Taken from article written by Martica Heaner, M.A., M.Ed., for MSN Health & Fitness
When your body moves (or any cell in your body does any type of ”work”) it needs energy. The amount of energy needed is measured as a unit of heator a calorie. The fuel to produce this energy comes from several sources, mostly fat and carbs (glucose), and occasionally amino acids (protein). How and when fat is ‘”burned” (or metabolized to provide energy for the body) and how that affects body fat levels and weight is a very complex area of physiology research. There have been hundreds, maybe thousands, of studies exploring the utilization of fat for energy under a variety of different conditions. There is still much to be understood, but this is how we know it works so far:
Whether you are watching TV or running around a track, the fuel your body uses to give you the calories your cells need for energy comes from burning mostly fat and carbs. Your body nearly always burns a mix of both fat calories and carb calories. So normally, for every calorie burned, the fuels are around a 50/50 split of both fat and carbs.
How hard you are moving during exercise is one major determinant of which fuel your body will use. Carbs provide a faster energy source. So whenever you need to do something fast or produce force, carbs are the better fuel. Fats are favored during long, low-intensity activities. It’s not that you stop using one or the other fuel, it’s that the ratio of both shifts depending on your activity. In more scientific terms, you alternate between aerobic (more fat-burning) and anaerobic (more carb-burning) metabolism.
Losing Weight by Burning Calories
When it comes to weight loss, it really doesn’t matter whether you are more or less fat burning. It doesn’t matter what your calories are made of, but it does matter how many calories you burnand the more the better. So when you are sittingand burning more fat–you are burning only about one calorie per minute. Clearly, even though you’re in a greater fat-burning state, no one ever lost weight by sitting! (How many calories you burn depends on many factors, including how heavy you arethe more you weigh, the more you burn.)
You do burn less fat when you work anaerobically, but it doesn’t matter because you are burning more total calories. You will always burn more calories the longer or harder you exercise, no matter what your intensity is. So doing cardio for only 15 minutes makes no sense unless you are short on time. Burning BOTH fat calories and carb calories can result in fat loss or pounds off the scale.
The Ideal Amount of Cardio
How much exercise you need to do depends on your goal. Even small amounts of exercise are great for your health. Just moving a little every day can improve metabolic functions, such as how sensitive your body is to insulin and how it reacts to excess blood sugar. Exercise can also reduce your risk of heart disease and other chronic conditions. That’s why the U.S. Centers for Disease Control and Prevention and the American College of Sports Medicine recommend that all healthy adults accumulate 30 minutes of moderate intensity activity on most, if not all, days of the week. Although all types of exercise count, generally the recommendations reflect more cardio activity than lower-calorie-burning moves like strength work or stretching.
The big question is whether you’ll drop weight from this amount of exercise. The answer is that some people can. If you work a little harder in that 30 minutes, you will burn more calories and that will have a bigger effect on weight loss. If you have not been active before, then doing 30 minutes of low-intensity activity would still probably help you lose weight.
The less cardio activity you do (or the fewer calories you burn per workout), the slower the weight loss, and that’s because it’s a numbers game. In 30 minutes of moving around, you might burn 150 to 300 calories. But, theoretically, it takes burning around 3,500 calories to lose one pound of fat. So, you’re going to have to do enough workouts, at say 150 calories burned per session, to add up to 3,500 calories, or 17,500 calories for five pounds of fat, and so on.
The problem is that most people give up if they don’t get concrete results fast. Also, different people have different physiological makeups. So this theoretical caloric equation may not work perfectly in every body. That’s why more realistic fitness guidelines for losing weight or maintaining weight loss have been established. The Institute of Medicine and the USDA’s Dietary Guidelines recommend that a person do 60 to 90 minutes a day of accumulated moderate-intensity physical activity at least five days per week.
Can Exercise ‘Burn Muscle’?
This claim is slightly misleading. You certainly do not burn muscle after only 15 minutes of exercise. You start using protein (perhaps from muscle, as well as other amino-acid-containing components in the body) for energy under extreme conditions such as starvation and at the tail end of long, hard endurance events where carb stores are in short supply. The average person who works out does not need to worry about this.
Will More Muscle Rev Up Your Metabolism?
In short, probably not. The average person won’t lose muscle from doing cardio and won’t burn more fat by building more muscle, mostly because one has to eat more than usual (not diet) and lift seriously heavy weights to build more muscle. And even then that person probably won’t build enough muscle to make a difference. Dr. Joseph Donnelly and other leading exercise physiologists conducted a comprehensive review of all the research on exercise and weight loss for the American College of Sports Medicine. While resistance training was recommended for its beneficial role in potentially improving muscle strength and power, the physiologists found no evidence that increasing muscle mass enhanced weight loss, especially when combined with dieting.