November 1, 2005

Fat Metabolism & Exercise Performance

BY David Bohn ("Fitness")

As we enter the holiday season, diet (what we eat, not necessarily how we restrict our food intake) and exercise will play an even greater role in how we look this holiday season. While people traditionally say, “winter is when I put on my winter weight,” who’s to say that added weight is necessary? And as many people know firsthand, as we age, the more you play that yo-yo game with your weight, the harder fat is to remove once it has been put on. For those over 35 especially, who has noticed that all of a sudden the brakes have been put on their metabolism? Because the body does change, whether we like it or not, it becomes ever more important to be conscious of our bodies and its use of energy. To explain this change I am going to talk about body composition for a bit and then outline a program for keeping the body in shape for the holiday season and beyond.

Men and women obviously are physically different and this extends down to how fat is stored in the body. As most are aware, men hold on to android (abdominal) fat, women have gynoid (thigh) fat. Fat cells have receptors built into them which signal them to either be stored or released as energy. Alpha receptors inhibit lipolysis (splitting of fat), beta receptors stimulate lipolysis. Women unfortunately have more alpha receptors than men, particularly in the thighs and they also have more of what’s called lipoprotein lipase (LPL – another hormone that signals the storing of fat).

While both men and women have these receptors and enzymes, how and what we eat and what we do during our day can affect how much of those hormones our bodies put out. Being under constant negative stress (work, bills, relationships, etc.) with no physical outlet stimulates the production of cortisol; another hormone that inhibits lipolysis. As fat is signaled to deposit in the midsection, it becomes more visceral; or internal, which is deadly because it can surround the organs. Further, visceral fat is highly associated with all causes of mortality because it is so metabolically active; it is more receptive to exercise than subcutaneous fat. This means that as more fat is released into the bloodstream, if it is not quickly utilized by the body for energy then it can deposit on arterial walls and lead to heart attacks, clots or strokes. So, while exercise helps, it’s best to not accumulate excessive fat in the first place.

Caloric restriction dieting is not the answer either because such dieting suppresses the body’s resting metabolic rate. If a person starves the body, the body responds by lowering its caloric need so that he or she will burn fewer calories at rest. This response is designed to make the body survive longer on less sustenance and the body does that by shedding muscle; one of your primary calorie burners. This explanation is why weight training, combined with cardio, is so important to maintaining optimal weight. In a recent study, people who dieted only and lost weight lost 28% due to lean muscle mass, those who did additional cardio lost only 3-4% and those who did resistance training lost 0% from lean muscle mass.

With that brief explanation of the mechanisms of fat and why dieting in the traditional sense does not work, how do you go about shedding that extra insulation while sparing your lean mass? Besides weight training (obviously), there are different ways to do cardio to burn those extra calories and some of those modalities depend upon your current physiology. This is where the bad news comes in for people who are currently overweight or obese because those individuals will have a harder time losing excess fat once it is already on and this fact is why it is so important to not add fat to the body in the first place.

While there are exceptions to every rule, in general obese people are less efficient at burning fat at resting rates than people who are not obese. This statement relates to the genetic coding of the mitochondria, the “fat burning” cells of the body. In lean individuals, the mitochondria through training and genetics simply burn more fat at rest than obese people. Therefore, when lean people do cardio, they can actually workout at any intensity they desire. I say that because we have all heard that “fat burns in the aerobic flame,” meaning, lower intensity = more fat burned. This is true…. to a degree. When your heart rate is 70% of max and down, the body burns the highest percentage of fat for energy. However, one does not necessarily burn the most total calories in the aerobic training zone. Let me show an example. Person X works out at 70% for 30 minutes and burns 250 calories, 60% are fat or 150 fat calories. Person Y works out at 85% for 30 minutes and burns 450 calories, only 40% are from fat or 180 fat calories. Now, does that make sense why high intensity exercise is so beneficial? Any way you slice it, high intensity is better than moderate to low intensity; the primary concern with always doing high intensity exercise is burnout (mental) and overtraining. Those risks are why it is still good to vary your workouts.

Since we know high intensity exercise is so good, why cannot people who are overweight just workout at a high intensity all the time just like everybody else? The reason comes back to their body’s efficiency at utilizing energy while exercising. Because lean people tend to be (though not always) more aerobically efficient than overweight people, they do not have to train their bodies as much at the cellular level to burn fat as the primary source of energy. When overweight people exercise, their body’s tendency is to use carbohydrate first because it is a faster energy source than fat. Once the carbohydrate has run out, the body quits operating at its current efficiency; this is when all exercisers experience “the wall.” Marathoners who don’t plan correctly know this feeling well, right about mile 22, yes? Basically the body says, “well, no more fuel, so I’m done moving for the time being.” This efficiency, or lack thereof, is in the mitochondria, and to improve one’s fat burning cells to burn more fat, both at rest and during activity, one has to train them. How is this done? Through aerobic cardiovascular exercise.

Wait, didn’t I just say you can workout at any intensity and burn fat? No, read closely - aerobic cardiovascular exercise. In all but elite athletes, people’s aerobic capacity maxes out around 75%. Go above that rate and the body begins to burn more carbohydrate because it has to go anaerobic; without oxygen (and without mitochondria). Therefore, overweight people need to stay in their aerobic training zone more than lean people to train their bodies to use fat as the primary fuel source. All this means is that their cardio sessions will have to be longer to get the same caloric benefit as an all high-intensity workout. Now, this is not to say that overweight people should never do high intensity exercise, nobody gets off that easy! The program that they should follow should be 80-10-10; 80% of the time in their aerobic zone (you can comfortably talk), 10% moderately uncomfortable (can talk briefly), and 10% in the NO-FUN-I-HATE-EXERCISE zone. Furthermore, in addition to working in different zones, people should use different modalities (the treadmill, the pool, the Precore, bike, etc.); variety keeps the body guessing and less able to adapt and become calorie-efficient at exercise (the one time in our lives we don’t want to be efficient with our time!) How much of this fun cardio-work are you supposed to do? For people looking to just maintain their weight, 150 minutes a week. For people desiring to lose weight, 200 – 280 minutes a week of cardiovascular exercise is needed. I know that is a lot of time, but the workouts do not have to be in 4 giant chunks; the total effects just need to be cumulative. It is a daunting task indeed, but with 65% of the American population officially overweight, it’s time fight the second helping and workout instead.

It can unfortunately take up to 12 months for the body to respond at the cellular to exercise change, so don’t give up after 6 weeks. Remember exercise is a lifestyle; plan to do it ‘til you die, so what’s a few more weeks? Work on all three areas; diet, weight training and cardiovascular and you won’t need to hide underneath any large bulky sweaters this holiday season.