To explain the theory of fat availability we have to start with a little
bit of basic physiology. Specifically, we have to look at how our body
fat actually works.
The main purpose of our body fat is to serve as an ‘energy reservoir’
for our bodies. In times of caloric excess (overeating) our body fat
expands to store energy by one of two processes:
They expand to store energy in the form of triglycerides, to be used later as energy to supply our metabolic demands when calories from food are limited (caloric restriction or dieting). In other words, our body fat has the unique ability to rapidly expand or contract depending on nutritional status (Calorie Surplus or Calorie Deficit).
Free fatty acids (FFAs for short) are what is released from adipose tissue when we need energy. This is happening throughout the day even if you’re not dieting, but it’s elevated when we are fasting, exercising or in a caloric deficit (dieting). This is a very good thing since most of us who are dieting and exercising are doing so because we want to decrease the size of our body fat!
When the fat is released from your body fat stores it can end up in a few different places.
1. It can go to the liver and gets stored there
2. It gets burned for fuel by other organs (heart, liver, kidneys etc).
3. It simply gets recycled back into your body fat cells to be re-stored.
The reality is that all three of these processes are happening on a daily basis. The difference is the % of fat that is being recycled back into fat tissue vs the amount that is being burned by other organs.
The movement of fat in and out of your fat cells is an extremely important process that needs to be tightly regulated. The FFAs that are not burned as fuel must quickly be taken up by the liver or recycled and stored back in your fat tissue. If there is an overabundance of fats that are not either burned or stored they can cause an inflammatory response, which chronically can lead to a whole host of health issues and chronic diseases [Sun K J clin invest 2011]. This is how excess fat and an inability to effectively burn the fat contributes to things like diabetes and heart disease.
If this extra fat remains circulating for too long it can also cause ‘ectopic’ fat storage, which is basically fat being stored in places it shouldn’t be stored (like your heart and liver). This can also lead to severe health consequences in the long-term.
It is extremely important that the body regulates the amount of fat that is entering your blood stream because having a high amount of fat circulating in your blood is linked to diseases. You can’t simply have every single one of your fat cells empty out all at once into your blood- the results would be catastrophic!
Each of your fat cells is able to release fats into your blood stream at a predetermined rate, and the ‘rate’ at which an adipocyte can do this is relatively fixed. It can increase to match the needs created by dieting and exercise, but even during exercise, higher energy needs simply cannot be met by fat (which is why we rely primarily on carbohydrate during high intensity exercise).
The amount of energy your fat can provide when you are dieting is dependent on how much body fat you have. The more body fat you have, the larger the calorie deficit you are able to ‘fill’ with the energy stored in your fat.
The word ‘fill’ is actually a good way of describing what your body fat does when you’re dieting. When you diet you create a calorie deficit – essentially a gap between the amount of calories you eat, and the amount of calories you’re burning on a daily basis. In an ideal world, this gap is filled by the energy that is stored in your body fat.
As you lose body fat the amount of fat that can be released also decreases. This means most people do dieting wrong, and specifically they do it backwards. In other words, the more fat you have, the lower and longer you can go in a caloric deficit without experiencing any ill effects. But, towards the end of a diet, when body fat levels can be extremely low, the deficit needs to be shorter and smaller.
When you are carrying excess body fat you can eat less for longer because your body has plenty of fat to burn as a fuel to ‘fill in’ your calorie deficit. This is the time for rapid fat loss. One way to imagine it is like having a full ‘tank’ of reserve energy. As your body fat levels decrease you can no longer handle as large a deficit for as long a period of time. This makes sense that the less fat you have to lose, the slower it comes off.
At the extreme low end, when your body fat cannot ‘keep up’ with the deficit the calories MUST come from SOMEWHERE. This is when you are at risk of losing lean body mass during dieting (commonly referred to as ‘starvation mode’). This happens at extremely low levels of body fat, under 6% in men and 12% in women [Friedl K.E. J Appl Phsiol, 1994].
Oddly enough, it seems as though some obese people have an unbelievable amount of fat available as a fuel, but a lower ability to burn that fat, whereas as they get leaner, they have less and less fat available as a fuel, but a greater and greater ability to burn the fat they do have. So at extreme levels of leanness it is the fat availability that limits a persons ability to lose fat.
When a large calorie deficit is maintained while there is limited body fat available that metabolic disturbances such as altered Testosterone, Thyroid and Cortisol levels begin to take place and negatively affect both your muscle mass, and overall muscle strength [Nindl B.C. Med Sci Sports Exerc, 2007; Freidl K.E. J Appl Physiol 2000].
This is the theory of fat availability – As body fat levels decrease so does your ability to handle a large calorie deficit. Most people forget about the Theory of fat Availability when they design a diet, so they decrease the amount of calories a person consumes as they get leaner. This leads to a whole host of problems, including feeling sluggish, depressed, moody, loss of muscle mass, water retention, metabolic alterations and a generally feeling of ‘not wanting to diet or exercise anymore’. [Freidl K.E. J Apple Physiol, 1994; Keys, A. Biology of Human Starvation, 1950; Keys, A. Science Washington, 1946; Taylor H.L. Science, 1950]
• There is a set amount of fat that can be released from a fat cell.
• The more fat you have, the more fat can be used as a fuel when dieting.
• The less fat you have, the less fat can be used as a fuel when dieting.
• Towards the end of a transformation, when body fat is extremely low you may not have enough fat to handle a large caloric deficit anymore.