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How Calories Work
By John P. Hussman, Ph.D.
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The next several paragraphs are going to look like 7th grade math. And the reason is, they're 7th grade math. I know. Ugh. As David Letterman says, 50% of Americans are mathematically illiterate, and the other two-thirds aren't much better. Bear with me. And grab your calculator.

A pound of fat is 3500 calories. And though the late-night ads for Ab-thumpers and mylar spacesuits may try to convince you otherwise, nobody has repealed the First Law of Thermodynamics - energy cannot be created nor destroyed, only converted from one form to another. What that means for you is that to burn a pound of fat, you have to metabolize as energy 3500 calories more than you take in. That difference between energy use and intake is called a "caloric deficit". Now here's where it gets tricky - severe caloric restriction triggers a fasting state that slows your metabolism down, and also causes muscle loss. Moreover, it causes your body to increase the level of "fat storing" enzymes (lipoprotein lipase) in the body. Exactly what you don't want.

Studies show that fat loss is generally smaller than would be predicted by a given increase in physical activity, because people too often counter the extra activity by increasing their food intake ("Wow! Great workout. Let's get a beer"). In short, as you lose weight, your body subtly tries to gain it back. The only way you're going to counter that is by increasing muscle mass, intentionally maintaining muscular activity (through cardio and weight training) while you lose fat, properly setting your portions, and being careful to avoid sitting around all day after your workouts. Extreme caloric restriction (e.g. less than 1000 calories a day) doesn't work because it ignores the body's tendency to "survive" by shedding muscle, increasing fat storage efficiency, and slowing metabolic activity. So you can't succeed at a fitness program while severely restricting your calories.

The way to lose fat, very simply, is to FOCUS ON THE DEFICIT. You won't lose fat by exercising more if you let your caloric intake creep higher. You won't lose fat by restricting your calories if you're skipping workouts. The goal is to create a deliberate and well-controlled caloric deficit between the energy you take in and the energy that you burn. You do that by planning carefully, keeping accurate records, and maintaining discipline. 

Your daily "caloric deficit" depends on all sorts of factors, including your height, weight, lean mass, gender, workout intensity, and portion sizes. It's unlikely for anybody to get the deficit beyond about 1500 calories a day by working out harder or eating less, because you'll either interfere with proper recovery, or throw yourself into a fasting state. If you're following effective workouts and specifically targeting fat loss and muscle tone/gain, it is not unreasonable to target fat loss of as much as 1 to 2 lbs a week for women, and as much as 2 - 3 lbs a week for men. Unless you score basketball points by throwing the ball downward, it's very difficult (and generally unadvisable) to lose more than about 3 pounds of fat per week.

When you're eating 5 or 6 times a day instead of 3, your portions had better be much smaller than they used to be. As a rule of thumb, it's generally advised to target between 8-10 calories per pound of desired weight, if you're shooting for fat loss, and up to 15 calories per pound of desired weight if you're shooting for muscle gain. The problem is that fat itself is metabolically inactive, so it's better to base your intake on lean weight rather than scale weight. If you want a quick rule of thumb, I prefer the following: shoot for 9-11 calories per pound of lean weight if your main goal is fat loss, and about 15-17 calories per pound of lean weight if your main goal is muscle gain without fat loss. Now, 9 calories per lean pound is almost certainly below your Base Metabolic Rate (see below), so you shouldn't go with much less than 9 even if you're aggressively targeting fat loss. [Example: If you weigh 180 pounds and are at 20% bodyfat, your fat weighs .20 x 180 = 36 pounds, so your lean weight is 180 - 36 = 144 pounds. So you might target 1300-1600 calories daily to achieve a fat loss goal].

Want the perfect number of calories? There is no perfect number. Your body is extremely efficient at adjusting its activity level in response to moderate changes in caloric intake, and all of that takes place unconsciously and involuntarily. That's why the portion rule is useful - it's simple, and excessively fine-tuning your calories is useless. Just keep your portions relatively small if you want to lose fat. The frequency of the rations, and the balance of high-quality protein and carbohydrate are the most essential aspects of  your nutrition plan.

Still, some people want a more scientific number. Alright, technically, your body needs a certain amount of intake to support your "Base Metabolic Rate" or BMR. Unless you're a Munchkin, your BMR is rarely less than 1100 calories a day. So let's estimate your BMR. I've replaced my original section with the Java-based calculator below, with the eventual goal - as in the Dilbert cartoon - of making this website into a complete Broadway musical.


To lose 1 pound of fat a week, you have to generate a weekly caloric deficit of 3500 calories, or a deficit of 500 calories per day. On a workout day you'll burn about 1.5 times BMR. On a free day, you'll burn about 1.3 times BMR (If you've got a sedentary job or think your metabolism is slow, just use multipliers of 1.4 for workout days and 1.2 for free days). When you look over the entire week (including a free day if you take one), you should be able to say that you ate fewer calories than you burned. But don't restrict your calories dramatically below BMR. About 9 calories per lean pound of bodyweight is about as far under BMR as you should go. As long as your intake on any given day is in the ballpark, you can set your portion sizes with confidence that your goal is in reach.

As a sidenote, BMR is regulated by the thyroid gland. For most of you, the calculation I've just given you is accurate. But if you have an underactive thyroid (hypothyroidism) and are undiagnosed and untreated, your BMR may be about 10% lower than this estimate. This is only a problem for about 5% of you, but before you yell at me about these BMR calculations, some of you may want to have your thyroid checked (simple blood test). See the thyroid section of my Q&A page for more on this. People who have emaciated muscles can have an even lower BMR, but since they also have very low lean weight, the BMR calculation (as my calculator computes it) will pick this up. 

Look. If your thyroid is normal (true for most of you) and these calculations still seem like way too many or too few calories, you're probably lousy at counting calories.  So focus on good food choices, eat a lot of vegetables and low-fat soups, and use your common sense. Many of you who have lost weight successfully have told me things like "I tried never to be hungry and never to be full." If adjusting your calories doesn't work, keep it simple and adjust your portions.

Portion size, and the choices you make about foods (particularly lean meats and unprocessed fruits and vegetables) are far more important than rigidly counting calories. Remember this: small and frequent rations for fat loss (an apple and a chicken breast is a perfectly acceptable ration), larger and frequent meals for muscle gain.

As noted above, you can take in as much as 1.5 times your BMR if your fat loss goals are less aggressive or if you are focusing on muscle gain. Beyond about 1.8 times BMR, you'll gain fat, with little additional increase in muscle growth.

Want to see more about caloric deficits? See my Caloric Deficit Page, which includes some actual data using your faithful lab mouse - me.


Related Articles

Caloric Deficits
If you accumulate persistent caloric deficits day after day, you'll lose fat.

BMR Calculator
Estimate your base metabolic rate. This is only an estimate, but it can help keep you on track.

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