Couch potatoes are ‘as fit as hunter-gatherers,’ according to several media outlets, based on research undertaken by anthropologists from Hunter College, New York, and several other academic institutions in the US.
The study compared the number of calories members of an African tribe burned in a day to the average ‘burn rate’ of those from developed countries. They then calculated the total energy expenditure and found it was about the same.
The study concentrated in particular on the metabolic rate of the African tribe and how it compared with westerners.
Metabolic rate is the total body energy expenditure per unit time. The normal metric unit of energy is the joule, but for metabolism the unit calorie is used. One calorie is the amount of heat energy needed to raise the temperature of one gram of water from 14.5C to 15.5C.
Therefore, basal metabolic rate (BMR) is the metabolic rate when a person is at mental and physical rest but not sleeping, at a comfortable temperature and has fasted at least 12hrs.
There are many factors that can affect metabolic rate including:
Sleep, as metabolic rate decreases during sleep.
It decreases with age. After 20 years, it reduces about 2% every decade; this is because there’s a strong link between exercise volume and your metabolic rate. In other words, the more exercise you do, the higher your metabolic rate.
Second, metabolic rate is also linked to total calorie intake. This means that the more you eat, the higher your metabolic rate. And as people get older they tend to eat less, hence they usually have a lower metabolic rate.
Gender. (Women have less than men at any size) Women naturally have a lower BMR, due to breast and gluteal tissue which is higher in fat than in men, due to natural processes such as child birth.
Fasting. This is because when fasting humans must maintain blood glucose above 2mM otherwise the brain will stop working- unconsciousness. This is done for example as fatty acids are released from adipose tissue. The fatty acids are used as fuels by most tissues in preference to glucose.
However the presence of or an increase in any of these factors can cause an increase in metabolic rate:
Height, weight and body surface area contribute a lot to BMR, as the greater the surface area the greater youre BMR, tall and thin people tend to have high BMR.
Pregnancy, menstruation, lactation
· Infection or other diseases
· Hormones (such as leptin and thyroid hormone.)
· Muscular Activity
The thyroid hormones are the most important determinant of metabolic rate, for people of all ages, size and sex. Thyroid hormones increases oxygen consumption and heat production of most body tissues excluding the brain. This is called the calorigenic effect, the ability to increase metabolic rate. If there too many thyroid hormones for a long time, for example with those with hyperthyroidism, a lot of effects other than the calorigenic effect will take place. For example, the increased metabolism will cause an increase in hunger and food intake. Whereas a lack of thyroid hormones can result in mental and physical lethargy and delayed reflexes.
However, the factor that increases metabolic rate most is altered skeletal muscle activity. Small increases in muscle contraction can affect metabolic rate and strenuous exercise dramatically, and can potentially cause an increase in energy expenditure by fifteen times. This is why during sleep the metabolic rate is extremely low because there is less skeletal activity.