Justify CICO w/ thermodynamics of food!

...The thermic effect of food (TEF, also known as specific dynamic action or SDA or Dietary Induced Thermogenesis or DIT) refers to the slight bump in metabolic rate that occurs after eating, due to processing and utilization of the ingested nutrients. For example, protein has to be broken down and processed in the liver which requires energy. As well, the simple act of eating protein stimulates protein synthesis in various tissues (organs, liver, muscle) as well. All of which takes energy. Carbohydrates get broken down to glucose, which goes through the liver, some processing, etc. Fat undergoes the least processing. There are exceptions such as medium chain triglycerides (MCTs) which undergo quite a bit of processing in the liver, causing a slight bump in metabolic rate (via TEF) in the process.

As it turns out, different nutrients have different individual TEF’s. Protein turns out to have the highest, to the tune of 20-30%. Meaning that of the total protein calories you eat, 20-30% is lost in processing. Carbohydrate stored as glycogen requires about 5-6% of the total calories. Carbohydrate converted to fat (which generally doesn’t happen in very significant amounts) uses up ~23% of the total calories as TEF. Most fats have a tiny TEF, maybe 2-3% (because they can be stored as fat in fat cells with minimal processing).

Since it’s usually impractical to sit and figure out the individual TEFs for each nutrient, the normal estimate used is 10% of total caloric intake. So if you consume 3000 calories per day of a relatively ‘normal’ mixed diet, you can assume that your TEF is about 300 calories per day or so. You also generally find that, with the exception of extreme diets (such as all protein), shuffling macronutrients has a pretty minimal overall impact on metabolic rate via TEF.

For example, consider the difference in TEF for carbs versus fat: 5-6% vs. 3%. That means that, for every 100 calories of each you ate, you’d burn 5-6 or 3 calories. So if you replaced 100 calories of fat with 100 calories of carbohydrates, you’d burn a whopping 2-3 extra calories via TEF. If you replaced 1000 calories of fat with 1000 calories of carbohydrates, you’d burn 20-30 more calories. If you were able, by some means, to replace 2000 calories of fat with carbohydrates, you would burn 40-60 more calories via TEF. One study found that metabolism was about 4% higher (100 calories per day or so) for the higher carb versus the higher protein diet. That still only amounts to an extra pound lost per month or so. Nothing to write home about to be sure.

About the only time that TEF can become considerable is when you replace carbohydrates or fat with protein. For every 100 calories of carbs/fat replaced with protein, you’d expect to burn about 25 calories more (30 cal for protein vs. 3-6 for carbs/fat). So a doubling of protein from 60 to120 grams/day might increase TEF by 80 calories/day. Triple it to 180 grams/day and TEF could increase by 150 calories. The 20-30% TEF of protein can become even more significant at extreme intakes. However, for the most part, such extreme intakes aren’t practical or used outside of the bodybuilding subculture. In all but the most extreme diets, protein stays fairly static and carbs and fats are shuffled around; the effect is typically minimal in terms of TEF.