You’ve Got Questions, I’ve Got Answers!

Answers to questions on relationships among metabolism, dieting, exercise, sleep

Posted Oct 17, 2013

I received some great questions on my recent post Sleeping Beauty concerning the relationships among metabolism, dieting, exercise, and sleep. I will do my best to answer them below. Bolded text represents the reader question.

Why would "reducing metabolism" increase appetite?

This is an often misunderstood illusory correlation. What research suggests is that when we chronically deprive ourselves of calories (e.g., via caloric restriction on a diet), our bodies up their production of ghrelin (I’m hungry hormone) along with several other hormones that make us hungry, like insulin, which regulates our blood sugar. At the same time, when we chronically deprive ourselves of calories, our bodies begin to lower our basal metabolic rate (the number of calories we need each day to function normally). In essence, the body becomes more efficient at doing its daily tasks. So it isn’t that reducing metabolism increases appetite. Depriving yourself of calories both increases appetite and causes your metabolism to take a nose dive. Thus, the illusory correlation.  

Gym bunnies claim that dieting "ruins your metabolism" and only exercise can help you lose weight.

Yes and no. Here’s what the research suggests. Extreme dieting (drastically cutting calories) can lower your metabolism by up to 45%. But less extreme dieting also lowers your metabolism—just not to that extent. Exercise can help you lose weight, but just because you exercise does not mean you will lose weight as many people compensate for exercise by eating more. The healthiest way to lose weight is to slightly lower caloric intake while increasing the number of calories burned through exercise.

Let’s say you weigh 200 pounds and you’d like to lose two pounds a week. Dropping 1000 calories per day is not healthy and you’ll only set yourself up for metabolic slowdown by doing that. In fact, studies with mice suggest that even a mild (5%) deprivation can increase fat storage. So what should you do? Exercise!

Exercise can help “off-set” the lowering of your BMR. How does this work?

  • Exercise (especially strength training) increases in muscle mass, and muscle burns more calories than fat while taking up less space. So increased muscle mass = increase in BMR!
  • Exercise causes enzymatic changes that promote lipolysis (fat loss) – the key is to switch up your workout after every six workouts to keep your body guessing

So let’s go back to our scenario. You weigh 200 pounds and want to lose 2 pounds per week. If you plan to lose half of that weight (so 1 pound per week) through calorie restriction, then that means you only have to cut 500 calories per day. One whole milk venti caffe mocha from Starbucks. Piece of cake! (Actually cutting out 1/2 Piece Colossal Carrot Cake from McAlisters Deli would work too.) And, best of all, your body doesn’t freak out and enter starvation mode. Of course this also means you need to get moving to burn the other 500 calories per day and keep your metabolism humming. But 30 minutes of high intensity interval training should take care of that. So would a hard hour on the treadmill. Or half an hour on the treadmill plus 20 minutes of weight training.

And it doesn’t even have to involve a go at the gym. Being more active in general being can help burn quite a significant amount of calories. Take the stairs instead of the elevator, park further away at the grocery store, bike to work, walk around more, and take frequent movement breaks during the day and you’ll end up burning 100-200 calories or more per day. So much better than starvation mode and fat accumulation!

Why didn’t people in concentration camps get fat?

I think this question stemmed from the fact that when you go on an extreme calorie diet, your BMR slows down to help ensure your survival. That said, that does not mean you will gain weight. Your BMR only slows down to the extent that you cut calories. Weight gain from a slowed metabolic rate will not occur until after you increase your caloric intake, especially if you do so quickly without giving your body time to adjust to the excess calories.

How can sleep deprivation increase calorie consumption and decrease metabolism?

There are two different things going on here. Lack of sleep is a stressor. When we are under stress, for any reason, our bodies respond by releasing cortisol and adrenalin. These hormones do many things for us, but among the digestive processes they activate are: 1) making you craving high energy foods as your body thinks you need them to fight or flee (so you’re hungrier  and may respond by eating more). 2) reducing your metabolism either because you feel sluggish and aren’t as active or by altering the way your body utilizes the hormone insulin (which can lead to insulin resistance and type 2 diabetes).

I hope this answers your questions!

About the Author

Mary E. Pritchard, Ph.D., is a professor in the Department of Psychology at Boise State University.

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