Starting an exercise program is a necessary part of a healthy lifestyle change.  Granted, this is no major revelation.  Burning more calories than you eat is a fairly obvious recipe for weight loss.  The question then becomes: How should you exercise for maximal fat loss and optimal long term changes in body composition?

Many magazine articles, trainers, coaches, and even the recommended heart charts on exercise equipment all suggest that you should exercise at a moderate intensity for long durations.  Conventional wisdom has said that this is the best intensity for two reasons: because you burn more fat at this intensity and because it avoids the dreaded lactic acid buildup.  This conventional prescription, however, is based on outdated ideas about how the body responds to exercise.

Your body can utilize several different fuel sources; carbohydrates, fat and proteins, and varies the proportion of each depending on the intensity and duration of an activity.  For the first couple of minutes of exercise, your body uses something called ATP – the most readily available source of energy.  But your supplies of ATP are limited. After 2 to 3 minutes, your body switches to carbohydrates stored in muscle tissue. This lasts for 15 to 20 minutes before you switch to fat.  Claims that moderately intense exercise burns more fat are based on studies that showed increased consumption of calories from fat over long workout periods.   The idea was to favor “long and slow” exercise, like a marathoner, instead of “short and fast” exercise, like a sprinter.

Research in recent years, however, is showing that the assumptions behind this recommendation are not entirely correct.  To the extent that a person exercising in this range is burning a greater proportion, the effect is small.  But burning fat during exercise is not the way to achieve long term changes in body composition and become leaner.

Moderate intensity durational exercise just doesn’t stress the energy-supplying systems efficiently.  The body has two paths to turn stored energy into work: the aerobic and anaerobic systems.  The aerobic system uses oxygen to break sugars, fats and protein into energy.  Anaerobic system works when not enough oxygen is reaching muscle cells.  You can get a very high-energy output from this system but not for very long.  When you are using your anaerobic system, you are building up reserve capacity in your heart, expanding your lung volume, triggering the production of growth hormone and melting away fat.  Moderate-intensity aerobic training improves aerobic power without changing anaerobic capacity, but high-intensity training improves both anaerobic and aerobic systems significantly[i].

Other drawbacks with this durational exercise method are numerous.  On a psychological level, this type of moderate exercise is monotonous.  This is important because boredom is one reason that people often give for quitting exercise programs.  If the workout is boring, is it any wonder most people consider going to the gym an unpleasant chore?  Higher intensity effort is more challenging and achieves exercise goals in less time.  More intense efforts also require more concentration, creating more interest in the workout.

The most important limitation is that, like crash dieting, “long and slow” exercise can have a counter-productive effect.  Drawing preferentially from fat stores tells your body you need that fat.  The body replenishes these “necessary” fuel stores the next time you eat, and becomes more efficient at both using and maintaining fat.  Top-level endurance athletes expend enough total calories to overcome this handicap, but most of us find ourselves fighting an uphill battle.  Significantly, although they may not accumulate fat, it has been noted that the muscles of marathon runners actually shrink. When the muscle biopsies of marathon runners were analyzed, researchers found their muscle fiber size had decreased and atrophied.[ii]  One has only to look at elite marathon runners to see the paucity of their muscle mass, which can accelerate some negative effects of aging.

We see patients actually gain weight in many long duration, moderate-intensity exercise programs.  Short bursts of exercise tell your body that storing energy as fat is inefficient, since you never exercise long enough to utilize the fat during each session.  Carbohydrates, which are stored in muscle rather than fat, burn energy at high rates. Exercising for short periods will use these carbohydrates and burn much more fat after exercising while you replenish the carbohydrate stores. Short interval exercise maximizes fat “after burn.”  High-intensity exercise stresses both the aerobic and anaerobic systems[iii].  By doing so, it forces the body to recover and rebuild once the effort ends. In fact, studies show that high-intensity short duration exercise increases fat oxidation long after the exercise is done[iv].   The most important effects of exercise occur after, not during, your exercise session.  If done correctly, it can affect your metabolism for several days afterward.

Prolonged endurance exercise causes inflammation and generates free radicals, which cause damage to cells and possibly accelerate aging.   Excess free radicals are especially damaging because they directly attack DNA, which can cause mutations, cell death, or cancer.  Free radicals also cause other unwanted reactions leading to cell damage, such as breaking down cell walls, interfering in protein synthesis, and more.  In such ways free radical damage, also called “oxidative stress” is involved in diseases like atherosclerosis[v] and liver disease[vi]. Free radicals produced by “long and slow” exercise, such as a marathon, can also damage “good” cholesterol (HDL) for up to 4 days after the exercise is over, further increasing heart disease risk[vii].  It is not uncommon for a marathon runner to collapse and die suddenly at an endurance event.

Other unwanted effects of prolonged moderate intensity exercise include things like destruction of bone mass.  Male long-distance runners have lower bone mass and higher bone turnover than control groups, which indicates bone loss[viii].  Another study of male long-distance runners showed that they had lower levels of testosterone and higher levels of the stress hormone cortisol, and that these levels did not return to normal after a break in training[ix].

With all these dire effects, it may be easy to conclude that exercise can do almost as much harm as good.  That isn’t the case at all, of course.  The right exercise for the right person is still an effective part of the lifestyle changes that will help your overall health.  What each person making such changes needs is an exercise program that treats them as an individual.  Books, health magazine articles, etc. give recommendations based on what works for large groups, but what if you do not conform to the “average” person?

The pros and cons of different interventions, and combinations of interventions, should be weighed and balanced.  What we need, in essence, is an exercise prescription.

Fortunately, there are professionals that can help cut through this morass of information.     Once a plan is in place, committing to long-term changes is a key attitudinal adjustment.  By focusing on making changes to the way we treat food and exercise instead of “going on a diet” we are much more likely to be successful.

[i] Tabata I. Nishimura K. Kouzaki M. et al.        Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max. Med Sci Sports Exerc. 1996 Oct;28(10):1327-30.

[ii] Trappe S, Harber M, et al. Single muscle fiber adaptations with marathon training. J Appl Physiol, 101:721-727, 2006.

[iii] Tabata I. Irisawa K. Kouzaki M.     Metabolic profile of high intensity intermittent exercises. Med Sci Sports Exerc. 1997 Mar;29(3):390-5.

[iv] Lee YS. Ha MS. Lee YJ. The effects of various intensities and durations of exercise with and without glucose in milk ingestion on postexercise oxygen consumption.J Sports Med Phys Fitness. 1999 Dec;39(4):341-7.

[v] Victor VM. Rocha M. Solá E. et al. Oxidative stress, endothelial dysfunction and atherosclerosis. Curr Pharm Des. 2009;15(26):2988-3002.

[vi] Muriel P. Role of free radicals in liver diseases. Hepatol Int. 2009 Nov 26. [Epub ahead of print]

[vii] Liu ML. Bergholm R. Mäkimattila S. et al. A marathon run increases the susceptibility of LDL to oxidation in vitro and modifies plasma antioxidants. Am J Physiol. 1999 Jun;276(6 Pt 1):E1083-91.

[viii] Hetland ML. Haarbo J. Christiansen C. Low bone mass and high bone turnover in male long distance runners. J Clin Endocrinol Metab. 1993 Sep;77(3):770-5.

[ix] Houmard JA. Costill DL. Mitchell JB. et al. Testosterone, cortisol, and creatine kinase levels in male distance runners during reduced training. Int J Sports Med. 1990 Feb;11(1):41-5.