Recommendation: engage in daily moderate-intensity exercise to stay healthy
Almost everyone will agree on the importance of getting regular exercise, but too many of us don’t follow that simple recommendation. Along with cultivating a lifelong practice of avoiding fast carbs, a lifelong practice of regular exercise is one of the baseline strategies for maintaining a healthy weight and lowering your risk of developing metabolic and cardiovascular disease.
As with diet, there are a lot of competing exercise regimens and recommendations that offer various and sometimes conflicting advice. My recommendation is straightforward: everyone should aim to exercise five days a week, for a minimum of thirty to sixty minutes, with an eye toward a weekly total of at least 150 minutes and preferably closer to 300 minutes. You can do this at a moderate level of intensity, or exercise more vigorously and gain the same benefit in perhaps half the time. Most of your activity should be aerobic, but strengthening the muscles through some form of resistance training at least twice a week is also important.
Jim Hill, a professor at the University of Alabama at Birmingham, says there are two reasons physical activity is so crucial. “First, it increases your energy expenditure and allows you to eat more,” he explained. Without exercise, people who lose weight have to sustain the caloric restrictions that got them there, despite a hormonal response that cries out for more food. The body compensates for reduced intake by lowering its resting metabolic rate, using less energy, and storing more fat. That process creates what Hill calls an energy gap that is hard for almost anyone to overcome.
“The more you try to [fill this gap] with food restriction alone, the more likely you are to fail. Some people can do it, but most people need to use exercise to fill at least part of this gap,” he said.
Second, exercise helps to regulate metabolism. Over the course of the day, there are generally periods when we eat fat, carbohydrates, or protein, and others when we are eating nothing at all. If you are healthy, your metabolic system adjusts easily, keeping caloric intake and physiological activity in tandem, and body weight remains stable. But if your metabolism is out of balance, then the body responds by slowing metabolism even as caloric intake increases.
Exercise, Hill said, can “fix your broken metabolism.” He later added, “Physical activity is the major predictor of metabolic flexibility and inflexibility.”
This is clear when we look at the problem of insulin resistance. When insulin acts on glucose, 80 percent of that glucose is taken up by the muscles; the liver, fat tissue, and brain tissue dispose of the rest. In a prediabetic or diabetic state, the efficiency of those processes is undermined. Glucose uptake by skeletal muscles and other tissue is insufficient, and the sugars continue to circulate in the bloodstream with rising blood glucose levels.
Because muscles are the dominant storehouse for glucose, many researchers believe that their resistance to insulin is the primary defect that allows prediabetes and diabetes to develop. Jacob Haus of the University of Michigan says that exercise helps the muscles to take up more glucose from the bloodstream and metabolize it. Exercise also helps to increase insulin sensitivity in fatty tissue and the liver, enabling those depots to dispose of glucose more efficiently as well.
In one study, Haus compared three groups to determine the influence of exercise on insulin sensitivity—healthy individuals; those with some degree of impaired sensitivity to insulin; and people with diabetes. After three months of aerobic exercise, sixty minutes a day, five days a week, at 75 percent of the body’s maximum oxygen intake capacity and 80 to 85 percent of its maximum heart rate, the investigators recorded improvements in all three populations.
It doesn’t even have to take that long to see results. Haus and his team also found that seven consecutive days of aerobic exercise raised insulin sensitivity by 45 percent, leading to more glucose uptake by the muscles and increased glucose metabolism.
How do we translate the science into an exercise regimen that works? First, keep in mind that moderately intense physical activity means different things to different people, and depends on your baseline level of fitness. Doctors and nutritionists have various ways of measuring exercise intensity. For example, they can assess how close you are to your body’s maximum capacity to absorb oxygen (VO2 max). But that’s not something you can do on your own. What you can do instead is establish an appropriate target heart rate, with guidance from your doctor, and then wear one of the many types of monitors available to help you measure it. With moderately intense activity, such as brisk walking, doubles tennis, recreational biking, or swimming, you can reach 50 to 70 percent of the heart’s maximum capacity. More vigorous exercise uses up more energy—running a ten-minute mile, for example, can raise your heart rate to 70 to 85 percent of maximum capacity.
Your own perception of how hard you are working is also a reasonable measure—if you are breathing hard, but can still carry on a conversation and are perspiring lightly after about ten minutes, your exercise is probably moderately intense. In a vigorous workout, you should be breathing too hard to talk much and perspiring within a few minutes after initiating exercise.
The good news is that if you are significantly overweight, or are new to exercise, the benefits accrue very quickly. But individuals with prediabetes or full-blown disease will find that they have to do more to get the same clinical benefit as someone who is metabolically healthy. The more dysfunctional your metabolism is when you start, the longer you will have to work to see the benefits. But exercise at any point, from any fitness baseline, will lead to results. In other words, it’s never too late.
Walk a little farther, or more often; climb a flight of stairs instead of taking the elevator; you can reduce your glycemic load somewhat beginning on the very first day you exercise. The effect of multiple activities is cumulative, and consistency is critical. “You’re only as good as your last exercise,” says Haus, emphasizing the importance of an ongoing commitment.
Frequency, intensity, duration, and type of activity all influence the effects of exercise, but the bottom line is that the relationship between cardiorespiratory fitness and insulin sensitivity is powerful and protective. Being fit allows you to take in more oxygen when you exercise, which increases your endurance and makes you more sensitive to insulin. Over time, sustained moderate-intensity physical activity—not once, but day after day for a lifetime—is essential for weight maintenance, metabolic control, and cardiovascular health.