The theory on diet, arthritis and acids goes something like this: proponents believe that arthritis is caused by the build up of too much uric acid in the body (as happens in gout) and that by avoiding certain foods (often dairy products, wheat, certain fruits and vegetables and animal fats are cut out) the body can be restored to its natural alkaline state and arthritis inflammation reduced. Doses of vinegar are supposed to help regulate acidity levels in the body and aid the 'alkalising' process.
So does it work? Well, these kind of diets might help arthritis but not for the reasons they suggest. Firstly, acid is not the cause of arthritis. Whilst it's true that in gout, joint inflammation occurs because of too much uric acid this isn't the case for most other types of arthritis. Moreover, the build-up of uric acid in joints isn't due to dietary acid levels but is caused by chemicals called purines. In fact, if you have gout eating an 'acidic' orange will help you because vitamin C can reduce the severity of gout attacks.
Secondly, the acidity levels in your body vary according to the function of different body parts. Saliva is slightly alkaline to help prevent the acids from food damaging our teeth. The stomach is highly acidic to breakdown food and kill bacteria. Your body happily controls and regulates all these different acidity levels independently of what you eat or drink.
Why are there so many fans of 'alkaline' diets for arthritis then? Put simply, the food you eat on these diets is good for you. Most plans encourage you to cut out foods that aren't so healthy, like processed meats, saturated fats and sweets, and instead make wholegrains, fruit and vegetables the focus of your diet - all things we know can help improve not just your arthritis, but also your overall health.