Thermogenesis is a term that describes generation, or production of heat. There are two types of thermogenesis: shivering and non-shivering. Shivering thermogenesis is always associated with muscular contraction and often occurs as a reaction to the lower environment temperature. Non-shivering production of heat is a type of continuous thermogenesis that occurs in both muscle and adipose tissues, and is dependent on the metabolic rate.
Metabolic rate is the rate at which energy is spent per unit of time by the body. It is measured in kcal/h, often called energy expenditure rate. Since many factors cause the metabolic rate to vary (such as different types of activity), basal metabolic rate (BMR) is most commonly evaluated. BMR represents energy expenditure rate at rest.
Since the amount of energy derived from oxidation of the same amount of fats, carbohydrates and proteins is the highest for fats (9 kcal/g), the latter substrate is the preferred and most effective form of stored energy in the body. Fat collected in the adipose tissue represents the depot of unutilized energy the body "puts away" when the need for energy is less than the caloric value of the nutrients the
body has at its disposal. Conversion of nutrients (such as fat) to energy occurs within the cells, more specifically, inside cell structures called mitochondria.
Mitochondria’s role in energy exchange is well recognized. Often referred to as cellular “power plants”, mitochondria are able to convert energy derived from oxidation of various substrates into the ultimate “mobile” energy unit, – ATP (adenosine triphospate) molecule for further use in cellular processes such as muscle contraction, or alternatively, dissipate in the form of heat (thermogenesis).
The most striking difference between the brown and white adipose tissues is in the amount of mitochondria they contain. These intracellular organelles make fat cells visibly darker. Brown adipose tissue plays an important role in the heat production and maintaining body temperature in human infants and hibernating mammals. In human adults, however, the amount of brown adipose tissue is insignificant and irrelevant in terms of maintaining homeostatic body temperature. Thus, the overwhelming majority of human adult adipose tissue is white, with an average of 2,000 mitochondria per cell(5).
The efficiency of the ATP generation process within the mitochondria is never 100%. So-called “mitochondrial proton leak” is a phenomenon responsible for heat generation that occurs as a result of not fully aligned (coupled) biochemical reactions aimed to generate ATP.
The balance between ATP production and heat generation, among other things, is regulated by specialized proteins called UCP (uncoupling proteins). Research shows that dietary fucoxanthin supports UCP-1 production resulting in a shift of the equilibrium toward “proton leak” and heat generation. Consequently, additional fat breakdown takes place to accommodate production of the same amount of ATP.(See Fig.3) Thus, UCP-1 uncouples the process of ATP production, making it “less efficient” in terms of ATP yield and more efficient in terms of heat generation.(See Fig.2)