What Happens When You Fast

Some individuals, in their quest for weight loss, will turn to a fasting approach. Whether it is a detox diet that involves mixing up some odd concoction of lemon juice, maple syrup and water or else a straight away fast that has you on nothing but water, it is very likely that you will lose weight. The duration of the fast however is what will determine the type of weight you lose and how detrimental it is to your health.

Most people tend to fall under the belief that if they go for longer than three hours without food, their body will start catabolizing its muscle. This is simply an incorrect belief that needs to be put to rest in this world. It is true however, that going long periods of time without food will put you at a high risk for muscle loss. For this reason, long duration fasting would not be a smart approach because you will only end up losing a lot of muscle mass, which will not help you out in the long run. As soon as you begin eating normally again your body will add back on most of the weight only now it is much more likely to be added on as fat mass rather than the initial muscle mass you lost.

During a fasting period, your body is going to primarily depend on endogenous substrates for energy. The essential fuel that you need in order for the CNS to function will be obtained from glucose initially (fuel for the brain). Other tissues in the body are able to oxidize free fatty acids however to meet their needs.

During the first 12 to 15 hours, the liver is going to predominately supply glucose to the body through a process called glycogenolysis. After this time, once these stores have been depleted, a process called gluconeo-genesis will start producing the energy needed. This is the process of converting some of the amino acids found in proteins in the body to energy and will be accompanied by an increase in nitrogen excretion in the urine. While this is going on, there will be about 20 grams of glycerol that is released through lipolysis (breakdown of fat tissue). As time goes on, more and more glycerol will be released from the fat stores and less amino acids will be broken down.

By this point, your body will now be mostly running on a substance called ketones for fuel and glucose use will be very limited. Muscle protein will also be spared, as your body will pretty much solely be running on fats for fuel.

If the fast continues beyond several days, then you will start seeing the drop in BMR of approximately 10-20% along with the body weight diminishing by about 300g/d (about ¾ of a pound). At this point you will be losing about 2/3 fat tissue and 1/3 muscle tissue. While that may not seem like much, do this for numerous days and it does account for a great deal of muscle loss.

If fasting continues beyond about 60 days, then fat stores are likely to become exhausted and the body will need to resort to protein for fuel. Since all that is left now is the rest of your lean muscle and organ tissues, death will likely result.

Obviously most people are not going to be fasting for 60 days straight or longer, but this gives you a good indication of what happens in the body when you go without food. While a few days of fasting may not be incredibly detrimental to the body, it would not be an advisable approach for longer than this. There are far superior ways to go about losing body fat that won't result in such a great loss of muscle tissue.