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How to 'fuel' the fire

written by Marsha Seidelman, M.D.
on Friday, 21st June ,2013

                                              How to 'fuel' the fire

I've always found it intriguing that the ellipticals and treadmills at the gym give options for working out in the 'fat burn zone'. Isn't all exercise meant to burn fat? And why would you try to stay at a lower intensity of exercise, where the 'fat burn zone' is, if you were trying to burn fat? They usually suggest that this zone is at 40% of your maximum VO2, which for our purposes today will be 40% of what you sense is your maximum exercise capacity.

To help explain why they suggest this we need to get back to basics. We'll look at a simplified version so it doesn't feel like we're in Biochemistry 101. As you may know, there are three major components of foods, carbohydrates (or carbs), fats and proteins. I'll only say a few words about proteins, since they are a source of energy only after hours of exercise- and even then account for only about 5% of the total. I'll concentrate on carbs and fats, since they supply the majority of fuel for your muscles.

Proteins are made up of amino acids and provide the building blocks for your muscles. They are essential in our diets. Without them, our muscle cells would break down and we would lose muscle mass and strength. You may have seen this happen in people with marked weight loss who have inadequate protein intake. It helps explain why people who have been bedridden with serious illnesses and poor nutrition can't even stand up when they're otherwise ready to leave the hospital.
In general, protein intake should be about 0.8 to 1.0 grams/kilogram of body weight per day. So a 150 pound or 70 kg person should eat about 56-70 grams of protein daily. It should be more with increased exercise, although how much it should increase is controversial. Good dietary sources of protein include nuts, beans, tofu, fish, lean meat and certain grains, like quinoa and buckwheat.

OK - onward to the primary fuels for exercise - carbs and fat - again taking advantage of simplified explanations. Carbs are stored in three forms - liver glycogen, blood sugar and muscle glycogen. Glycogen molecules are large chains - sometimes up to thousands - of the simple sugar glucose. Initially, with exercise, the sugar in the blood is used for energy, and then glycogen stored in the muscles; meanwhile, the glycogen in the liver is broken down to glucose to re-supply the sugar in the blood so you don't get hypoglycemic and feel like passing out.

Fat, as we've mentioned in prior entries, is made up of excess energy (known as calories!) that we eat. 3500 extra calories in is stored as 1 pound of fat. Fat is stored in fat cells (adipocytes), and somewhat in muscle, as triglycerides (TG). Think of the structure of TGs as the capital letter "E".
The backbone is glycerol and the three 'arms' are free fatty acids (FFAs). These FFAs can be burned for energy - not only when you're in the 'fat burn zone'.

Your use of carbs versus fat for fuel is determined by your diet, your stores of glycogen and the intensity and duration of your exercise. With lower intensity exercise, like walking, you're likely to use more fat than carbs, percentage-wise. So at 20% of maximal energy use, not working too hard, you might get your energy 60% from fat and 40% from carbs. Because of the 'slow' muscle fibers and the hormone status in your body, you would preferentially be breaking down fatty acids instead of glycogen. With higher intensity exercise - say at 60% of your maximum - you might use the opposite combination, 60% carbs and 40% fat. You'd be using more 'fast' muscle fibers and your epinephrine would be reacting to your exertion, both contributing to more use of sugar rather than fat. But realize that the fat burn calories are 40% of a higher number. I should mention that during prolonged exercise of moderate intensity (40-60% of your maximum for over an hour), there is a return to increased reliance on fat.

Let's look at an example using these numbers. If you burn 200 calories at low intensity exercise, you will burn 120 calories from fat (60% of 200). If you 'turn up the heat' and exercise harder, you might burn 300 calories, with 120 calories from fat (40% of 300). You've burned the same number of fat calories, but more calories overall. If you can continue to exercise at a higher intensity, or with intervals , you will burn more total calories and more fat calories. In terms of weight loss, a calorie is a calorie - better to burn more with a given duration of exercise. When you're early on in your training, you will need to stay at a lower intensity in order to spend a long enough time exercising to burn a significant number of calories. Later on, you may not want to stay on the bicycle, elliptical, treadmill, whatever, for longer and longer durations (even though you could), so you can step up the intensity, exercise for a shorter time, and still 'burn fat'. Stay tuned for a future entry on 'interval training' to get the most out of your workout time.

Now you can understand what it means to 'hit a wall' in your workout - that feeling that your legs are like jelly and you can't continue. Assuming you don't have chest pain, shortness of breath, or other medical reason to be concerned, it often means that you've used up your glycogen stores - there isn't any instant fuel available to your muscle cells. A snack bar containing carbs should help within the next few minutes. Eating this before you 'hit a wall' will allow you to work out longer and more comfortably. The snack that works best varies from person to person, and can change over time. I find that a large glass of water and either oatmeal with nuts and berries or an apple with peanut butter about 30-60 minutes before a workout works well. Some people can't tolerate more than 100 calories or so in that time frame. Then, to avoid bottoming out, I use a Clif KidZ Bar or a Health Valley Oats and Honey Bar after an hour or so. Next month, it may be something different. Finding a good snack and the best amount for you is just trial and error.

Also relating to glycogen stores, you may have heard of 'carb loading' before a big race. There is no consensus about how many days before the race to start, nor how much to load. The purpose of it is to maximize the glycogen stores that will provide fuel for the first hour or more of the race. Not just for marathoners, but for all of us, it is important to eat carbs after a big workout to replace those stores so you can exercise longer the next time. Whether you try to exercise in the 'fat burn zone' or not, your glycogen stores can be the limiting factor in your workout. Adequate nutrition overall, with a good balance of fats, carbs and protein is essential to feeling energetic, building muscle, and feeling motivated to advance in your exercise program. More on all of that to come ----

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