For the Best Gains, Make Moderate the Goal

Fri, 4 Nov. 2011 - 6:34 p.m. MT
Credit: ARA Staff - American Running Association

Workout recommendations for both dedicated distance runners and the novice enthusiast share a common theme: on most days, favor a moderate level of exertion. Whether your goal is to set a new marathon p.r. or just shed a few pounds for summer, the lion’s share of your activity should occur at this level of effort. Why is this? What is the physiological mechanism at play here that makes moderate the great universal aim?

For starters, let’s look at the way a distance runner uses the various energy systems. As a seasoned runner, gradually building up your training to include vigorous intensity workouts will improve your oxygen consumption, strength, power, coordination, and lactic acid threshold—all of which will make you faster, leaner, and tougher in the face of fatigue. But other concerns, such as the way our bodies use stored energy and from what sources, are still best addressed with moderate exercise over vigorous.

Exertion Level                                                           Signs

Light (25% maximal)                                                 normal to slightly increased breathing
Moderate (50% maximal)                                           deeper breathing and sweating
Vigorous (75% maximal)                                            gasping, heavy sweating
High (85% maximal)                                                  gasping, muscle fatigue, cramping

Looking to P.R.? Any runner, seasoned or not, burns carbohydrate, fat, and protein simultaneously when exercising—and this is also regardless of the intensity of the workout. Similarly, you use all three primary energy systems: the immediate, the anaerobic, and the aerobic. However, as the intensity and duration of your activity changes, the usage percentages of each system shift—just as the relative amounts of fat, protein, and carbohydrate you burn also shift.

In distance running, in general, the amount of protein that’s burned is negligible, and mainly for the production of enzymes that facilitate the burning of carbs and fat. The latter two energy sources will therefore be our focus. For a discussion of protein’s somewhat complicated role in running nutrition, see Protein: How Much is Too Much? in this issue.

The energy system with the least undesirable byproduct is the aerobic system, which of course utilizes oxygen. While the immediate energy system is also clean, it can only supply energy for about 8 to 15 seconds and so, like protein, we will not dwell on it here. The anaerobic system gives you the power to sprint or to push a heavy weight, but it’s self-limiting and creates the byproduct lactic acid. Distance training, then, should favor use of the aerobic system.

As an athlete, as opposed to someone with a weight-loss goal, even though your goal is not primarily related to burning fat, do recall that the goal of any endurance training is to increase the work your aerobic engine can do without increasing your intensity. And so you want to increase the training load provided to the aerobic system. This means moderate—and sometimes vigorous—exercise will always win out over low- and high-intensity exercise.

As we will detail below, low-intensity work doesn’t increase the aerobic training load enough. On the other side of the equation, if you get up to 85 percent maximal effort, your anaerobic system has taken over. For some people this occurs at as low as 70 percent maximal effort. You will fatigue as the lactic acid in your blood cannot be cleared as quickly as it is produced, which in turn will shorten your aerobic training not only by the very definition of anaerobic threshold, but also because fatigue will cut the workout short.

Looking to get moving? For people simply looking to burn fat, health gains may seem more out of reach but they are even easier to attain on at least one level: any increased energy expenditure yields results. Unlike the seasoned marathoner who is pleased to shave just seconds off her race time, the previously sedentary exerciser has greater latitude in what they do to see gains. To lower all-cause mortality, lessen cardiovascular disease risk, and decrease incidents of type-2 diabetes, moderate exercise will do the trick. Moderate exercise has been shown repeatedly to be as effective as vigorous exercise for reducing blood pressure. In short, physical activity does not need to result in a training effect to elicit a health benefit.

Given these facts, why not favor light exertion? And isn’t that the level at which fat is mostly metabolized anyway?

With light exercise, your body does burn a higher percentage of fat than it does carbohydrate. This is surely why people sometimes confuse light exertion as ideal for fat burning workouts. It isn’t, and the reason is this: while the ratio of fat to carbohydrate use is highest in the modest 20 to 25 percent range of maximal effort, the absolute amount of any energy being used is much smaller than for moderate exercise (45 to 65 percent of maximal effort). If your goal is to lose fat, therefore, light exertion is not a good way to reach it. When you increase your intensity to 45 to 65 percent of maximal effort, you burn more total calories and more fat than you do going lightly, even though fat relative to carbohydrate burn is percentagewise higher under light exertion.

Another important point is that in order to burn any fat at all, your system needs carbohydrate present. The amount of fat you burn decreases with the depletion of carbohydrate over time. Because fat can only be burned aerobically but carbohydrate can be burned both aerobically and anaerobically, as intensity increases (thereby moving you closer to your anaerobic threshold), the amount of carbohydrate you burn increases and the amount of fat you burn decreases. Also, you require energy more quickly than it can be liberated from fat—a good source of energy, but not an efficient one.

Offset intensity with duration. Energy expenditure—the linchpin of fat reduction—increases with walking speed. Yet regardless of self-selected walking intensity, to some extent, exercise duration offsets intensity. In one study, the post-meal storage of fat in subjects was reduced by the same amount with 180 minutes of exercise at 30 percent VO2max as it was with 90 minutes at 60 percent VO2max. Another study looked at insulin sensitivity in diabetic women and found a similar trade-off between intensity and duration.

Furthermore, the hazards of physical activity, including injury and, rarely, cardiac events, appear to be associated more with the intensity than with the duration or frequency of the exercise. For example, in the 2002 Aerobics Center Study of over 5,000 men and nearly 1,300 women, injury prevalence in walkers was found to be similar to that of sedentary people. Increased walking duration did not increase injury risk. Not surprisingly, those unaccustomed to intense physical activity are at the greatest risk. So as a practical matter, public health recommendations target sedentary individuals, and stress gradual increases after an exercise regimen is in place. An injury early on may, after all, do much to discourage beginning regular exercise in the future.

If someone you know is thinking about getting moving, encourage them by setting regular walking dates. And remember that several 10-minute bouts of walking in a day offer nearly all the health benefits of one 30-minute bout. Work schedule and beginning fitness level may make one plan more feasible than the other, but you needn't worry which. Just urge friends and family to first see their doctor before beginning any regular physical activity.

Food for Fitness by Chris Carmichael, 2004, G.P. Putnam’s Sons, New York, NY, pp. 20-32

Physical Activity and Health: The Evidence Explained by A. Hardman and D. Stensel, Routledge, London, 2003, 289 pp.

Med. Sci. Sports & Exerc., 2002, Vol. 34, No. 5, pp. 838-844

Exercise: A Program You Can Live With, Harvard Health Publications, Feb. 2002

(RUNNING & FITNEWS®  May / June 2011 • Volume 29, Number 3)

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