Claim: To remove body fat, you need to learn to use it as fuel. The muscle fibers fueled by fat (“slow twitch” fibers) are the ones that produce easy movements

Status: You don’t have to learn how to use fuel. Are you aware that you’re burning fuel 24 hours per day? When you’re sitting around doing absolutely nothing, you’re burning fuel. Many tissues can use free fatty acids for fuel, not just slow twitch muscle fibers. Losing body fat involves more than the activity of the slow twitch muscle fibers (how about calorie deficit). To use slow twitch muscle fibers, nervous stimulation is required (the central nervous system requires calories although not fat calories). High intensity exercise often results in a lower RQ (indicating higher proportion of fat) than low intensity exercise post-workout.

Below is a brief description of what occurs during mobilization of stored fat and the oxidation of fatty acids. This information has been excerpted from Fat Burning: How it Works by Jamie Hale.
The body’s two major stores of fat that provide energy are adipose tissue and intramuscular triglycerides (IMTG). Adipose tissue stores fat in the form of triglycerides (triacylglycerols or TG). TG is composed of a glycerol backbone with three free fatty acids (FFA) attached to it. IMTG are droplets of fat stored within the muscle fiber. They are contained within the muscle and can be used directly. FFA from adipose tissue must be carried through the bloodstream to the muscles to be used for energy.

Fats are broken down to fatty acids and glycerol. Glycerol enters the glycolytic/glucogenic pathway via glyceraldehyde 3 phosphate (can be used to from TAG in liver as well). The FFA move through the cell membrane of adipocytes and bind to albumin in plasma. They are then transported to the tissue where they enter cells. Keep in mind, regardless of free fatty acid blood levels, the brain (although the brain can use ketone bodies) and erythocytes can’t use FFA for energy. The breakdown of TG is initiated by hormone sensitive lipase (HSL), which is primarily influenced by insulin and the catecholamines. HSL removes a fatty acid from carbon 1 and/or 3 of TAG. Additional lipases including diacyclglycerol and monoacylglycerol remove the remaining fatty acids (Harvey and Champe 2005).
Adrenaline and noradrenaline bind to beta adrenergic receptors in fat cells, stimulating HSL and causing the release of FFA. FFA are burned in the mitochondria to produce ATP and acetyl-CoA.

Claim: The best exercise for fat loss is low intensity, long duration aerobics.

Status: Dietary factors excluded, the proportional use of fat during exercise is related to training intensity. The lower the intensity, the greater the proportion of stored fat burned. The higher the intensity, the greater proportional use of glycogen and/or the phosphagen system. The real question should be, what type of exercise promotes chronic fat burning? The actual time spent training takes up a small portion of an entire day. Even if you trained two hours per day every day that still means you have 22 hours per day when you aren’t training. Keep in mind, any training regimen must be supported with a proper nutritional protocol that matches training objectives.

As you’re sitting there reading this, you’re burning proportionally more fat than you would be sprinting 100 meters (you’re relying primarily on the phosphagen system). Does that surprise you? It’s commonsense that sprinting 100 meters would be more beneficial than reading for net fat loss. However, the key word is net fat loss. Net fat loss depends on more than proportional fat oxidation while training. Don’t forget the total calories burned during training. Also, consider the absolute fat oxidation during training. Often, an exercise may burn a higher amount of proportional fat, but due to the low calorie expenditure when compared to a higher intensity exercise (up to 75–80 percent), the absolute amount of fat oxidation may actually be lower. Higher intensity activity also generates a more significant effect on excessive post-exercise oxygen consumption (EPOC).

Calories burned while exercising

Most trainees overestimate the significance of caloric expenditure while training. The amount of calories burned while training is generally very low relative to total calorie consumption. During low intensity exercise, approximately 5 kcals per minute are oxidized while increasing intensity could result in burning up to 10 kcals per minute.

In general, weight training results in a caloric expenditure of about 7–9 calories per minute including rest periods. Significant gains in skeletal muscle tissue can result in higher calorie expenditure over time.

Fat oxidation during and immediately following exercise

Fat oxidation during exercise tends to be higher in low intensity treatments, but post-exercise fat oxidation tends to be higher in high intensity treatments. Phelain’s (1997) team compared fat oxidation at three hours post-exercise of 75 percent VO2 max versus the same calories burned at 50 percent. Fat oxidation was insignificantly higher during exercise for the 50 percent group but was significantly higher for the 75 percent group three hours post-exercise. Lee’s (1999) team compared the thermogenic and lipolytic effects of exercise in college males pre-fueled with milk plus glucose on high versus low intensity training. Pre-exercise intake of the milk/glucose solution increased excess post-exercise oxygen consumption significantly more than the fasted control group in both cases. The high intensity treatment had more fat oxidation during the recovery period than the low intensity treatment.

Fat oxidation: The 24-hour effect

Melanson’s research team (2002) carried out a study that compared an even mix of lean, healthy men and women aged 20–45 with identical caloric expenditures at a 40 percent VO2 max training intensity to a 70 percent VO2 max intensity. There was no difference in net fat oxidation between the low and high intensity groups at the 24-hour mark.

Saris and Schrauwen (2004) conducted a study on obese males using a high intensity interval protocol versus a low intensity linear one. There was no difference in fat oxidation between the high and low intensity treatments at 24 hours. In addition, the high intensity group actually maintained a lower respiratory quotient (burned higher proportion of fat) post-exercise.

Fat oxidation: Long-term, chronic effects

Long-term tests are the most important when looking at total fat loss. A common finding with long-term testing is that when caloric expenditure is the same during training between high and low intensity exercise minimal differences are seen in fat loss. Another significant finding is that high intensity training usually results in maintenance or growth of muscle tissue. Low intensity training usually results in loss of muscle tissue.

The majority of research indicates that high intensity interval training (interval training alternates periods of short near maximal intensity activity with low to moderate intensity activity) is superior for both fat loss and lean mass gain/maintenance. Tremblay’s team (1994) did a study comparing HITT versus steady state endurance training on young adults over a 20-week period. The HITT used a progressive program working up to five, 90-second intervals near their max heart rate thee times per week. The steady state endurance group worked up to 45 minutes of exercise five times per week. Although the interval training group only worked out one hour per week compared to 3.75 hours in the steady state group and expended only half as many calories during the interval workouts, fat loss, as measured by skin folds, was nine times greater in the interval training group. In the HIIT group, biopsies showed an increase of glycolytic enzymes as well as an increase of HADH activity, a marker of fat oxidation. Researchers concluded that the metabolic adaptations in muscle in response to HIIT favor the process of fat oxidation.

Contrary to hearsay, you don’t have to do steady state, low intensity endurance training to enhance fat loss. In reality, fat oxidation while training is only part of the picture when attempting to maximize fat loss. Post-workout, 24 hours, and chronic fat oxidation must be considered. One final thing to look at is the physiques of 100-meter sprinters. Generally, they perform minimal to no low intensity aerobic activity. They also burn a minimal proportion of fat while training. Think about it.

In reality, no exercise (aerobic or anaerobic exercise) is required to drop body fat. Creating a calorie deficit on a regular basis will result in body fat loss. The amount of body fat lost or gained also depends on the P-ratio. The P-ratio is the amount of weight stored or mobilized as protein during weight gain or weight loss. People with higher P-ratios tend to gain and lose higher percentages of weight as protein. Lower P-ratios result in less weight deposition as protein and less weight loss in the form of protein (Henry 2008). The P-ratio can be altered to a degree (with exercise, nutrition, drugs) but is largely dependent on genetics.

Claim: Movements requiring effort (either for power or acceleration) are done by muscle fibers that burn sugar.

Status: This depends on the intensity and duration of the movements. There are two anaerobic energy systems—the adenosine tri-phosphate, creatine phosphate (ATP/CP) pathway and the glycolytic pathway. Adenosine tri-phosphate (ATP) is the basic energy unit for all living things, and the body has a limited amount of ATP in storage. After 3–4 seconds, ATP stores are depleted. After ATP levels are depleted, creatine phosphate (CP) comes into play. CP gives phosphate molecules to adenosine di-phosphate (ADP) to convert to ATP. After about ten seconds of maximal effort, ATP and CP become depleted.

Some sources suggest that the ATP/CP system can fuel intense effort for 20–30 seconds (Siff 2000). The glycolytic pathway becomes the primary contributor to muscular energetics after depletion of the ATP/CP system. The glycolytic pathway involves the breakdown of glycogen to produce ATP. Pyruvate is the end product of glycolysis, which is converted to lactic acid when insufficient levels of oxygen are present. When sufficient levels of oxygen are present, pyruvate is able to enter the Krebs cycle. Exercises that are used to enhance power (the rate at which work is done) and/or acceleration (rate of change of velocity) are generally short in duration and intense (e.g. plyometrics, Olympic weightlifting, sprints, throws, and speed squats). These movements are dependent on the ATP/CP system. The chemical fuel used in this pathway is creatine phosphate (Janssen 2001).

When the word “sugar” is used, I assume the person is referring to glycogen (long chain of glucose stored in muscle and liver) and glucose as sugar. Technically, sugar could be one of many different molecules including glucose, galactose, fructose, maltose, sucrose, lactose, or a few others. The newer classification system (described in The Carbohydrate Files by Jamie Hale 2007) classifies carbohydrates according to degree of polymerization (polymerization is a chemical process that combines several monomers to form a polymer or polymeric compound) and may be divided into three principal groups, namely sugars, oligosaccharides, and polysaccharides. Using the word sugar is vague and imprecise.

Claim: Working out vigorously tires you and makes you hungry because you run out of sugar quickly.

Status: This is another example of a fallacy or “hasty generalization.” Fatigue depends on numerous factors. Refer to Skeletal Muscle Fatigue: Cellular Mechanisms by D. G. Allen (2008) for a comprehensive discussion of various factors. It isn’t unusual to see Olympic weightlifting sessions last 3–4 hours (which is a predominantly ATP/CP sport). Activities that are primarily glycolytic (I’m helping you out here because I think that’s what you meant to say above) such as boxing and mixed martial arts are performed with rest intervals. If the activities are performed without rest, they become lower intensity and more aerobic in nature.

The rate at which you run out of glycogen or glucose depends on numerous factors including pre-workout glycogen levels, amount of glycogen you’re utilizing, dietary intake while training, and so on. Fatigue can be induced by aerobic activity as well. Guess what sugar (glycogen and glucose) can be depleted with aerobic activity? Aerobic exercise can rely on multiple fuel sources including glycogen, glucose, free fatty acids, intramuscular triglyceride, ketone bodies, and protein (McDonald 1998). The storage of carbohydrates is limited while the storage of fats is almost unlimited. Their contributions to the energy supply are different and depend on glucose availability, level of exertion, training, and duration of activity. What are the effects that exercise has on hunger? Vigorous activity often causes decreases in hunger. The effects that vigorous workouts have on hunger are variable among individuals.

Research says

A study conducted by Erdmann et. al. (2007) investigated the effect of exercise intensity and duration on ghrelin (ghrelin is produced primarily in the stomach and has been shown to increase appetite and food intake) release and subsequent ad libitum food intake. Bicycle exercise on an ergometer for 30 minutes at 50 W, which was below the aerobic/anaerobic threshold, led to an increase of ghrelin that remained unchanged during the higher intensity at 100 W. In a second group, seven subjects cycled at 50 W for 30, 60, and 120 minutes. Ghrelin concentrations rose significantly above baseline for the respective periods of exercise. The researchers concluded that low rather than high intensity exercise stimulates ghrelin levels independent of exercise duration.

In my personal experience, I’ve noticed a decrease in hunger following high intensity activity. Many of my clients have also reported decreases in hunger following vigorous exercise. It is also important to consider factors other than just exercise that influence food consumption.

The following is an excerpt from Popular Diets: A Scientific Review (Freedman MR, King J, Kennedy E (2001) Popular diets: A scientific review. Obesity Research 9(S1): 1–40.):

“Many factors influence hunger, appetite, and subsequent food intake. Macronutrient content of the diet is one and it may not be the most important. Neurochemical factors (e.g. serotonin, endorphins, dopamine, and hypothalamic neuropeptide transmitters), gastric signals (e.g. peptides and stomach distention), hedonistic qualities of food (e.g. taste, texture, smell), and genetic, environmental (e.g. food availability, cost, and cultural norms), and emotional factors (e.g. eating when bored, depressed, stressed, or happy) must be considered. These parameters influence appetite primarily on a meal-to-meal basis. However, long-term body weight regulation seems to be controlled by hormonal signals from the endocrine pancreas and adipose tissue (i.e. insulin and leptin).”

On the other end, let’s assume vigorous activity causes a huge increase in hunger. So you eat some food. That isn’t a bad thing. In fact, many athletes strategically plan big meals around training time to take advantage of nutrient timing. I think most people would agree that food needs to be eaten sometime throughout the day.

Claim: Fat calories and sugar calories aren’t interchangeable. Doing a lot of work will make you tired and hungry. You’ll need to stop sooner and you’re likely to eat more. The waste products from sugar burning will also interfere with the processes that release fat from storage and burn it.

Status: I’m not sure what is meant by “they aren’t interchangeable,” so I’ll address this statement from various angles. If this implies that the caloric value is different between the two, this is correct (approximately 9 kcals for 1 gram of fat and approximately 4 kcals for 1 gram of carbohydrate). If this suggests that some tissues use fat while others use glucose, this is correct (or prefer one or the other). If this suggests that activity that’s primarily dependent on glycolytic activity doesn’t contribute to body fat loss, this is incorrect. A calorie is a unit of energy. It is the amount of energy or heat that it takes to raise the temperature of one gram of water one degree Celsius (1.8 degrees Fahrenheit). One calorie is equal to 4.184 joules, a common unit of energy used in the physical sciences. The energy derived from foods when they are oxidized in the body is measured in kilocalories (thousands of calories). A kilocalorie is the amount of energy required to raise 1000 grams of water one degree Celsius. Kilocalorie is written as “Calorie” (with a capital C) or it may be abbreviated to “Kcalorie” or Kcal. The definition of a calorie doesn’t change whether it comes from fat or sugar (as you say) calories.

I’ve already addressed the tired and hungry issue (refer to the information above). I assume one of the waste products referred to is lactic acid (or lactate). Contrary to popular belief, lactate is not a toxic byproduct of metabolism accelerated by glycolytic exercise. Lactate is produced even while resting and can serve as a valuable extra substrate (Siff 2000, Janssen 2001). Lactic acid formed in muscle during exercise can be used to manufacture glucose (gluconeogenesis) in a process known as the Cori cycle (Siff 2000, Hale 2007). High lactate levels have been associated with a decrease in the burning of fat. I’m not sure if this is due to lactate per se or an increase in blood acidity, decreased insulin, or some other factors. The elevation in blood lactate is acute and has minimal effects on body fat loss. After high intensity activity, blood lactate returns to normal after approximately 60–75 minutes.

As I indicated earlier, the utilization of fat during exercise has little influence on fat loss. Zelasko (1995) stated, “Although exercise does increase energy output during and after exercise and can expend energy from fat for many overweight persons, excessive caloric expenditure has limited implications for substantially reducing body weight independent of nutritional modifications.” No matter what type of exercise regimen you’re following or how many calories and fat you oxidize while training, you must create a calorie deficit to lose body fat.

Claim: Although a vigorous workout will keep your energy use going for several hours afterward, the recovery processes are mainly powered by sugar, so this extra “metabolism” isn’t burning much fat.

Status: The human body constantly uses energy (vigorous workout or not). Once your body stops using energy, you’re dead. The recovery process is a different subject matter than what we have been discussing. The key goals for the recovery phase are to replenish depleted energy sources and remove metabolic byproducts. We can further distinguish the recovery process into various phases including ongoing recovery, rapid recovery, and delayed recovery (Zalessky 1979).

Burke (1999) divides the recovery process into three phases including the rapid, intermediate, and longer phase. Substrate utilization during these phases varies. Factors that influence substrate utilization include nutrients consumed, nutrient storage, hormones, and enzymes. This “extra metabolism” (increase in calorie expenditure) can utilize calories from various sources. Saris and Schrauwen (2004) found there was no difference in fat oxidation between the high and low intensity treatments at 24 hours. In addition, the high intensity group actually maintained a lower respiratory quotient (burned higher proportion of fat) post-exercise.

Claim: If you spend a lot of time moving around, you will burn a lot of fat. If you move easily enough to keep it up all the time, this will burn the maximum amount of fat. Any activity that you can do continuously without getting ‘fatigued’ is only using fat for fuel.

Status: What is meant be “a lot?” This is a relative term. A lot to you may not be a lot to me or vice versa. No one can move all the time. This statement implies if you move too hard and burn too many calories, you won’t maximize fat burning. The primary scientific data and mounds of anecdotal evidence say that this is incorrect. Another thing to realize is that burning a higher proportion of fat doesn’t mean you’re burning a higher absolute amount. With increased intensity, the absolute amount of fat used is often greater. I will say it again. The fuel used during exercise is of secondary importance compared to the amount of calories expended over a day’s time. Some people will burn more calories with lower intensity, longer duration activities. Trainees attempting to maximize the fat loss benefits of exercise need to find a balance of duration and intensity that allows them to maximize caloric expenditure.

Claim: By spending a lot of time moving around, you’ll stimulate the release of stored fat, so more fuel is always available. This will cause your stored (“adipose”) fat to be reduced, but it’s a slow process. Rapid weight loss is never caused by reduced body fat.

Status: For an explanation of how and why fat loss occurs, refer to the information I’ve provided above. Rapid weight loss is due to the loss of glycogen, water, minerals, bodily proteins, adipose tissue, intramuscular triglycerides, and decreased gastrointestinal tract storage. A large weight loss in the short term is generally due to a high proportion of water loss.


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