Introduction

Over the last few years there has been much conjecture as to how to optimize fat metabolism (burning). At the end of the day, it isn't really rocket science provided a few key points are kept in perspective.

Firstly, the two main sources of energy during muscular exercise are fat and carbohydrate. Protein does play a minor role in the energy metabolism pathway but only in extreme cases when exercise is exhaustive in nature. 

For a hard training athlete the main reason why glycogen stores (both muscular and liver) are essential is that athletes can only convert their fat stores into energy during exercise very slowly.

Sure training can improve and elevate the exercise intensity this occurs at, but once a person ups the ante and exercises more intensely carbohydrate becomes the crucial fuel. Consequently, once muscle glycogen and blood glucose concentrations are low, exercise intensity must be lowered to a level where the body's energy needs can be met by converting body fat into energy.

With training, endurance athletes can significantly increase the rate at which body fat can be oxidized, therefore allowing them to exercise longer before becoming exhausted due to glycogen depletion. However at "some" stage, regardless of how well trained a person is, eventually the limited carbohydrate stores of the body will be exhausted forcing the person to slow down. 

FAT STORES

Adipose Tissue

Fat is stored in the body in the form of triglyceride - a complex comprising three fatty acids attached to a molecule of glycerol.  There is more than twice the stored energy (9 kcal) in a gram of fat than in an equal weight of carbohydrate or protein (4 kcal/g). Approximately 100 kcal of energy are expended per 1500 metres of walking, your "average" person has sufficient stores of body fat to walk 800-1600 kilometres. As this vast supply of energy is stored in a relatively small mass it provides a great way for people to carry fuel as they move from one place to another. In contrast, if all of this energy were stored as muscle/liver glycogen along with the three grams of water stored with every gram of glycogen, the weight of our stored energy would be considerably more.

The ability of humans to store fat is no doubt a evolutionary adaptation which allowed people to store energy as they wandered over vast distances in search of food and to give them a "store-house" of fuel which during times of famine could be drawn upon and during times of plenty could be added to.

Intramuscular /Blood Triglycerides

Triglyceride is also stored as tiny droplets within the muscle fibres ensuring that this fuel source is in close proximity to the muscle mitochondria (energy house) where aerobic energy metabolism takes place. Intramuscular triglyceride accounts for 2,000-3,000 kcal of stored energy, making it a larger source of potential energy than muscle glycogen, which can contribute only about 1,500 kcal. In comparison to muscle glycogen metabolism, relatively little is known about intramuscular triglyceride as a fuel source and how the body adapts to using it as an energy source after periods of endurance training. It is known however that intramuscular triglyceride can only provide energy for intense exercise at less than a third of the speed muscle glycogen can. In other words, during intense training or competition intramuscular triglyceride should only be considered as supplementary fuel source to that supplied by muscle glycogen. Plasma triglycerides also form another source of energy for muscle that can be accessed in a fasted state, however the contribution to energy metabolism, compared to the other sources is relatively small.

FAT METABOLISM DURING EXERCISE

Accessing Free Fatty Acids (FFA)

The large stores of triglyceride within adipose tissue are mobilized relatively slowly during exercise. Exercise stimulates an enzyme, hormone sensitive lipase, which dissolves the triglyceride molecule into three molecules of free fatty acids (FFA) and one glycerol molecule. This is known as "lipolysis" and is the process anyone wanting to "lean down" or lose body fat is striving for. The primary factor responsible for the stimulation of lipolysis during exercise appears to be as a result of the endocrine system releasing greater amounts of adrenalin (or as it is correctly known these days - "epinephrine") into circulation, which activates the beta-receptors in fat cells (or adipocytes) stimulating the whole process.

At rest, about 70% of the FFA released during lipolysis are reattached to glycerol molecules to form new triglycerides within the fat cells. However, during low-intensity exercise (say 60-75% of maximal capacity), the overall rate of lipolysis increases and the rate of circulating FFA in the blood increases dramatically, some of which are ultimately transported to mitochondria sites for use in energy production.

Studies of endurance-trained athletes who had fasted overnight found that the rate of appearance of FFA in the blood decreased significantly as exercise intensity progressively increased. While the rate of carbohydrate metabolism increased in an inversely.

Muscle Fat Stores During Exercise

It has long been recognized that intramuscular triglyceride plays an important role in fat metabolism during exercise up to certain intensities. During low-intensity exercise, (e.g., Walking) it would appear that blood borne FFA are almost the exclusive fat source as a fuel as the rate of appearance of these substances closely matches their rate of disappearance under such exercise conditions. However, during exercise at higher intensities, total fat oxidation in endurance-trained people is far in excess of the rate of plasma FFA disappearance, suggesting additional fat metabolism must be coming from the intramuscular triglyceride stores.

As a consequence it is often assumed that the intensity of exercise must be kept very low to burn fat optimally (e.g. Walking pace). However, research would suggest that the rate of fat usage is optimised around 65% of maximal capacity. The reason for this is that even though the rate of fat use "proportionally" is higher at lower exercise intensities, the total amount of energy expenditure is greater at higher exercise intensities meaning "in total" there is a greater amount of fat burned.

Therefore to optimise fat use as a fuel source exercise needs to be of a low to moderate intensity level. 

FAT SUPPLEMENTS DURING EXERCISE – A USELESS UNDERTAKING

Long-Chain Triglycerides

As a consequence of their acidic nature it is not possible to consume FFA. You can however raise blood fat content by consuming triglycerides. From time of ingestion through breakdown to availability for energy metabolism is about 3-4 hours and the uptake of this fuel source by muscle tissue is relatively low. Consequently, ingested long chain trigs don't offer a viable fuel substrate for athletes.  

Medium-Chain Triglycerides (or MTCs)

During the early to mid 1990's MCTs where all the craze. Unlike long-chain triglycerides, medium-chain triglycerides (or MCTs) can be absorbed directly into the blood and liver and are rapidly broken down to fatty acids and glycerol. They therefore provide a theoretical means of rapidly elevating plasma FFA. Additionally, MCTs appear to be readily transported through cells and into the mitochondria for energy metabolism. Recent studies suggest that a considerable portion of MCTs ingested are metabolised for energy production and that this rate of metabolism can be increased if the MCTs are taken in combination with carbohydrate. The downside is that MCTs (and indeed all fat) slows the rate of gastric emptying. Given that nothing is absorbed from the stomach itself loading up on MCTs, in many athletes, results in gastric problems and diarrhoea - not exactly what you want to experience when you're out racing! Additionally, it is well known that carbohydrate feeding also stimulates insulin secretion which inhibits lipolysis meaning that after an MCT/carbohydrate feed the body may struggle to access its "own" fat stores resulting in a significant reduction in the rate of fat metabolism compared to simply exercising in a fasted state.

ENDURANCE TRAINING INCREASES FAT METABOLISM

One of the main adaptations to endurance training is an increase in the size and number of muscle mitochondria which greatly improves aerobic metabolism and consequently the ability of muscles to use oxygen to metabolise fat and carbohydrate for energy. Further, endurance training means that at the same power output the body uses less of its precious stored carbohydrate and more of its abundant fat stores. 

Where is this increased fat metabolism coming from?

When measuring the rate of blood borne FFA use researchers have found that it actually decreases following endurance training. This suggests that body adapts and becomes more effective at using intramuscular triglycerides as the primary source of fat for energy production following endurance training and that it is this increased rate of usage of intramuscular fat stores that reduces muscle glycogen utilization and as such improves endurance performance.

DIETARY CARBOHYDRATE EFFECTS FAT METABOLISM DURING EXERCISE

The rate of fat metabolism during exercise is very sensitive to the duration of time between eating carbohydrate and the commencement of exercise coupled with the duration of the exercise. This is partly due to the plasma "insulin response" that occurs as a consequence of eating carbohydrate which, as mentioned above, lowers lipolysis lowering the release of FFA into the blood. In fact if a high glycemic food (high GI) is chosen the insulin response can stay in effect for as long as four hours! It appears that the body prefers carbohydrate to fat and when it is consumed during the previous few hours prior to exercise the body will tap into it to the detriment of pulling on fat stores.

This reduction in fat and an increase in carbohydrate usage rates is not usually detrimental to performance if all of the increase in carbohydrate use comes from glucose in the blood from the ingested meal, as this has little effect on muscle glycogen stores. With this in mind there is no reason to suggest that people don't eat carbohydrate before exercising, indeed this circulating blood glucose can help to top up liver glycogen stores after an overnight fast and help ensure that a person doesn't "bonk" as a result of lowered blood glucose levels - which is the only energy source that the brain can tap into as no glycogen is stored in the central nervous system.

From a diet and fat loss perspective, blood FFA mobilization is extremely sensitive to even small increases in blood insulin concentrations which directly effects lipolysis for extended periods of time. Low carbohydrate diets or ingesting diets that contain low glycemic index carbohydrates (that cause less insulin secretion), probably still produce enough of an insulin response to reduce blood FFA mobilization. Therefore, if you are considering any commercially available product or diet that claims to increase FFA mobilization and oxidation it would have to almost totally eliminate the insulin response to any carbohydrate in the product, which is unlikely. Even if this where the case and the product increased blood FFA concentrations there is still no evidence to suggest that this is of any use to untrained people because elevated FFA concentrations generally exceed their (the untrained) ability to use it anyway.

What about eliminating carbohydrate from the diets of endurance trained people to improve fat metabolism further I hear you ask?

Given that even small amounts of dietary carbohydrate influence fat metabolism through the insulin response, a group of researchers fed endurance-trained men a high-fat diet containing almost no carbohydrate for 4 weeks. This diet lowered muscle glycogen stores by 50% and significantly increased fat metabolism at around 65% of maximal oxygen uptake. However, the diet did not increase the length of time that exercise could be maintained, despite the fact that fat oxidation was increased. Additionally, subjects involved in this investigation were not capable of exercising at higher intensities, which in the real world of elite sport is a distinct disadvantage. Of course the other downside of a high fat diet is the elevated risk cardiovascular and other degenerative diseases.

CONCLUSION:

1. People store large amounts of body fat in the form of triglycerides within fat (adipose) tissue as well as within muscle fibres (intramuscular triglycerides).When compared to carbohydrate stored as muscle glycogen, these fat stores are mobilized and oxidized at relatively low rates during exercise.

2. As exercise progresses from low to moderate intensity, the rate of fatty acid delivery from adipose tissue into the blood decreases, while the rate of total fat usage increases as a consequence of a greater reliance on intramuscular triglycerides stores. It is also these intramuscular triglycerides that are most readily accessed when a person undertakes an endurance training program.

3. Dietary carbohydrate intake has a significant effect on fat availability and use during exercise. When dietary carbohydrate available it becomes the preferred fuel source lowering fat use. This becomes increasing important the higher the exercise intensity because only carbohydrate (and not fat) can be metabolised quickly enough to meet the energy demands for repeated fast muscular contractions.