Obesity may be a risk factor for the frail bone disease osteoporosis, a study suggests. US researchers have discovered that some people who are overweight have hidden fat inside their bones that could make them weak and prone to fractures. The Harvard Medical School team in Boston did body scans on 106 obese but healthy men and women. The findings are published in the journal Radiology. The scans reveal some people carry fat in hidden places like the liver, muscles and bone marrow as well as their belly, hips or thighs. Dr Miriam Bredella, who carried out the work, says apple-shaped people who carry weight around their waist may be at greatest risk. Fat bones The bone marrow is where the cells responsible for new bone formation - osteoblast cells - live. Dr Bredella reasons that if more of the marrow is taken over by fat cells then this will weaken the bones. She said: \"If you have a spine that\'s filled with fat, it\'s not going to be as strong. \"Obesity was once thought to be protective against bone loss. We have found that this is not true.\" Given that none of us can choose where we put on weight, the only answer is to stay slim, say the researchers. Almost three million people in the UK are estimated to have osteoporosis. The condition is normally associated with being slight of frame and frail. Bone marrow fat has been found in higher-than-normal levels in people who have osteoporosis.