LONDON, Jan 11 (Reuters) - Consuming too much sorbitol, a sweetener widely used in "sugar-free" chewing gum and sweets, can cause serious bowel problems, German doctors said on Friday. The warning follows the cases of two patients who suffered chronic diarrhoea, abdominal pain and severe weight loss after ingesting large amounts of sorbitol. Writing in the British Medical Journal, the doctors from Berlin said the patients -- a man and a woman -- had consumed some 15 to 20 sticks of chewing gum a day. When they kicked the habit, both regained normal bowel function. Sorbitol, also known as E420,...