As Krista stated a few weeks back, we're constantly using the food we eat (fats and carbohydrates) as fuel, both during exercise and at rest. Understanding how our body uses that fuel can make a huge impact in knowing how to tailor exercise to create an effective weight loss program.
During lower intensity cardiovascular exercise, we mostly burn fat. However, we can't burn huge amounts of calories in short periods of time at low intensity. Contrarily, during higher intensity cardiovascular exercise, we use a much higher percentage of carbohydrates, but we also use all of the fat that is expended at the lower intensities as well. Therefore, we burn more of calories. During resistance training, we utilize a much larger amount of carbohydrates, even at moderate intensities, and the maximum amount of carbohydrates at higher intensities. So, how can we use this information to our benefit?
Theoretically, if we exercise at maximum intensities for extended periods of time, we would be using a large amount of carbohydrates, as well as fats. In this way, we burn the most calories and therefore lose the largest amount of weight loss. Unfortunately, this doesn't always work, because of many reasons.
First of all, exercising at high intensities for extended periods of time can be a hassle and is certainly not enjoyable day after day. Secondly, exercising this hard, coupled with resistance training can use a large amount of the carbohydrate stores. This, in turn, lowers blood glucose, which is a good thing for people with prediabetes or diabetes, but not good for those of us with normal blood glucose. Low blood glucose can cause hunger, which we all know is difficult to ignore. The hunger associated with low blood sugar usually results in a person eating all the calories back in a desperate frenzy. Lastly, using such high amounts of carbohydrates causes muscular fatigue and lowers the pH of our body, which results in even more fatigue. Therefore, even if we wanted to exercise at high intensities every day, our bodies could not handle it due to soreness and fatigue.
The alternative option is to exercise at lower intensities and just burn fats, but this isn't a realistic option. Most of us don't have three hours to spend on the exercise it would take to burn the amount of calories required to lose weight.
Armed with this information, how can we make the most efficient workout for weight loss?
The answer is that everyone is truly different, and the workout that works for YOU is the best! Don’t trust anyone that says they have the perfect exercise program if they don’t know the physiology of your body. Everyone is different, and their bodies use different amounts of fuels for different exercise intensities.
That being said, a very effective approach for new exercisers is to start slow. This will make things manageable in the short term and reduce muscular fatigue and soreness. As you become acclimated, work towards a moderate intensity program for large portions of your workouts. Moderate intensity will burn fat, of which we have an essentially endless supply (~20,000 calories). Then, integrate intervals which burn high amounts of carbohydrates for shorter periods. This causes the body to burn large amounts of calories, along with forcing the body to adapt to a higher intensity exercise.
Physiologically, the reason our bodies can adapt to this high intensity is due to increased capillarization: the increased ability of the muscle to remove wastes, along with increased mitochondria. In layman’s terms, mitochondria are energy factories within every cell in the body, with the sole purpose of using oxygen to convert food to fuel for cells. By increasing mitochondria, we have a greater capacity to utilize fats instead of carbohydrates, even when exercising at higher intensity for longer periods of time.
The bottom line is that a balanced approach to fitness will yield results without being so intense that you hate it. I've had clients who hated exercise and hated to be pushed. But, because we started slow, they can do things today that they never imagined possible. And that continued participation yields results 100% of the time.