The average adult human consumes close to 1 million calories per year. Despite this huge calorie intake, most healthy individuals are able to strike a remarkable balance between energy intake and energy expenditure, resulting in a state of energy balance. This accurate balance between energy intake and energy expenditure is an example of homeostatic regulation and results in the maintenance of body weight and body energy stores. Regulation of energy balance is achieved over the long term despite large fluctuations in energy intake and energy expenditure within and between days. The accuracy and precision by which the body maintains energy balance is highlighted by the fact that even a small error in the system can have detrimental consequences over time. If energy intake exceeds energy expenditure by as little as 25 kcal/d, over time, a person becomes obese. The achievement of energy balance is driven by the first law of thermodynamics, which states that energy can be neither destroyed nor created. This principle necessitates that when energy intake equals energy expenditure, body energy stores must remain constant.
The BMR (Basal metabolic rate)formula Men is (18-30 yr )=(0.0630x actual weight in kg +2.8957 )x 240 kcal /day
Men is (31-60 yr )=(0.0484 x actual weight in kg + 3.6534 )x 240 kcal /day
Women is (18-30 yr) = (0.0621 x actual weight in kg +2.0357 ) x 240 kcal /day
Women is (31-60 yr) = (0.0342x actual weight in kg + 3.5377) x 240 kcal /day
Estimated total energy needs depending upon Activity low
Low (sedentary) ;1.3 Intermediate ,1.5, High :1.7
Energy expenditure = BMR X activity factor
The human metabolism is dependent on interplay between fat , carbohydrate and protein metabolism .To maintain normal body proportion of these three stores dietary nutrients must be oxidized in the body in proportion they are in the diet. Carbohydrate intake affects usually is equal to the body stores of glucose and its more vulnerable to changes more than protein or fat.
The regulation of Energy metabolism happens by Brain, Pituitary, adrenal glands. There is a feedback mechanism of afferent and efferent signals. Factors which increase hunger include a decrease in blood glucose, increase in gastric contractions or abdominal uneasiness. Olfactory or gustatory factors,hormones like cystokinin, gastrin related peptide These peripheral signals are integrated by neurotransmitters in the central nervous system to regulate food intake. Some neurotransmitters are specific modulators of intake of macronutrients (fat, protein, carbohydrate), and they cause an increase and decrease in intake of these macronutrients.
Oxidation of food is calculated from Respiratory quotient (Ratio between carbon dioxide produced to oxygen used). The corresponding ratio in food is referred to as the food quotient. As the percent of dietary fat increases, the RQ must decline if weight has to stay stable. If the RQ does not decline, the body continues to oxidize carbohydrate stores and must replace these by eating more food to obtain more carbohydrate or synthesize endogenous glucose from protein stores. Whether this adjustment comes into play depends upon genetic determinants. Physiologically, the adaption to High fat, western type diet requires decrease in carbohydrate oxidation to preserve carbohydrate stores. In such condition the oxidation of fat increases to provide nutrition needs and lowers the RQ. So to maintain body weight, heavier individuals must on average ingest more food to provide necessary energy leading to Obesity.
However based on BMR calculations and future models of the person can be calculated by low metabolic rate,high RQ,indicating carbohydrate oxidation and need to eat to replace carbohydrate and insulin resistance, these factors can be regulated by dietary modifications and lifestyle modifications.