Calories and Your Metabolism Slows?

We’ve all heard that you can slow your metabolism by eating too few calories. But at what point does this “slow down” in your metabolism happen? I recently got asked an excellent question from a CGB reader regarding this subject.

Question from Dax: Hey, I really like your website . . . and I’m a guy, is that okay?  I understand the science behind the slower metabolism on a restricted diet.  Your body becomes more efficient, which sounds like a good thing, but actually means that you run on few calories and, therefore, more calories would become fat.

However, I get confused by the numerous examples of the opposite.  Why did Christian Bale in the Machinist get so thin!  He claims he lost 63 pounds by eating nothing other than a can of tuna and an apple per day.  Why didn’t his metabolism keep him from losing the weight? (Pic is of Christian Bale before and after his weight loss)

Also, what is the fewest calories a man can eat without going too low?

Answer from Martha: First, I think it’s great that you enjoy my blog! I actually have a lot of male readers. I try to make many of my posts of interest to both guys and girls. Second, you asked an excellent question that I have actually wondered myself.  While I don’t have a scientific answer for you, here are my thoughts…

Studies have demonstrated that the metabolism does slow when the caloric intake has been decreased to a very low level, especially when this restriction has been ongoing AND a fair amount of weight loss has occured. The act of losing weight in itself will slow the metabolism. In addition, the new lower body weight will require fewer calories that the initial higher body weight. So this is a double whammy so to speak.

But you have raised a good point. How can some people, such as Christian Bale or anorexics, get to such a low body weight if their metabolisms slow down? My answer would be that they consume SO FEW calories that they still lose weight despite the slow down in metabolism. Many anorexics consume only 400 calories a day and may exercise for several hours a day. Christian Bale’s diet doesn’t sound far off from 400 calories.  BTW – check out this link for more info on Christian Bale’s 63 pound weight loss for the movie The Machinist. Another example would be liquid diets. I worked with these medically supervised fasts, such as Optifast, for many years when they were popular. The participants generally started out obese and ate only 500-800 calories a day (in the form of shakes) for months at a time. If they stuck to the diet, the majority of them lost a large amount of weight.  So despite the slow down in their metabolic rate, they were still able to lose weight.  But here is the problem –  when they start to eat even somewhat normally … their weight skyrocketed back up – usually rapidly. This is because their metabolism had slowed down and even small increases in caloric intake caused rapid weight gain.

So to summarize, weight loss can still occur in many people if very low calorie diets are adhered to. However this is not recommended as very low calorie diets are inadequate in nutrients and will make weight regain likely once the low cal diet is discontinued. The best way to lose weight is a moderate reduced calorie diet and regular weight training and cardio.

To answer your second question – I can not tell you exactly how low you can go with your calories before your metabolism will start to slow. This really depends upon your age, weight, activity level and your own metabolism. But I can tell you that you should not consume fewer calories than your BMR (basal metabolic rate). Check out this link to find out your BMR.

Hope this answered your question Daz and thanks for reading CGB!


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I especially love problem-solving, whether it’s helping women defeat issues plaguing them for years, helping a busy executive find practical ways to get heart healthy, or providing tips to help you reverse diabetes. That’s why I’m on a constant quest to expand my knowledge by staying on top of the latest research.

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