Unintentional weight loss can occur because of an inadequately nutritious diet relative to a person's energy needs (generally called malnutrition). Disease processes, changes in metabolism, hormonal changes, medications or other treatments, disease- or treatment-related dietary changes, or reduced appetite associated with a disease or treatment can also cause unintentional weight loss.[26][27][28][32][33][34] Poor nutrient utilization can lead to weight loss, and can be caused by fistulae in the gastrointestinal tract, diarrhea, drug-nutrient interaction, enzyme depletion and muscle atrophy.[28]
Digestive metabolism, or thermic effect of food (TEF): Simply digesting food—turning carbs into sugar and turning protein into amino acids—typically burns 10 to 15 percent of your daily calories. Digesting protein burns more calories than digesting carbohydrates or fat—about 25 calories for every 100 consumed. Digesting carbohydrates and fat burns about 10 to 15 calories for every 100 consumed.
The prevailing formula for a long time on how much fat you’re going to burn was calories in minus calories out, based on your basal metabolic rate (BMR) and exercise efforts, explains strength and performance specialist Joel Seedman, Ph.D., owner of Advanced Human Performance in Atlanta. But with all the different biochemical reactions in the body, hormonal response, and endocrine function, there are an infinite number of factors that can affect how your body is storing and breaking down calories.