It has been a central tenet of dieting that a calorie is a calorie, whether it comes from chocolate or kale. Eat 300 calories of something naughty and it should have exactly the same effect on your weight as eating 300 calories of something completely virtuous and devoid of fun. It’s a proposition that has gotten me through many late nights when a bowl of Ben & Jerry’s Chunky Monkey seemed so much more appealing than yet another meal of grilled skinless chicken breast and salad. From a purely thermodynamic perspective, it should work. Except that a recently published study shows that we might be missing one important part of that equation: what you eat might change how many calories you burn each day.
The Science Behind Losing Weight
One pound of fat contains about 3,400 calories of energy. (Strictly speaking it is actually 3,500 kilocalories, but for diet purposes everyone refers to them as just calories). If your calorie intake
averages 2,000 calories per day and your body burns an average of 2,000 calories per day, your weight should remain exactly stable. You can decide to go on a weight-loss diet and decrease your calorie intake to 1,500 calories per day. With a net deficit of 500 calories per day you should lose around 1 pound per week since you are now short by 3,500 calories each week. Note that achieving that goal has required you to drop your calorie intake by 25%. That is not an easy regimen to maintain and accounts for why losing weight is generally not easy.
The flip side of this is even more depressing. Add just an average of 10 calories a day to your diet and by the end of a year you have taken in 3,650 calories more than you burned. So from just 10 calories a day, your weight could increase by about one pound. Enjoy a glass of wine with dinner each evening? Well, that’s about 125 calories in a glass. So that glass of wine could be worth about 12 pounds a year.
But all of this assumes that your body burns exactly the same number of calories every day if your activity level remains constant. We have all had the experience of starting a diet and losing weight for a while only to reach a plateau where further weight loss seems to practically stop. If your calorie intake is still lower than before your diet, it could be that your metabolism has slowed down and now you are not burning as many calories per day as you were before your diet. So, your resting metabolic rate (the number of calories you burn each day exclusive of physical activity) may not be such a fixed number after all.
How the Study Challenges Thermodynamics
In a study published in November, 2018, in the British Medical Journal, researchers found that what you eat, whether your calories came from fat or carbohydrates, had a measurable effect on how many calories you burn each day.
Their study was based on the carbohydrate-insulin model of obesity. According to this theory, consumption of a meal with a high load of carbohydrates directs metabolism away from burning calories and towards storage in fat tissue. This is also though to contribute to an overall increase in hunger, food cravings and weight gain, especially among people who have an underlying condition known as “insulin resistance” (People with insulin resistance typically have other conditions such as high blood pressure, elevated triglycerides, low HDL or increased belly fat). Some estimates put as much as 40% of the American population into this category.
The researchers started with a weight loss diet designed to reduce body weight by about 12% over a 10 week period. Then they assigned participants to a high (60% of calories), moderate (40%) or low (20%) carb diet for the next 20 weeks. The goal at that point was not further weight loss but to maintain the weight they had already lost. Each participant was provided with meals that had similar protein and fat content.
What they found was that for every 10% decrease in the amount of carbohydrates they ate, their daily calorie burn increased by about 52 calories. Overall, those on a low carbohydrate diet burned about 209 to 278 calories per day more than those on high carbohydrate diets even though their physical activity levels were about the same. In those with the greatest degree of insulin resistance the additional calorie burn was as high as 478 calories per day.
Of course, the study had some shortcomings that may affect these conclusions in future research. The study looked mainly at overall carbohydrate intake and did not differentiate the quality of the carbohydrates. Those in the high carb intake group appear to have had more high glycemic index carbs. Also, the caloric intake of the participants was adjusted to maintain their weight loss after the initial diet phase and that may have had some effect on metabolic rates.
However, if this study holds up in future research then the composition of your diet may have a huge effect on how many calories per day you are burning.
So, while that bowl of ice cream and that chicken salad may have the same number of calories going in, one of them may cause your body to burn a lot more calories than the other. In that case- a calorie is definitely not just a calorie. A 200 calorie per day difference in your basal metabolic rate can translate into a very substantial difference in your weight over time.
The Laws of Thermodynamics remain intact: calories are neither created nor destroyed but there is a price you pay for eating something that is more fun.