Retrospective #52: Thermic Effects of Food

Thermogenesis is the process of heat production in organisms, and heat is a form of expended energy. The unit of measurement is a calorie. Human metabolism is comprised of three components of energy expenditure: 1) the energy expended by the basal metabolic rate (to keep the resting organism alive), 2) the energy expended through exercise (motion), and 3) the energy expended due to the “cost” of processing food for use and storage. This last component is known as the thermic effect of food.  

It is estimated that the thermic effect of food is about 10% of the caloric intake of any given meal though, according to Wikipedia, “the effect varies substantially for different food components.” Of the three macronutrients, fat has minimal thermic effect. Carbohydrates – especially simple sugars and carbs in highly processed, manufactured foods – are very easy to process and have very little thermic effect. Proteins – and whole foods and complex carbohydrates to a lesser extent – are harder to process and have a larger thermic effect. The ratio of protein to carb energy expenditure is between 2:1 and 3:1, depending on how “simple” the carb is.

So, total daily energy expenditure includes a resting metabolic rate component (60-70% of total), the physical activity component (15-30% of total), and the thermic-effect-of-food component (~10%). While 10% is a small percentage, if the energy expended to digest, absorb and eliminate protein is two or more times the amount required for carbs and fat, replacing carbs with protein in an isocaloric diet would nevertheless increase the metabolic rate and help burn more calories, including stored fat. And if the difference in the “burn rate” was even greater for simple sugars and processed carbs, made easier to digest by manufacturing, that would account to a degree for why our metabolisms burn less on today’s modern processed-food diets.

Thus, we can naturally increase our metabolic rate by 1) increasing the amount of protein in our diet and 2) replace simple sugars and processed-foods in the diet with whole foods, that is, unprocessed, complex carbohydrates. Note that 2) above assumes that you don’t already have Insulin Resistance (IR), Pre-diabetes, or frank Type 2 diabetes, or are obese. It you do (are), you should severely restrict/limit carbs in the diet anyway, whether whole or processed.

The typical American diet currently gets about 15% of calories from protein. The Standard American Diet (SAD for short), espoused by the USDA/HHS and promoted on food packages in the Nutrition Facts Panel, is 10% protein. (On a 2,000kcal diet, 10% = 200kcal = 50g/d of protein. My Very Low Carb diet is about 20% protein by calorie.

Barr SB and Wright JC examined the issue in, “Postprandial energy expenditure in whole-food and processed-food meals: implications for daily energy expenditure,” published online July 2010 in The Journal of Food and Nutrition Research. An abstract on PubMed explains: “Empirical evidence has shown that rising obesity rates closely parallel the increased consumption of processed foods…in the USA. Differences in postprandial thermogenic responses to a whole-food meal vs. a processed food meal may be a key factor in explaining obesity trends…” Their conclusion: “Ingestion of the particular processed food meal tested in this study decreases postprandial energy expenditure by nearly 50% compared with the isoenergetic whole food meal. This reduction in daily energy expenditure has potential implications for diets comprised heavily of processed foods and their associations with obesity.”

Wow! 50% is a big difference in thermic effect. And, in the meals they were comparing, “either ‘whole’ or ‘processed’ cheese sandwiches (multi-grain bread and cheddar cheese) were deemed ‘whole,’ while white bread and processed cheese product were considered ‘processed.’” Imagine if they had chosen a real whole food instead of ‘multigrain bread’ which is a far, far cry from a real whole food. And what if it had been a protein food instead?

The authors explained, “A more strict whole food would be one devoid of any processing, such as a specific fruit, vegetable, or meat. However, we sought to compare two meals that were familiar to the Western diet, and could be easily interchangeable.” And while the much higher thermic effect of protein is probably too small to have a noticeable effect on weight in the short term, over a period of months or years this difference would be significant.