Burn After Running

Exercise can seem like a long-tail strategy. We assume we must develop patterns of activity that can be sustained for years to ensure health benefits. That is certainly true, yet the acute effects of exercise should not be overlooked because they can serve as motivational reminders when willpower recedes in the face of the frustratingly longer view.

Another important point is that exercise is the gift that keeps on giving, long after the workout session is completed. These two ideas combine powerfully to help keep us on track. We will see here that the decision to exercise or not is far from a zero-sum game. The difference between participating in a session and remaining sedentary, on any given day, has more resonant consequences for good or ill than you might have considered. Three aspects of human health are affected in particular long after—not to mention during—a run, walk, ride, swim or other workout session. These are metabolic rate, blood pressure, and fat burning.

Metabolic Rate
Exercise raises metabolic rate, helping to prevent weight gain. For up to half a day following exercise, oxygen consumption remains elevated. Post-exercise, the body is consuming more oxygen in part to lower body temperature and in part to replenish muscle and liver glycogen stores. The duration of this elevated oxygen consumption is positively correlated with the intensity and duration of exercise.

Elevated post-exercise oxygen consumption is measured immediately and not, as with resting metabolic rate, after an overnight fast and 8 hours of sleep. It therefore offers an important glimpse into exercise's acute effects. People who exercise twice a day enjoy an almost permanent elevation in metabolic rate due to the acute effects of frequent exercise bouts.

Blood Pressure
During endurance exercise with the body's large muscles the cardiac output increases by several times its normal output. The dilation of the blood vessels that occurs to accommodate this attenuates the increase in blood pressure that would otherwise result.

The great benefit to blood pressure comes after exercise: during recovery, the cardiac output immediately falls back, but the blood vessels continue to remain dilated for several hours. One study found that 13 hours after just 30 minutes of low-to-moderate exercise, systolic blood pressure in moderately hypertensive men decreased by 6 mm Hg, and diastolic by 9 mm Hg. Mean arterial blood pressure reduced by 8 mm Hg.

Like the effect on metabolic rate, the astonishing duration of this positive effect implies that someone who regularly and frequently engages in endurance exercise may well spend most of their lives in a state of post-exercise hypotension.

Fat Consumption
Insensitivity to insulin ultimately determines whether a person has impaired glucose regulation, leading to diabetes and other major health problems. The lack of insulin's effect on glucose disposal is termed insulin resistance syndrome (or metabolic syndrome). Many of the abnormalities associated with insulin resistance syndrome can be linked to things going wrong metabolically after a meal. After eating, triglycerides in the blood are elevated; over time, as this increase in fat in the blood repeats day in and day out, low HDL levels and high concentrations of small, dense LDL particles can result. When triglycerides are high, then, blood has a propensity to clot.

Exercise does wonders for lipid metabolism—and the results are not only long-term but immediate. Post-meal blood fat concentrations are lower in endurance-trained athletes than sedentary people. As mentioned in previous issues of this publication, one study found the rise in triglycerides in endurance athletes to be 43% lower than sedentary subjects after consuming a McDonald's breakfast.

What is particularly noteworthy in the discussion of exercise's acute effects, however, is that after interrupting training for just two days, the metabolic response to fat intake was compromised by 45%. As with the positive effects on metabolic rate and blood pressure, the characteristic ability of trained bodies to respond aggressively to fat intake by metabolizing it quickly and efficiently seems to be due in part to the fact that they have always recently just exercised.

Another study looked at middle-aged men and found that after walking briskly for 90 minutes they improved their insulin sensitivity and increased whole-body fat oxidation by up to one-third. This, again, is after a single session of moderate (60% VO2max) activity.

The science makes it clear that we enjoy benefits hours after completing exercise, another reason it continues to reveal itself as an ancient, simple and elegant solution to a vast array of modern health concerns.

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

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