You know too much body fat is bad for you, because the media keeps harping on about it, but have you ever wondered why? We evolved the ability to store food energy during the eras when food was scarce and seasonal, but now it comes from supermarkets, many people eat too much. Even so, the connection between your energy stores and big diseases like cancer and heart attacks is not obvious. It was not obvious to scientists either until 1994, when they discovered leptin. Leptin is a molecule fat tissue makes, which floats away and stirs up your metabolism at distant sites. Since then fat has been shown to make a further 600 or so metabolically active molecules. Fat is a signalling station, not a fuel depot. One of the key targets of its signals is the immune system. Fat is inflammatory. Obese people have higher levels of inflammatory marker molecules in their blood than people of normal weight. They also get more cancer and heart disease. Inflammation is the link.