You've heard it over and over: a calorie is a calorie is a calorie. While there is truth to this statement, there is also more details to consider when we compare the effort our body puts into digesting calories from different sources such as macronutrients. 1,000 calories of fat has a different effect on your metabolism than 1,000 calories of protein. Eating food costs energy, and different foods cost more or less depending on the macronutrient content. I know what you're thinking -- This is great news because it means I'll burn calories just from eating. And you will, but if you are eating too much of the wrong kind you won't reap the benefits. So what should you be eating to maximize your calorie burning from the food you eat? First let's get a little science-y.
The cost of energy used to digest, transport, metabolize, and store food is referred to as the thermic effect of food (TEF). Basically a fancy name for a small rise in body temperature from ingesting and digesting food. The TEF is a component of your overall metabolism, which also includes your resting metabolic rate and activity level, and should be factored into someone's total daily calorie burning. The cost of the TEF is estimated to be about 10% of your total caloric intake, meaning that it could range from 150-400 calories burned per day, depending on what you eat.
Does Eating Fat Burn Calories?
In short, eating fat isn’t a weight loss hack as many have hoped for. High fat intake has a low metabolic cost and results in excess calories which stimulates fat storage. Our bodies don't have to work very hard to store dietary triglycerides and convert it into adipose tissue, but does have to use more energy to convert excess carbs to stored body fat. We shouldn't avoid eating fat but rather focus on replacing trans and saturated fats with better fats. Polyunsaturated (PUFA's - fish oil), monounsaturated (MUFA's - olive oil), and medium chain triglycerides (MCT's - coconut oil) are in general healthier, higher quality, and recent research is pointing to essential fatty acids and medium chain triglycerides as having higher lypolytic properties than other fats.1 Another highly researched fatty acid tied to metabolic benefits is conjugated linoleic acid (CLA). In general, fat intake accounts for only 5-15% of total TEF, with the 15% side of the estimate being the rare exception.2
Does Eating Carbs Increase Fat Burning?
Once again, carb consumption contributes to overall TEF, but similarly to fat probably not very much with estimates ranging from 5-15%.2,3 Carbs have been compared directly to fats and in general have a higher thermic effect than fat.1 One variable that plays a direct role in the thermic effect of carbs is the type of carb. In general lower glycemic carbs have greater thermogenic properties when compared to high glycemic carbs.1
If in Doubt, Increase the Protein
As you may have guessed by process of elimination, protein has the highest thermic effect and keeps us feeling fuller longer (increased satiety). The estimated thermic effect of dietary protein is 20-35%, which is more than twice that of carbs. High and low protein, and high protein vs. high carb diets have been compared in studies and the high protein diet wins out every time for greatest thermic effect. Even when macronutrients were infused via IV into the body the greatest thermic effect was clear; carbs 6-8%, fat 2-3%, and aminos 30-40%.1
Why The Big Difference With Protein?
Scientists suggest that protein comes with a metabolically higher energy cost because of the rate of nitrogen turnover from protein metabolism.1 The difference in macronutrient content in your diet does play role in weight management. Protein exhibits a higher thermogenic effect than carbs and fat. Overeating high protein meals will not lead to the same amount of weight gain as overeating high carb and high fat meals.1
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1. Antonio, J. Smith-Ryan, A. Sports Nutrition & Performance Enhancing Supplements (2013). p289-293.