed an animal as an exception, for a feast."  In medieval times, there were far more feast days for saints or changes in the season, and Prof Lang said he was not advocating eating meat only once a year, but once a week at the most.  Prof Lang, who advises the World Health Organisation, as well as the Department for Environment, on food policy, said eating too much meat can cause obesity, heart disease and type 2 diabetes. Last year, Sir Liam Donaldson, then Chief Medical Officer for England, said cutting consumption of meat by 30 per cent would prevent 18,000 premature deaths a year.  Producing meat is harmful for the environment as growing animals requires energy and water, and cows produce the greenhouse gas methane.  Sir Paul McCartney has advocated Meat-Free Mondays but Prof Lang said: "I am saying instead of having one day where you do not eat meat, eat meat once a week and have really good, grass fed meat."