As a newly diagnosed diabetic and something of a rational scientific type, after my diagnosis I became intensely interested in everything to do with blood glucose. Previously I had not given much thought to what happened to food in my body after the chewing and swallowing stage. One thing I had certainly never looked into was what happens when you restrict the amount of carbohydrate you put into your body and rely on your fat stores and the fat in your food. I started to become aware of the different metabolic pathways the body has for providing energy through diabetes forums. People seemed to be having some success at helping the management of their diabetes (primarily type II but also some type I's) through a low carb high fat diet which involves the body primarily burnign fat for fuel.
Multiple Energy Sources
I had always thought that whatever food you took in, be it sugar or anything else, it all got broken down and turned into glucose for your body to use as fuel. It turns out it is much more complicated than that. The first thing to realise is that the body uses many different things as sources of energy and has lots of slightly different chemical reactions it uses to make fuel. Here is a non-exhaustive list of the things that humans can use as fuel:
- Other carbohydrates
- Lactic Acid
So you can see there are a bunch of different ways of getting energy for the body. The bit I am interested in today though, is how we break down and use fat.
Fat is a really dense source of energy, providing around 9 calories per gram vs 4 calories per gram for sugar. In order to understand how it is used by the body, we should follow it from the stomach and see what happens.
Fat Absorbtion in the Gut
Most of the gunk floating around in your stomach is water based and this includes the enzyme lipase that the body uses to break down fats. Fats and oils are hydrophobic, so lipase can only act on the surface of any fat droplets where it can get access to the individual molecules. In order to speed up the process we need to increase the surface area of the fat and to do that we need to reduce the size of the fat droplets. To make it happen the body releases bile salts which help emulsify the fat in your stomach and small intestines. If you are having your lunch, I apologise, it gets more chemical after this so you are past the worst. The lipase breaks down the fat and the bile salts bind to the bits and form teeny tiny droplets called micelles. These are small enough to float up next to enterocytes which are the cells that absorb nutrients from the gut. The bits of fat that are hoovered up by the enterocytes are recombined and move through the lympthatic system and then on into the blood. Phew!
Non-Energy Uses of Fat
Once the fat gets into the blood stream, it is used for a number of things. Most visibly it is used by the body as padding which also acts as energy storage and insulation. The fat we store in our bodies can come from any excess calories though, just because we eat fat doesn't mean we get fat. Myelin that forms part of our nervous system is made from fats, as are parts of the walls of our cells. Fats are also used to store and transport the fat soluble vitamins A, D, E and K. Cholesterol which is a lipid is also used in the synthesis of steroid hormones.
One of the most important uses of fat is as an energy source. Many cells in the body can directly use fatty acids, the building blocks of fats, as an energy source. Red blood cells cannot metabolise fatty acids because they do not have mitochondria and the brain cannot use fatty acids because they are too big to cross the blood brain barrier. From what I can see though, most everything else can use fatty acids for fuel just fine, but the preference is to use glucose if it is available. Insulin which is released to help regulate the levels of blood glucose slows down the reaction that breaks down your fat stores and releases the fatty acids for fuel and instead encourages the body to store fat, so high levels of insulin can encourage you to gain weight.
Because of the effects of insulin, the only time you would typically see people burning significant amounts of fat is when they have not eaten any carbohydrates for a while. This typically happens at night, after about 8 hours most of your excess carbohydrates have been used up and your insulin levels drop. To provide energy your body will release some of its fat stores to make up the shortfall. The blood transports the fatty acids to the cells that use receptors to grab the molecules and start to convert them into energy the cell can use via the citric acid cycle.