Metabolism

Whether you want to show off in a bikini or set a great example for your kids, exercise is a crucial component for health. Although this is a given considering past decades of scientific research on weight loss and physiology, not everyone understands just how much time in the gym is essential. You might not need to put as much effort into the weights or treadmill as you thought.

Your BMR 

To really get how much time you should be exercising, you first have to look at your basal metabolic rate, or BMR. This is the amount of energy (calories) you burn at rest and which your body uses up just surviving. The basic rule of thumb is that you absolutely must net at or above your BMR in terms of caloric intake. This means that, when you subtract the amount of exercise calories you’ve burned from your total calories for the day, the difference is at least what your body needs to support fundamental physiological processes.

Why BMR Matters 

When you are trying to exercise and eat right to lose weight, netting at least your BMR is non-negotiable because your body eventually will think you are starving if caloric intake isn’t meeting energy requirements. As the body tries to figure out how to deal with the perceived starvation, it can use stored carbohydrates, fat or protein for energy sources. Protein is what makes up lean muscle mass. Normally the body uses metabolizes muscle as a last resort, but lean muscle tissue requires more calories than other tissue. The body will metabolize muscle as a way of reducing the number of calories you need per day. That’s hardly what someone looking to get ripped wants, and it can be especially dangerous considering that one of the most vital organs of the body—the heart—is a muscle. The heart actually can decrease in size, slow and eventually fail. As your metabolism slows and you lose lean muscle, it becomes harder and harder to eat “normal” amounts of calories without gaining weight, simply because you’ve trained your body to make do with less and to hang on to anything “extra.”

American College of Sports Medicine Recommendations 

The American College of Sports Medicine takes BMR requirements seriously because of the potential dangers of insufficient caloric intake. Subsequently, they endorse a minimum net daily caloric intake of 1,200 for women and 1,800 for men. Factors such as age and height affect the exact number of calories a person needs per day however, so these numbers truly are only a guide.

Considering that each person has a minimum BMR they have to accommodate, ACSM also recognizes that it is not always possible to create enormous caloric deficits while dieting. The ACSM recommendation is to aim for a combined dietary and exercise deficit between 500 and 1,000 calories per day, which translates to 1 to 2 pounds per week (3,500 calories makes one pound). More than this drives net calories below BMR requirements in most cases.

Real Life Example 

Joe Schmoe has a BMR is 1,800 calories. His activity level is low, so he only needs about 500 more calories to support himself above his BMR. This means he needs 2,300 calories total. Then Joe resolves to lose weight. He cuts 500 calories from diet and does tough workouts each day that burn an average of 500 calories. The total deficit each day is 1,000. Subtracting the deficit from his total daily energy expenditure (TDEE) yields just 1,300 net calories. This number is below Joe’s BMR requirements. This means that it is not really safe for Joe to restrict his calories and to exercise quite so much. The most he should cut back is 500, or 2,300 – 1,800.

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Now, Joe has options about how to create his safe 500 calorie a day deficit. His first option is just to exercise without cutting calories from his normal diet. That would probably mean about 60 to 90 minutes in the gym daily, assuming high-intensity workouts. Another option is to get half the deficit from a dietary reduction and half from workouts. He’d eat a little less and might not tone up as fast, but he would only spend about 30 to 60 minutes in the gym. The one thing Joe shouldn’t do is just cut calories from his diet. Exercise inhibits the loss of lean muscle tissue during a weight loss plan and therefore keeps metabolism high, so although Joe doesn’t need to be shackled to his weights, he does need to use them.

What If I Want to Exercise More? 

The “rules” associated with BMR, caloric intake and safe weight loss do not mean you are limited in terms of how much exercise you do. They simply mean that, if you exercise a great deal, you might have to eat more to keep your net caloric intake high enough. Some people call this “eating back” calories, but this isn’t truly accurate given that you still have a caloric deficit when all is said and done. Let’s use marathon runners as an example. Marathons can take 2,000 or more extra calories. Using Joe Schmoe above, if Joe wanted to lose weight, he would need at least 1,800 calories (his BMR) plus 2,000 (the calories needed to marathon), or 3,800 calories—he’d be able to eat over 7 500-calorie meals and still have a deficit going at the end of the day, because he’s cutting out the calories needed for non-marathon activities. If he didn’t want to lose weight, he’d still need to tack on an additional 500 calories to support his non-marathon activity for a total of 4,200 calories!

Even if you are eating enough, you still have to remember that exercise is a source of stress for the body. You must give your body time to rest and recover to avoid hormonal imbalances and injury while seeing the best results. This is why good weight loss and exercise programs have one or two rest days per week guaranteed and alternate different muscle groups on different days. Don’t assume that you can kick butt at the gym for hours for days in a row just because you ate like a horse.

Find out your BMR here with our BMR calculator.

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