Many of us are under the false impression that it is possible to cut body fat while gaining muscle. Is this possible or is this just another gym myth? To arrive at an answer to this question, we need to know exactly what happens to cause fat to be mobilized from it's storage place in the adipose tissue.
The large stores of triglyceride within adipose tissue or fat are mobilized at relatively slow rates, even during exercise. In this process, exercise or a negative energy balance caused by caloric restriction, stimulates an enzyme hormone sensitive lipase, to dissolve the lipid or triglyceride molecule into three molecules of unbound or free fatty acids (FFA) and one glycerol molecule. This process of breaking down triglycerides is known as lipolysis. The glycerol released from this reaction is water soluble and diffuses freely into the blood. It's rate of appearance in the blood provides a direct measure of the amount of triglyceride hydrolyzed in the body.
The primary factor thought to be responsible for the stimulation of adipose tissue lipolysis during exercise or a negative energy balance is the increasing plasma concentration of epinephrine, which activates beta receptors in adipocytes (Arner et al. , 1990). Of course exercise greatly affects the mobilization of FFA in to the blood stream. During prolonged sub maximal exercise FFA oxidation is about 10 fold from a resting state and reesterification of FFA is also decreased.
What happens when the intensity of the exercise is increased ->70& VO2 max-? Sure the oxidation rate of FFA increased but lactate production also increases. Lactate decreases the FFA mobilization rate and increases FFA reesterification rate. Carbohydrates then become an increasingly important source of fuel. However, during prolonged sub maximal exercise blood lactate levels are very low, thus not affecting FFA mobilization significantly. Thus, carbohydrates are used much less as a fuel and oxidated fats become the most abundant source of energy.
When we cut the calories so as to create a negative energy balance are we still able to build muscle? Well, to answer this we must next understand that if our body does not have enough calories to meet the demands of exercise and basal metabolism it must start breaking down stores. Of course, as we have seen, fat stores are broken down to meet these energy demands. However, the body is not selective in its needs, it also breaks down protein and carbohydrate stores. During this period of negative energy, the body's insulin production also is much lower. Thus, the body releases glucagon and other glucocorticoids such as cortisol. These catabolic hormones quickly go to work by breaking down the whole body tissue and converting it into usable energy. So obviously, low insulin levels and high glucagon levels for long periods of time are not very conductive to building muscle. Thus, the longer we are in a negative energy balance, the more our body degrades protein to meet energy needs.
Most of us are also mislead by the 'gym myth' that says by increasing dietary protein during a negative energy period, you will stop this protein degradation from ever taking place. Is this myth true? Our body has a choice of three places to get these reserves of amino acids needed for energy: 1) dietary protein; 2) tissue free amino acid pools; and 3) endogenous tissue protein (muscle). Unfortunately dietary protein is a relatively minor source of amino acids even when consumed in large amounts. So eating more protein when dropping weight will not slow this degradation process significantly.
The free amino acid pool is next best source of amino acids. However, it is also used quickly in a negative energy balance as it is only estimated to supply less than 1% of the metabolically active amino acids. The largest, most abundant supply of amino acids is from the endogenous protein breakdown. This simply means that the majority of the amino acid broken down for energy, due to a negative energy balance, comes from the breakdown of muscle.
By increasing the dietary protein intake, we may minimize the loss of lean tissue, but not completely stop it. However, it is widely accepted that the larger the energy deficit the more the body must utilize its stores of energy. So slow weight loses are perhaps the best. We lose much less muscle that way.
What are the causes of protein degradation? Of course, as described, the decreased energy state of the cell due to either a negative energy balance created by caloric restriction or a negative energy balance created by exercise is the most obvious cause. Also the rise in glucocorticoids, reduced dietary protein, and the decreased infusion of leucine cause higher rates of protein degradation.
Once again, it is obvious that calories are most important. So will doing resistance exercise while cutting body fat stop lean body mass loses? NO! Exercise decreases protein synthesis and it also increases protein degradation. Exercise also stimulates growth hormone production as well as other anabolic hormones which combined with calories (ie. amino acids) result in stimulation of protein synthesis and a decrease in protein degradation. However, if there is not enough energy to the cell, as in enough calories, protein synthesis is slowed or stopped and degradation rates are increased. So exercise might well help to protect or slow degradation, by causing increases in synthesis afterwards. However, it doesn't stop degradation in periods of negative energy, because rates of protein synthesis are very dependent on caloric intake and, once again, energy to the cell.
What about eating these new low carbohydrate diets? Newsholme, E.A., et al. (1983), as well as many others, reported that decreased availability of total carbohydrate sources, also result in a decrease in FFA oxidation. Low carbohydrate availability impairs high rates of aerobic energy production because the shift to fats as the predominant fuel leads to either a reduction in the rate of acetyl-CoA units or an inability to maintain an adequate level of Krebs cycle intermediates. Not only is FFA oxidation reduced, but it seems that plasma FFA mobilization is remarkably sensitive to even small increases in plasma insulin (Jensen et al., 1989), and it seems that lipolysis is influenced for a long time after eating carbohydrate (Montain et al., 1991).
Diets that are lower in carbohydrate or that contain carbohydrates that cause less insulin secretion (low glycemic foods), probably still elicit enough of an insulin response to reduce plasma FFA mobilization. Therefore, any commercially available product or diet that claims to increase FFA mobilization and oxidation would have to almost totally eliminate the insulin response to the carbohydrate in their product, which seems unlikely. At any rate, any increased FFA mobilization would certainly not seem to be of any value for untrained people because their mobilization of FFA normally exceeds the ability of the muscles to oxidize FFA.
So is it possible to gain mass while cutting fat and losing weight? No! If you want to lose body fat you are going to have to face the fact that you will lose some muscle in the process. It is possible to lose fat and minimize muscle loses by increasing dietary protein consumption and by avoiding severe negative energy balances through either exercise or caloric restriction or the combination of the two. So slow weight loses (~1lb/wk) are best. Another way to minimize protein loses is by increasing protein synthesis by taking advantage of induced post-exercise insulin rebound. Within two hours post-exercise there is a time frame where protein synthesis can be greatly stimulated. The consumption of simple carbohydrates and amino acids during this time period increases the body's insulin production and forces nutrients (amino acids) in to the cell at a much quicker rate. So take advantage of this time!
What can we do to increase FFA oxidation? Well, sub-maximal aerobic exercise seems to be the best method. This intensity causes the rise in the hormones epinephrine, norepinephrine, and growth hormone inhibit the release of insulin from the pancreas. This hormonal environment enhances liver glucose output and promotes greater fat utilization.
In summary, it's the caloric deficit resulting from the exercise session that best helps promote body fat loss. A gradual loss of body fat comes from burning more calories during exercise than you take in at the table, not from some special dietary ratio. Since aerobic training already creates a metabolic environment advantageous for fat metabolism, the best way to increase the body's fat-burning proficiency is to start doing exercise . Keep pumping the weight! But expect to lose some muscle in the process.