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How body fat works

Professor David Cameron-Smith explains how our bodies store excess fat

The inner journey of digested nutrients is a baffling, complex process. Possibly the simplest part is the convoluted transit route fat passes from the lips to the hips. Fats cover an incredible array of different types of compounds that give cooking oils their glowing sheen and cookies their delicious crunch. Almost all are fuel that can be stored in adipose tissue, the slightly sanitised and more medical way to describe body fat.

Fat begins the journey as an almost tasteless component of food. However, it’s what that fat does to the flavours and aromas of almost everything else, including proteins and carbohydrates, that makes it a real wonder.

The journey from here requires bile, digestive enzymes and the specialised cells of the small intestine. Bile is a bright yellow secretion from the gall bladder that acts as a detergent. The digestive enzymes split the fats into smaller pieces and the cells lining the small intestine transport fats into the lymphatic system.

Lymph is a largely hidden circulatory system that helps drain the body’s organs, but importantly it is also an alternative route for immune cells to get to sites of infection and to combat diseases. Fat leaving the intestines makes its way through the lymph, escaping into the blood via a duct located near the top of the heart.

“Each adipose cell is a fat-filled reservoir, waiting for the chance to either take up more fat or release that fat again back into the blood.”

 

The journey of fat from a meal quickens in pace. It takes several hours to get to this point, but once fat reaches the blood after a meal it lasts just a few minutes. It is whisked through the blood into the smaller blood vessels (capillaries) that drain into the tissues of the body. This includes the capillaries of adipose tissue, where enzymes and specialised detection systems go to work. Within a few minutes the majority of the fat from a meal has been rapidly taken up by the enormous sponge-like properties of the adipose cells.

Each adipose cell is a fat-filled reservoir, waiting for the chance to either take up more fat or release that fat again back into the blood. With populations in all countries gaining weight and the rates of obesity continuing to go up, the fat balance is leaning towards increasing fat storage. This is where the remarkable ingenuity of human physiology is apparent. Fat cells can swell, increasing in size till they are ‘set to burst’.

At this point, signals activate the genetic machinery to turn tiny stem cells, which are abundant, but are normally content to sit dormant, into new baby fat cells. Maturation can be rapid, and in a few short days, otherwise quiet little stem cells have reprogrammed themselves into a whole new army of fat storing cells. This process is meticulously maintained, with new fat cells being ‘born’ as required. The contrary side is weight loss (or more correctly fat loss), with adipose cells shrinking in size and a proportion becoming programmed to die.

If this all sounds like an elegant solution to store away excess and unwanted fat, then there are a few unexpected flaws. These tiny stem cells are found everywhere. When primed to act, fat cells start turning up in all the wrong places, causing metabolic mayhem.

The full damage fat causes to the normal functioning of the liver, pancreas and muscles is increasingly being realised as body fatness is the driver of the most dangerous epidemic of all time – the rise and rise of non-communicable disease, including diabetes, heart disease, stroke and cognitive decline. The soft centre of all of these serious diseases is stored fat – too much and in all the wrong places.

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