Funny calorie math.

Have you ever heard that a few extra calories each day—an extra soda, for example—can add up to significant weight gain over time? Or that making small changes in what you eat, such as skipping dessert, can promote weight loss? If so, you are familiar with the concept of energy balance. And if you were ever surprised by those claims, you are familiar with what I call funny calorie math.

This is the topic of my Health & Fitness column in the Aiken Standard this week.

Your body weight at any time is determined by the balance between the energy you take in and the energy you expend. The “energy in” part is simple, it’s the calories in the food you eat.

Your energy expenditure is determined by your basal metabolic rate (BMR), the calories burned keeping you warm and alive, as well as the energy you expend in activity. Of these, the BMR accounts for the majority of your energy expenditure, but you have the most control over your level of activity.

According to this energy balance model, eating or drinking an additional 250 calories per day would add up to about one pound of weight gain every two weeks and 26 pounds after one year (using the rough estimate that to gain a pound requires 3500 extra calories.

+250 calories/day  x  7 days/week  =  1750 calories/week  =  3500 calories/2 weeks

3500 calories/2 weeks  x  1 lb. weight gain/3500 calories  =  1 lb. weight gain every two weeks  =  26 lbs. weight gain per year!

Calculations like this are common, typically used to point out how fattening a particular food or beverage can be. But the same principle can be applied in reverse to determine weight loss.

Example: Eat (or drink) and extra 250 calories per day

250 calories/day  x  7 days/week  =  1750 calories/week  =  3500 calories/2 weeks

3500 calories/2 weeks  x  1 lb. weight loss/3500 calories  =  1 lb. weight loss every two weeks  =  26 lbs. weight loss per year!

The same formula predicts that cutting back by 250 calories per day should lead to losing 26 pounds in one year. This idea is the basis for suggesting that making small changes to your diet can lead to significant weight loss over time.

You can do the same thing with physical activity, too. Adding a 45-minute walk, which burns approximately 250 calories, each day should lead to the same 26 pounds of weight loss in a year.

Example: walk 2.5 miles in 45 minutes each day, using the rough estimate that you will burn 100 calories per mile.

2.5 miles/day  x  100 calories/mile  =  250 calories/day

250 calories/day  x  7 days/week  =  1750 calories/week  =  3500 calories/2 weeks

3500 calories/2 weeks  x  1 lb. weight loss/3500 calories  =  1 lb. weight loss every two weeks  =  26 lbs. weight loss per year!

The assumption, of course, is that you aren’t changing anything else as you eat 250 fewer calories or burn an additional 250 calories per day through exercise. It would be relatively easy to offset the energy expended though a walk by even a small change in what you eat. This is the biggest weakness of this energy balance model—in order for it to accurately predict weight loss or gain, nothing else can change.

Unfortunately, this isn’t the way it really works. Changes in body weight through eating or exercise also lead to changes in total energy expenditure. As you lose weight, the total calories you burn in a day drops, mostly due to a decrease in BMR, since it is based on your body weight. The result is that over time you don’t lose weight as quickly. The exact opposite occurs with weight gain, which causes BMR to go up, limiting weight gain.

This leads to a different outcome: the extra 250 calories per day is likely to lead to a weight gain of closer to 10 pounds (maybe less) due to your BMR increasing as you gain weight over the year. And the estimation of weight loss will be different, too, given that BMR will drop slightly over time.

This is one of many reasons why exercise is important for weight loss. You can offset the lower BMR that occurs as you lose weight by increasing energy expenditure through activity. Additionally, regular exercise can add to the weight you lose through a diet and help keep the weight off later.