Is it better to eat 1/2 hour before a workout? An hour before a workout? Do not eat? Yes, eat. Eat what? When? As Personal Trainers, we get these questions all the time.
To answer them properly, one would need to ask a lot of questions about the habits of the person seeking the advice. Often, the client needs to be educated in nutrition and their eating habits. This can be challenging.
Most people would not be able to intelligently break down the percentages of macro nutrients (carbohydrates, fats and proteins) and their amounts. Therefore, they could only make visual examples of how much food they consumed and attempt to paint a mental picture of what the foods looked like. Even if they've prepared the food themselves, they would not be able to assess the caloric value of sauces and spices of the meal either. Even educated ones without a calorimeter (an apparatus that literally burns food to determine the caloric content) really do not know for sure. Remember, a calorie is when the heat from the food burned inside a calorimeter heats 1 gm of water up 1N C. Often, the information they provide can be vague at most.
Even if you knew the specifics, there are so many "diet plans, meal plans, eating plans or philosophies" about eating in general. Most of them are probably valid and will work. These philosophies, principles, doctrines or "tenets" can range from the scientific to the spiritual. Try them all if you want. The focus of this article is for you to understand how the body burns sugar compared to fat. Once you understand those basics, you'll apply this knowledge to any food plan.
How the body uses the food or energy.
Did you know that to decrease body fat you do not need to burn fat during exercise?
Adipose tissue (fat) is lost during the hours after exercise is completed. (1) Triglycerides or serum lipids (fat in the blood stream) are mostly used during exercise. The amount of fat mobilized post exercise depends, in part, on the intensity of the exercise. Following moderate-high intensity exercise there is a greater increase of fat mobilization than with low-intensity exercise. Protein only accounts for 5 to 10 percent (2) of energy production during exercise. Proteins are primarily used for building and repair. However, the body's requirement for energy is higher up the requirement ladder for survival, so protein can be used as an energy source if there are not adequate depths of the other nutrients. At 50-60 percent of your V0² max, fat will provide 50 percent of the energy required. At 70-75 percent of your V0² max, fat only provides 33 percent of the energy required. (3) For example, a study conducted on experienced cyclists by Dr. Jack Wilmore at the University of Texas went like this: One 23-year old subject rode a bicycle ergometer for 30 minutes at 50 percent of V0² max and, on another day at 75 percent of V0² max. At 50-percent intensity, the cyclist burned a total of 222 calories and at 75 percent the cyclist burned a total of 332 calories. The math will show you that 50 percent of 222 calories is 111 fat calories and 33 percent of 332 calories is 110 fat calories. Almost exactly the same amount of fat calories. Double the duration of the exercise bout and you can see it's better to go harder than easier. Progression, and I'll repeat progress, is the key here. The reason for this is EPOC (post-exercise-oxygen consumption). Higher intensity exercise causes great EPOC, therefore burning more calories after the workout. Liken the body's carbohydrate stores to energy "credits". If the stored credits were used during exercise then a "debt" was incurred. The more credits used the greater the debt. The EPOC has to repay those debts by 1) re-establishing the original carbohydrates stores (credits) by re-synthesizing the lactate back to glycogen and 2) catabolize the remaining lactate through the pyruvic acid – Krebs cycle pathway. This phase of recovery, depending on the duration and intensity of the exercise, may take up to 24 hours to replenish. (4) How does this happen and how does this work?
Let's examine how fat is metabolized. For fat to be used for energy, triglycerides stored in the adipose tissue are broken down (Lipolysis) into fatty acids and glycerol. Once in the bloodstream, the free fatty acids (FFA) are attached to a protein carrier, albumin and mobilized to the skeletal muscles. There, they are then oxidized (burned) by a process called beta-oxidation in the mitochondria of the cell. Because this energy system requires so much oxygen the exercise intensity has to be low in order for the pairs of carbon molecules to be beaten away or broken down from the FFA to make acetyl coenzyme (acetyl CoA) which is an intermediate molecule of oxidation and synthesis Of fat. The interesting thing is fat somewhat depends on the continual breakdown of carbohydrates. (5) Acetyl CoA enters the Krebs cycle (aerobic oxidation) by combining with oxaloacetic acid which is generatively primed by carbohydrate catabolism. When carbohydrates levels fall, oxaloacetic acid levels may be inadequate to sustain a high level of fat breakdown. Here, fat burns in the flame of a carbohydrate. As a result of the low carbohydrates stores an imbalance occurs between triglyceride formation and lipolysis caused by low blood sugar. (6) The liver then picks up FFA's and forms Acetyl CoA faster than the liver mitochondria can oxidize it. This compromises the rate which Acetyl CoA can enter the Krebs cycle (aerobic). The excess Acetyl CoA is used to form ketone bodies in the liver. Exercising in this state causes a condition know as ketosis which increases blood acidosis, which can lower exercise performance. Without enough carbohydrates, amino acid (protein) metabolism is increased, which can also be toxic to muscle cells. In summary, it's not a good idea to exercise in carbohydrates depleted state.
Carbohydrates are a readily available, more available source of energy.
When carbohydrates are used to produce ATP, either blood glucose or muscle glycogen (the stored form of glucose or carbohydrates) is metabolized by a process know as glycolysis. Its main function is to regenerate ATP (our most available energy "currency") and to generate pyruvate for oxidation in the ninth and final chemical reaction of glycolysis. When the exercise intensity is too high and the demand for oxygen is greater than the supply (ie, anaerobic conditions), pyruvate is converted to lactate. Lactate may also be used as a source of energy, by converting back into glycogen by a reversal of the reactions of glycolysis or by the liver to make new glucose by a process called gluconeogenisis. If there is enough oxygen available (ie, aerobic conditions) lactate is not formed and pyruvate enters the mitochondria. With or without oxygen carbohydrates can be used for energy, either 1) anaerobically, with the glycolytic pathway repeating or 2) aerobically, with metabolism continuing on to the Krebs cycle. Therefore, carbohydrates are readily available, more available source of energy. Think of carbohydrates as kindling and fat as the log on a fire. The kindling will always burn easier and a lot faster than the log. Total carbohydrate storage capacity in the liver and muscle range from just 200 – 500 grams, (about 800 – 2000 calories of energy). (7) Again, this depends on the individual's capacity for storing carbohydrates. A highly trained endurance athlete will be able to store more in their muscles than an untrained person. This is important. Most people think they'll burn more fat if they do cardio after they strength train. That depends. Keep in mind who you're dealing with. A strength workout is
not likely to deplete carbohydrates stores for a few reasons. First, you're usually resting more that half the time. Secondly, a strength workout is not usually intense enough nor long enough to deplete the glycogen stores. Even if you could deplete your glycogen stores, you would not want to exercise in a glycogen-depleted state for the reasons described earlier. Keep in mind some people are better "fat burners" than others. This has to do with their body composition (body fat vs. lean tissue) and how long and consistent they've ingrained those energy pathways.
Diet will affect which fuel is used.
Fat metabolism is very sensitive to the interval between eating carbohydrates and the onset of exercise. A high carbohydrate meal will cause the pancreas to excrete insulin, which inhibits the breakdown of fat. Here, the body will be forced to rely on carbohydrates for fuel. The body will always go for the kindling before the log. Therefore, if you're trying to burn fat, then carbohydrates should not be consumed within a couple of hours of the activity. If the exercise or cardio session is going to be a long one (1 hour +), than carbohydrates will have to be consumed at some point during the activity. It's the ability of the muscles to oxidize FFA's, rather than the increase in the mobilization of FFA's from adipose tissue. Like anything else, the more you do something, the better you get at it. You must first learn to become a great "fat burner" through adaptation of exercise. Keep the exercise intensity low, so you can learn these energy pathways. Periodization is key here. Give the body time to adapt before you increase the intensity to take advantage of the effects of the EPOC. If you're exercising on an empty stomach in the morning and want to lose fat, go easy and for not longer that 30 – 45 minutes. Then, eat a carbohydrate that is easily digested before you lift weights. Again, it depends on the individual and their ability to store carbohydrates whether or not they'll need to eat before a strength work-out following a cardio session of that length and intensity.
Following an exercise session it is important to elevate glucose as soon as possible to provide a substrate for glycogen synthesis. Glycogen synthesis can occur more rapidly if carbohydrates are consumed within a half hour to an hour after exercise. It helps to have a bagel or sports drink available after a hard work out. Full muscle glycogen replenishment for a competitive endurance athlete requires a diet maintaining of greater than 60% of total calories over a two day period. 8 Also, consider the person and the goal. Endurance athletes such as marathon runners and road cyclists or collegiate wrestlers have a problem keeping body fat. They will not be interested in body composition as opposed to pure weight compared to a body builder. What is the experience of the exerciser, what gender, what age, etc .. Keep in mind these concepts though. An endurance trained athlete relays less on carbohydrates and more on fat as fuel. Fit people are simply more efficient at using fat for energy. A novice exercisers body is inefficient, relying primarily on carbohydrates as a fuel source until the body "learns" to use fat. I hope this helps you reach your fitness goal.
1. Fitness Management, December, 2001
2. Fitness Management, December, 2001
3. Burke, R. Edmund, Velo News, February 23, 1998
4. McCardle, Katch & Katch, Essentials of Exercise Physiology, pgs.70 – 71 Lea & Febiger, 1994
5. McCardle, Katch & Katch, Essentials of Exercise Physiology, pgs.53-54 Lea & Febiger, 1994
6. Fitness Management, December, 2001
7. Newell, Vicki, NCCPT Personal Training Manual, 2000
8. Fitness Management, December, 2001