How long does it really take to lose your fitness?

With five kids age 6 and under — including a set of twins — Karyn Ryan could barely find the time to sleep, let alone exercise. After a nine-month hiatus from her fitness routine, Ryan was so overweight and out of shape that she got winded just going up the stairs. “Mentally, I needed to find myself again,” she says.

As her running shoes sat idly by the door of her Gaithersburg, Md., home, Ryan was taunted by a worry familiar to anyone who has taken an extended workout break, whether due to work or family obligations, illness or injury, or just plain fatigue: Am I losing my fitness? How long does it take to get out of shape?

The short answer: It depends. Generally speaking, though, it takes longer than you might think.

For a recreational exerciser — someone who works out two to three times a week and is “fit enough to keep up with a 3-year-old” — it takes two to four weeks of inactivity for there to be a notable change in your conditioning, says Jo Zimmerman, an instructor in the department of kinesiology at the University of Maryland. If you are a more serious athlete, perhaps training for a marathon, you may feel this decline more acutely, but de-conditioning happens in proportion to how much effort you put into getting in shape in the first place.

Regardless of your fitness level and goals, “detraining” (a fancy way of saying you’ve logged more hours on the couch than at the gym) affects different parts of your body — your cardiovascular system, your muscles, your waistline — in different ways.

CARDIOVASCULAR SYSTEM: The first thing to slide is your aerobic fitness. After 10 to 14 days with little or no physical activity, the body’s ability to effectively consume and use oxygen, sometimes referred to as VO2 max, begins to decline, says Jessica Matthews, a senior adviser for health

and fitness education for the American Council on Exercise. When you detrain, the heart gradually loses its ability to handle extra blood flow, and those new capillaries wither. It sounds dire, but, Zimmerman says, “it’s safe and normal.” It can also be reversed once you start exercising regularly again.

MUSCLES: Detraining has a less immediately dramatic impact on muscular strength and endurance. During the first few weeks off, the effects are slight, Matthews says. After about four weeks off, however, muscle fibers begin to shrink, and sometime between then and eight weeks, that decline becomes measurable, Zimmerman says.

WEIGHT: Despite what many people think, “when you get fit, you are not turning fat to muscle,” Zimmerman explains. Fat and muscle are two different types of tissue. If you stop working out, your muscles will eventually shrink back to where they started; if you eat more calories than you burn, the extra calories are stored as fat. But the fat and the muscle are not replacing each other. “And it doesn’t go in reverse, either,” Zimmerman says. “You don’t turn muscle into fat.”

DEFEND YOUR FITNESS: How do you stop the slide? The answer is both harsh and obvious: Try not to stop exercising in the first place. Of course, if you are seriously injured or very ill, by all means rest. (And remember, too, that rest and recovery are a vital part of any exercise regimen.)

Running guru Hal Higdon says he tells runners that for every one day of inactivity, it takes two days to return to their previous fitness level. Runners who have lost several weeks on a strict regimen like his want to know how they can regain what they lost. “My advice to them is not to try,” he says, “Resign yourself to the fact that you have taken a hit in your conditioning, and maybe this half marathon is not the race you want to peak for.”