Basal metabolism

Basal metabolism is a term that we hear very often when we are in a process of losing weight . However, it is not the only element to take into account when it comes to our energy expenditure.

Daily energy expenditure consists of four components , the so-called total energy expenditure –TEE– (additional energy is needed for metabolic processes that take place during growth, pregnancy and lactation).

1. Basal metabolism

Basal metabolic rate is the energy your body expends to exist, to carry out vital bodily functions, such as keeping your heart beating, breathing, circulating blood, producing new cells, or absorbing nutrients. Basal metabolic rate (BMR) is often used interchangeably with resting metabolic rate (RMR). While BMR is the minimum number of calories needed for the basic resting functions listed above, RMR—also known as resting energy expenditure (REE)—is the number of calories your body burns while resting. Although BMR and RMR differ slightly from each other, RMR is an accurate estimate of BMR.

2. Energy expenditure induced by physical activity

The second element to consider is the energy expenditure induced by physical activity, that is, the number of calories that the body burns not only in relation to the sport that is practiced, but also during daily activities (walking, cleaning, taking care of children, shopping, etc.).

3. Thermoregulation

The third element is the thermoregulation of body temperature, that is, the process that allows the body to maintain its core internal temperature. All thermoregulation mechanisms are designed to bring the body back to homeostasis, that is, to a state of balance. The average body temperature is 37ºC, and by thermoregulating the body will try to always have this average. When we are very hot, the body lowers the temperature thanks to sweating and the dilation of blood vessels. On the contrary, if it is cold, there will be vasoconstriction to heat the body, the thyroid will send hormones to increase metabolism (hormonal thermogenesis), and the muscles can contract to create body heat.

4. Thermogenesis

Finally, the fourth element that contributes to our energy expenditure is diet-induced thermogenesis, that is, the energy that we will use to assimilate the different macronutrients that we consume (proteins, carbohydrates, lipids). The ingestion of food leads to – in humans, but also in animals – an increase in oxygen consumption. This increase in the metabolic rate, originally called “specific dynamic action” (SDA), is now called the “thermic effect” (TE) of food or “diet-induced thermogenesis” (DIT). This effect generally begins one hour after ingestion, reaches a maximum three hours later, and continues at this level for several hours. In quantitative terms, DIT represents about 10% of total energy expenditure (15% in the case of cold-induced thermogenesis), basal metabolism, between 60 and 75%, and the remaining 10 to 20% comes from physical activity.

The effect of food intake on energy expenditure occurs at two levels, the first and most important being the expenditure required to digest, absorb, distribute and store the ingested nutrients; the second is the expenditure derived from the production of additional heat due to the activation of brown adipose tissue (BAT).

Thermogenesis factors

The thermogenesis factor of each type of macronutrient is different, and therefore the amount of energy required will depend on each type: carbohydrates, fats and proteins.

Lipids

Fat is the least “expensive” in terms of DIT, because it requires relatively little hydrolysis (3 to 4% of ingested calories).

Carbohydrates

Carbohydrates are intermediate to DIT, requiring considerable metabolism when converted and stored as triglycerides and less when converted to glycogen (10-15% of ingested calories). Carbohydrates and fats can also cause increased heat production unrelated to energy use for digestion, transport, and storage of nutrients. While carbohydrates have a somewhat higher thermogenic effect than fats, they are also responsible for raising insulin, which is in itself one of the reasons for fat storage in the body. Similarly, higher protein consumption in the standard American diet can trigger metabolic diseases.

The protein

Protein is the most “expensive” for DIT, requiring expenditures of up to 30% of the inherent energy to metabolize it, which includes nitrogen elimination, urea synthesis and gluconeogenesis (on average, 15 to 20% of ingested calories). Unlike carbohydrates and lipids, protein – a chain of amino acids – cannot be stored by the body, so it uses more energy to digest it. Protein consumption therefore involves a significant energy expenditure (if 100 kcal is consumed, 30% will be spent to metabolize it, so 70 kcal will be assimilated).

The primary effect of protein on energy balance is thought to be satiety related to DIT. Postprandial thermogenesis was increased 100% with a high-protein/low-fat diet compared with a high-carbohydrate/low-fat diet in healthy subjects . DIT increases body temperature, which may translate into feelings of satiety. High-protein diets are therefore preferred for weight loss and maintenance, by promoting the maintenance or regain of lean mass, decreasing energy efficiency through higher thermogenesis, and reducing intake through increased satiety . Due to the enhanced thermic effect, high protein intake tends to boost metabolism, burning more calories around the clock, including during sleep .

Some studies compare, in healthy men, average BMI of 23 (+/- 0.8) a diet composed of 30% protein + 70% fat versus 12% protein + 55% carbohydrates + 33% fats ¬–very similar to the classic recommendations– and their results concerning energy expenditure. The results show that endogenous glucose production was lower in the zero carbohydrate group, and that glucose creation came from gluconeogenesis, higher in this group. This group also increased resting metabolic rate (RMR), and therefore TEE; 42% of this increase is explained by the need to create glucose from protein (33% of the energy from glucose produced).

Conclusion :

Basal or resting metabolism is not the only factor to consider when talking about the body's energy expenditure. Physical activity, thermoregulation and food-induced thermogenesis are also to be taken into account: the latter will have a different impact depending on the macronutrient consumed.