Energy expediture and its role in weight gain and lose

We use our net energy for maintenance (average daily expediture is 2000 kcal for female, 2500 kcal for male), for physical activity (exercising) and for growth. Our daily energy expediture can be higher or lower, depending on body size, illness or physical activity. The maintenance part is essential for our survival (things such as heart beat, brain function, respiration).

Now i will introduce some new terms. Basal maintenance in fasting state is called Basal Metabolic Rate (BMR). To this we will add the energy cost of digestion and absorption (dietary induced thermogenesis - approx. 10% of the caloric value of the meal). BMR covers 60-70% of the daily energy expediture and it is different between individuals, depending of the lean body mass (body mass remaining after we subtract the fat). Tall people with lot of muscle will have a much higher BMR than short fat people.

Physical activity is measured by PAL (Physical Activity Level) and is defined by the daily amount of physical activity of a person. This vary from 1.4 for sedentary people to 2.0 for athletes, with 1.6-1.7 being the average Joe. Can be measured though the oxygen consumption (indirect calorimetry).

As for growth, we will think at the net energy required to make extra tissue. It is close to zero for weight stable adults, relevant for children and pregnant women.

We store energy as fat, a dense and easy to use form of energy. The average amount of fat stored by a normal individual is 90.000 kcal (enough to support us for 30 days of fasting, but this can last for months if you are obese). Average amount of proteins stored is 36000 kcal, but you can use only 12000 kcal in case of emergency, before you start to compromise some of the normal body functions. Average amount of carbohydrates stored is 1400 kcal in the muscle and 350 kcal in the liver as glycogen, and it is used to maintain the blood glucose level in between meals.

When the energy intake is exceeding the energy expediture, the excess is stored as fat and we will experience weight gain. After a certain margin (let's say 600 kcal), the body weight tends to level off after a while, even if initially will go up, as the energy expediture also increase with the weight gain, until a new equilibrium is reached at a higher body weight. Why, you will ask? Because the BMR will increase as the body weight increase, even if the BMR is mainly determined by the lean body mass. The energy cost of the physical activity will be higher with greater body-weight (you need more energy to use your body as your overall body mass increase.

The opposite happens if we lose weight. Weight loss is rapid at first, but progressively slows down as the energy deficit decrease. Losing weight, the energy expediture decrease as well as BMR and the energy required for physical activity decrease, until a new equilibrium point is reached. This is the reason why every diet will work well at first, but lose efficiency over time.

Next post will be about weight management and obesity.