Being inactive for as little as a few days makes muscles weaker and smaller, but that is not all you lose. Two studies show that just two weeks of decreased physical activity brings you closer to becoming diabetic by decreasing your body's response to insulin, raising blood sugar levels after meals and making you fatter.
In the first study, researchers at the University of Liverpool in England asked 45 healthy active adult men and women who averaged walking more than 10,000 steps a day, to reduce their steps to fewer than 2,000 steps per day and to sit three and a half additional hours each day for two weeks (Diabetologia, Jun 2018;61(6):1282-1294). All of the participants had significant rises in blood sugar, lowered response to insulin, raised blood cholesterol, decreased leg muscle size and increased fat in their bellies.
In the second study, 22 diabetic, overweight adults (average age 69), who took 7000 or more steps a day, were asked to take fewer than 1000 steps a day for two weeks (J Gerontol A Biol Sci Med Sci, Jul 9, 2018;73(8):1070-1077). They all had changes predicting loss of muscle size and were less able to respond to insulin, and some had very high rises in blood sugar after meals. Most of the subjects did not have their markers of diabetes return to their more normal levels two weeks after they resumed their normal activity levels.
How Inactivity Can Cause Diabetes
When you reduce or stop your usual amount of exercise, you lose a major control of high blood sugar levels. Resting muscles do not pull sugar from your bloodstream, and when muscles become smaller and weaker they lose much of their ability to remove sugar from your bloodstream when you do exercise. Your muscles and liver are the only places in your body in which you can store significant amounts of sugar. When your blood sugar rises, and your muscles and liver are full of sugar, all extra sugar is converted to triglycerides, a fat that is stored in your body. Once you fill up your muscles and liver with sugar, all extra sugar is converted to fat and stored in your body to make you even fatter.
Aerobic Exercise is Not Enough
After you have stopped exercising for just a few weeks, your muscles are smaller and weaker and cannot remove as much sugar from your bloodstream when blood sugar levels rise too high. To regain your muscle size and strength, you need to exercise intensely enough to damage the muscle fibers so they will be stronger when they heal. When you use your muscles, you contract the muscle and shorten its fibers. However, you do not contract a muscle fiber equally throughout its length. Muscle fibers are made up of blocks, called sarcomeres, touching end to end to form the long stringy muscle fiber. Each block touches the next block at a point called the Z-line. You have to damage the Z-line to make a muscle grow larger and stronger. If you pedal a bicycle with great pressure or run very fast, you can damage the muscle fibers at the Z-lines, but most people do not run or cycle hard enough to do this. Adding weight training to the recovery program will help to regain the lost strength and muscle size. See my report and diagram on Inactivity Causes Muscle Loss
If you have to stop exercising even for just a few days because of an injury, vacation or illness, expect to lose strength and endurance and increase your risk for diabetes. When you resume exercising, you should add some form of strength training to regain your lost strength and muscle size. A general rule of thumb is that it usually takes at least three times as long as the period of inactivity to recover full strength in an inactive muscle (J Am Med Assoc, 2007; 297: 1772–1774).
Caution: Pain at the site of an injury may mean that you are tearing muscle fibers and should stop exercising immediately. Check with your doctor or physical therapist.