Health&Nutrition

THE SCIENCE OF RUNNING: Weight Loss, Stretching and Pollution

June 25, 2010
By Alex Hutchinson

Keeping weight off: go harder rather than longer

When you lose weight, your body responds with an array of physiological and behavioural tactics to burn fewer calories and push you back to your starting weight. According to a study in the January issue of the American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, one of the key elements in this conspiracy is a change in the efficiency of your muscles. It’s not just because they have less weight to carry around - it appears to result from changes in the ratio of enzymes that determine whether the muscle burns carbohydrate or fat.

In a long-term study where volunteers were checked into an in-patient facility and fed only a liquid diet for months at a time, researchers from Columbia University found that when the subjects lost 10 per cent of their total body weight, their muscles became about 15 per cent more efficient. Better efficiency may sound like a good thing, except that it means you burn significantly fewer calories when you move around, which pushes your weight back up. The opposite happens when you gain weight: your muscles get less efficient.

Since the ratio of carbohydrate-to-fat utilization depends on the intensity of physical activity, there’s reason to believe that the efficiency changes are only relevant at very low intensities - equivalent to the activities of day-to-day life as opposed to exercise. As a result, the researchers suggest, “the weight-reduced individual might ‘escape’ this increased efficiency by altering the intensity of exercise.” In other words, exercising harder rather than longer might be the most effective strategy for keeping weight off.

How often (and quickly) should you eat?

A long-standing staple of dietary wisdom is that you should eat frequently rather than cramming all your calories into three big meals. The idea is that you prevent large hunger swings, possibly by keeping the levels of appetite-determining hormones in your gut relatively constant. Studies over the past half-century have reached conflicting conclusions about whether this actually works. Most recently, researchers at the University of Ottawa put 16 obese volunteers on diets with identical caloric deficits for eight weeks. Half of them ate three meals a day, while the other half ate three meals plus three snacks. The results, published this year in the British Journal of Nutrition: no difference in weight, dietary compliance or gut hormone levels.

That doesn’t mean you can’t influence your gut hormones. Another study, published by Greek researchers in the Journal of Clinical Endocrinology & Metabolism in January, had volunteers eat identical bowls of ice cream either in five minutes or in 30 minutes. The slow eaters displayed higher levels of peptide YY and glucagon-like peptide, two hormones that signal you to stop eating.

The myth of the fat-burning zone

The “fat-burning” zone is familiar to anyone who uses cardio equipment at the gym. Don’t go too hard, the machines warn, or you’ll burn carbs instead of fat. This premise has several weaknesses, including the fact that it ignores the question of how much energy you burn in total. There’s not much point in burning 80 per cent fat if your leisurely stroll only burned 50 calories in total. But a new study from researchers at the Garvan Institute of Medical Research in Sydney suggests that the focus on fat-burning suffers from an even more basic problem.

In a study published in Cell Metabolism, the researchers genetically altered mice to burn more fat instead of carbohydrates. They found that the mice simply ended up converting unburned carbohydrates into fat for storage, so there was no overall difference in weight or energy expenditure compared to control mice. “Your body will use the energy it needs and store the leftover fats, proteins or carbohydrates as fat,” explains Greg Cooney, the senior author of the study. “When you do the sums, it’s ultimately a matter of calories in and calories out.”

Rethinking stretching

The hallowed tradition of pre-exercise stretching has come under attack in recent years, thanks to a series of studies showing that extending a muscle to the edge of its range of motion and then holding it there causes a temporary decrease of strength and power that can last for an hour or two. But few of these studies have addressed endurance sports like running - until now.

Researchers at Florida State University asked 10 male runners to complete a pair of one-hour tests consisting of 30 minutes running at a constant pace, followed by 30 minutes as fast as possible. Before one of the tests, the subjects did 16 minutes of “static” stretching, holding each stretch for 30 seconds. Sure enough, the non-stretchers burned about 5 per cent fewer calories in the first part of the run, and ran 3.4 per cent farther in the second part. The results appeared in the Journal of Strength and Conditioning Research.

The new findings fit with earlier studies showing that the better you do on a sit-and-reach test of flexibility, the worse your running economy is. This effect likely stems from the remarkable ability of your muscles and tendons to store energy like coiled springs, providing an estimated 40 to 50 per cent of the energy you use for each step. “If you decrease the stiffness of the muscles and tendons, then you can’t store and reutilize energy as well,” explains Jacob Wilson, the study’s lead author. Interestingly, preliminary results from a follow-up study suggest that “dynamic” stretching doesn’t produce the same slow-down, Wilson says.

Air pollution and marathons

The good news from a new study that appeared in the March issue of Medicine & Science in Sports & Exercise is that major big-city marathons have fairly high air quality overall. This makes sense: they’re held on weekend mornings so they avoid rush-hour car pollution, and they’re generally early enough in the day that they dodge secondary pollutants like ozone that are produced by intense solar radiation. And indeed, an analysis of nearly three decades of environmental data for the Boston, Chicago, New York, Twin Cities, Grandma’s, California International and Los Angeles marathons found that levels of six key pollutants almost never exceeded EPA guidelines.

The researchers also examined the top times in each of these marathons, to see if air quality - even within safe limits - had any influence. After all, a runner completing a three-hour marathon inhales as much air as a sedentary person would in two full days, and breathes more deeply so that pollutants penetrate deeper into the lungs. The only correlation they found was with levels of particulate matter smaller than 10 micrometres - and only for women, not men. It’s thought that women may be more sensitive to certain pollutants than men because their airways are narrower.