Energy balance is a fundamental thermodynamic principle that represents bodyweight alterations in response to changes in energy intake and expenditure.¹ A positive energy balance is when energy intake exceeds expenditure, and a negative energy balance is when energy expenditure exceeds intake.¹ Energy expenditure or components of total daily energy expenditure (TDEE) can be compartmentalized into resting energy expenditure (REE) which is the body’s basal metabolic rate (BMR), and non-resting energy expenditure (NREE).² NREE is further categorized into non-exercise activity thermogenesis (NEAT), thermic effect of food (TEF) and exercise activity thermogenesis (EAT).² BMR contributes ~70% of TDEE, NEAT ~15%, TEF ~10% and EAT ~5%.² Energy intake is dictated by food composition in one’s diet²; 1 g of carbohydrates and proteins is equal to 4 kcals and 1 g of lipids equates to 9 kcal.² Therefore, a diet high in fatty foods can easily induce a state of positive energy balance²; especially since fat has lower satiation compared with protein and carbohydrates.² Dietary nutritional content can change the contribution of TEF/MIT to the TDEE.² A protein-rich meal has a higher diet-induced thermogenesis gram for gram than a fat-rich or carb-rich meal³; with 20~30% of energy intake form protein, 0~3% of energy intake from fat an 5-10% of energy intake from carbohydrates are spent as MIT.³ Therefore, it is harder to induce a state of positive energy balance with a protein rich diet.

     Exercise can increase or decrease TDEE via time and intensity of physical activity.³ A more sedentary life, results in EAT having a low additional contribution to NREE.¹ Thus, contributing to a positive energy balance. Long intervals or short high intensity exercise can increase EAT’s additional contribution to NREE by over 500 kcals (due to the increased metabolic demand of tissues), resulting in a significant negative energy balance or a greater protection against positive energy imbalance.³ It should be noted that a diet low in carbohydrates would reduce energy expenditure because it is the main source of energy to fuel exercise.⁴

     Energy input and expenditure are interdependent, regulated by complex physiological control systems to maintain a steady state of energy balance.¹ Changes in one factor will result in compensatory changes in the other¹; however, the compensation response towards positive energy is weaker than the opposition.¹ When the body is in a constant state of positive energy, swelling of subcutaneous adipose tissue (SAT) occurs, is not continuous but rather gradual.¹ This is because weight gain is followed by a new steady state of energy balance at the new higher bodyweight, mediated via changes in BMR creating a ‘ratchet effect’.¹ When the body is under negative energy balance usually via hypocaloric diet, an endocrine response occurs.² This response promotes reduced BMR (reduced metabolically active tissue), reduced leptin and an increase in ghrelin to conserve energy and promoted intake to restore weight homeostasis.² However, under consistent negative energy balance, SAT and lead body mass will decrease.² Without optimal protein requirements and resistance exercise, lean body mass will significantly be lost.²

References

1. Hill JO, Wyatt HR, Peters JC. The importance of Energy Balance. European Endocrinology. 2013;9(2):111-115. doi:10.17925/ee.2013.09.02.111.
2. Trexler ET, Smith-Ryan AE, Norton LE. Metabolic adaptation to weight loss: Implications for the athlete. Journal of the International Society of Sports Nutrition. 2014;11(7). doi:10.1186/1550-2783-11-7.