For years, scientists have believed that the body mass index – weight divided by height squared – was a reliable indicator of overweight and health. New insights are now rapidly undermining this consensus. Terms such as ‘fat’ and ‘thin’ tell us much less about health than we thought.
Bell’s results pointed convincingly in one direction: it is not the amount of fat but its location that matters for health. He believes it is especially fat around the organs - which cannot be seen externally – that carries health risks.
So is the BMI totally out of date as a health marker? It’s not that simple, suggests Michael Müller, Professor of Nutrition, metabolism and genomics at Wageningen: ‘BMI is certainly useful as a measure of the quantity of fat, and it does correlate with a lot of health factors.’ He thinks Bell’s insights will be particularly useful for giving us a more detailed picture of possible health risks.
‘Thin in the right places’ looks set to become the new health gospel. Bell’s interest in the importance of invisible, internal fat began when he discovered during his research that thin people could get diabetes type 2, a disease generally associated with overweight. More than 800 British people went through Bell’s MRI scanner, and his team recorded where their various fat reserves were. This fat mapping led to the discovery that people of the same age and sex, and similar BMI levels could differ dramatically in the size and location of their fat reserves.
To the researchers’ surprise, even skinny people could have considerable reserves of internal fat – in some cases as much as seven litres. These TOFIs – thin outside, fat inside – run serious health risks. And on the other hand there are the FOTIs – fat outside, thin inside – whose BMI is far too high but who have little internal fat. Like Japanese Sumo wrestlers, they have nothing to fear from their overweight.
At the Wageningen department of Human Nutrition, Müller is also working on unhealthy internal fat. He stressed that fat is not just a storage place for energy; it is an organ that plays an active role in the production of hormones and inflammatory and anti-inflammatory substances. ‘Subcutaneous fat is actually protective for the body, as it makes a lot of helpful substances, including hormones and anti-inflammatory susbtances’, says Müller. Abdominal fat, and particularly fat in organs such as the liver, is very dangerous. ‘Fattiness makes the liver much more vulnerable to infection, for example’, says Müller. ‘Mice with fatty livers did not survive exposure to infection-bearing substances, whereas the group with healthy livers could cope with them.’
Müller believes that the effects of fat are closely related to the immune system, with a key role being played by special white blood cells called macrophages. Unhealthy food releases a great stream of nutrients which the body cannot actually cope with, and which are stored as fat. This increases the size of fat cells enormously, and they can absorb less oxygen, which in turn disrupts their functioning. The dying unhealthy cell causes an inflammatory reaction and attracts white blood cells. With chronically unhealthy eating habits, the infection spreads and the balance between inflammatory and anti-inflammatory substances is lost.