How quickly are calories turned to fat?

A friend of mine, lets call him Davy Gorilla just for arguments sake, asked me whether I supported the theory that it takes a week or two for diet and exercise to 'filter through' and make a change to your weight. Meaning you could act like a saint all week and see little or no improvement, or conversely act like a sinner but see little or no detriment. The benefit (or negative consequences) would largely be seen the following week.

It sparked the idea for this post, an interesting question indeed…

How do I answer it? Firstly, I have never seen or read any studies to suggest the above to be true, and the data I hold on myself would suggest that changes in caloric intake effect weight far more quickly than a week. I am a science guy, I like to see studies and hypothesise why things happen.

Secondly, the digestive process of food in humans is about 2 days for the average person. In the 1980s, Mayo Clinic researchers measured digestion time in 21 healthy people (1). Total transit time, from eating to elimination in stool, averaged 53 hours (although that figure is a little overstated, because the markers used by the researchers passed more slowly through the stomach than actual food).This would suggest that the energy and nutrients from the food eaten are used by the body a lot quicker than a week, or are somehow stored by the body and used later or then stored as fat.

This is partly true, your muscles and liver are able to hold a large amount of energy in the form of glycogen. This is something to consider in this argument, because if these “stores” are not full, glucose is shuttled from your blood stream and stored in your liver and muscles as glycogen.

Another thing to consider is the storage of water along side the glycogen. For every 1g of glycogen stored, the body needs to store about 4g water with it. This will effect weight dramatically and verty quickly.

Ypu will also store, for short periods of time, glucose in the blood. This is then shuttled out by insulin and used as energy, stored as glycogen, or then stored in the adipose (fat) tissue.

You also need to consider the types of food you eat, each macronutrient (carbs, proteins and fats) you consume is dealt with different by the body. Energy is used to digest and process foods you eat, this is called diet induced thermogenesis. Thermogenesis is higher when protein is consumed over fats and carbs, so this means more calories are burned processing proteins than fats and carbs.

Take home point – I can’t really answer it. Having not seen any studies to back this up, I can’t really confirm either way but my experience and what I know about the biochemical processes in the body, I would suggest that calories are stored as fat a lot quicker than a week. There are so man variables which equate to either fat loss or fat gain, to worry about the time it takes is somewhat futile.

It’s important to remember that your body is running various processes at the same time, and it’s not a case of shutting off one to do another. It’s a constant cycle, and you don’t just go from fat burning to another process instantly. Your body is complex, it adapts well and does amazing things.