*Geek Box: Measuring Diet-Induced Thermogenesis
Measuring DIT is a methodological challenge, given that numerous variables may influence the results. The first is the timing of the measurement, and there are generally two ways this may be done:
- measure RMR before each meal, and calculate DIT as the difference between this pre-meal value and the post-meal values, and;
- 2) calculate DIT as the difference between the pre-breakfast RMR only [i.e., true fasted baseline RMR], post-meal values for all subsequent meals.
The issue with option 1) is discussed further below under Key Characteristic. The issue with option 2) is that it assumes that RMR remains constant throughout the day. Both options have limitations. Then there is the issue of meal size: measuring DIT over 4hrs captures 10-20% less DIT than if measured over 6hrs. The size and timing of the meals relative to the timing of RMR measures is therefore a potential confounder to elucidating a true effect.
The macronutrient composition of the diet is a critical factor, as a higher protein intake will result in significantly greater DIT than carbohydrate or fat. Age is another factor: there is an age-related decline in resting energy expenditure, which may relate to loss of lean body mass – and lean body mass is another factor to consider!
All of these factors can, of course, be controlled for, however it makes comparisons between studies sometimes difficult, resulting in seemingly inconsistent findings. The reality is that is it a methodological challenge, one that researchers will continue to improve upon. Insofar, however, as DIT is a relatively small component of total daily energy expenditure, it still attracts significant interest from the research community.