There are some basic terms used in the literature.
- Total Daily Energy Expenditure (TDEE ) – How many calories you burn in a day
- Basal Metabolic Rate (BMR) – The number of calories your body burns at rest
- Thermic Effect of Food (TEF) – How many calories are required to deal with the food you eat in a day
- Nonexercise activity thermogenesis (NEAT) – The calories burned by the things you do above the basal rate
TDEE = BMR + TEF + NEAT
There’s very little that can be done to affect your Basal Metabolic Rate (BMR). In fact, weight loss by definition, lowers your BMR.
If you eat less food you will have a lower level of TEF (depending on the mix of the food types). If you fast you get no TEF since you are not eating.
From (James A.Levine MD, PhD (Professor of Medicine), Non-exercise activity thermogenesis (NEAT), Best Practice & Research Clinical Endocrinology & Metabolism Volume 16, Issue 4, December 2002, Pages 679-702):
Physiological studies demonstrate, intriguingly, that NEAT is modulated with changes in energy balance; NEAT increases with overfeeding and decreases with underfeeding. Thus, NEAT could be a critical component in how we maintain our body weight and/or develop obesity or lose weight.
From (James A. Levine, Mark W. Vander Weg, James O. Hill, Robert C. Klesges, Non-Exercise Activity Thermogenesis: The Crouching Tiger Hidden Dragon of Societal Weight Gain, Arteriosclerosis, Thrombosis, and Vascular Biology. 2006;26:729-736):
Classically, there are 3 components of total daily energy expenditure (TDEE) in humans (Figure 2): basal metabolic rate (BMR), thermic effect of food (TEF), and activity thermogenesis (AT). BMR is the energy expenditure (EE) when a postabsorptive individual is laying at rest. BMR is can be measured with errors of <1% generally using a high-precision, validated, indirect calorimeter.6 In sedentary individuals, BMR accounts for ∼60% of TDEE and can be well-predicted by lean body mass (∼80% of the variance) within and across species.7,8 TEF is the increase in EE associated with the digestion, absorption, and storage of food and accounts for ∼6% to 12% of TDEE. TEF can be measured by making repeated measurements of energy expenditure after a meal, using an indirect calorimeter.6 Activity thermogenesis is typically derived by summating BMR plus TEF and subtracting this product from TDEE. Physical activity level (PAL) is frequently calculated also and is TDEE/BMR.