If you already read my previous post, you should know that there are many different ways of activating our brown adipose tissue (BAT) depots, and thus increase our overall energy expenditure or EE (remember that BAT converts energy into heat to help body thermoregulation). This increase in EE has arisen as a novel target for controlling body weight, as an alternative to the obvious strategy of restricting caloric intake. Among the different approaches discussed before, a potential increase of BAT volume and activity has been postulated through the intake of some dietary compounds. Several studies suggest that food-derived components, particularly polyphenols, may play a role in preventing and managing obesity by increasing EE through BAT activation. Based on a recently published review on polyphenol supplementation and BAT activation, in this post I will talk about different dietary sources for these compounds, and about the current knowledge on the effects of their administration. But before getting started…what is a polyphenol?
“the role of polyphenolic compounds on energy expenditure enhancement represent a promising target in the fighting against obesity and its related diseases”
Polyphenols – also known as polyhydroxyphenols – are compounds characterized by the presence of large multiples of phenol units. Depending on the number and characteristic of these phenol structures, polyphenols can have different physical, chemical and biological properties. These compounds are secondary metabolites of plants and are generally involved in the defense against ultraviolet radiation or aggression by pathogens.
Chemical structure of phenol, the structural unit of polyphenols; and catechin, a flavan-3-ol present in several food sources
There are more than 8000 polyphenolic compounds identified in several plant species, but only a handful of them has been linked to an increase in energy expenditure. So, which are the sources for these compounds? What do we have to eat or drink to include them in our diet?
Flavan-3-ols. This group includes the most consumed polyphenols in western populations. The main sources of flavan-3-ols are dark chocolate, green tea, berries, nuts, red wine and grape seeds, which contain the flavan-3-ols known as proanthocyanidins and catechins. A diet rich in flavan-3-ols has been associated with a reversal in BAT dysfunction and stimulation of thermogenesis, together with an increase in EE and fatty acid oxidation stimulation, both in rodents and humans. However, we have to take into consideration that in most of these studies, the supplementation is made at extremely high doses, amply exceeding the mean dietary exposure to these compounds.
Tea flavan-3-ols. Within the flavan-3-ols, a separate group is that composed by the tea flavan-3-ol monomers. These polyphenols can be found at high levels in green tea. Supplementation with green tea for two weeks has been linked to a reduction in body fat and an increase in EE and BAT protein content in rats with induced obesity. Also, the intake of black tea has been shown to suppress adiposity and promote browning of white adipose tissue (WAT) in mice. Moreover, some studies carried out in humans point out to an increase in BAT density, with the concomitant increase in EE. Unfortunately, tea extracts have also a high content in caffeine and the effect of this molecule on EE was not evaluated in these studies, making the interpretation of results very difficult.
Resveratrol. Probably the best known and most famous phenolic compound over the last years. Resveratrol is a phenolic compound found only in trace amounts in red wine, peanuts, berries, red cabbage and spinach. Although its concentration in food is extremely low compared to other polyphenols, there is a great interest in this compound due to its remarkable effects on energy metabolism in mammals. It has been demonstrated that, in rodents, resveratrol improves glucose homeostasis, reducing the effects of obesity, diabetes and metabolic dysfunction. Also in rodents, it seems that supplementation with resveratrol at high doses reduces weight gain in animals fed a high fat diet, and decreases the size of white adipocytes. Moreover, resveratrol has been linked to an increase in basal EE and improved glucose tolerance, and also to a higher expression of specific thermogenic genes in BAT. Apart from the studies in rodents, there are also some evidences that link resveratrol to an increase in EE and decreased WAT adipocyte size in nonhuman primate models of obesity. However, it still remains to be determined whether resveratrol can exert the same effects in humans, since no changes in body weight have been reported in human trials so far.
Chemical structure of resveratrol
Other (poly)phenols. Apart from the several studies carried out on flavan-3-ols and resveratrol, there are a few reports that support the role of other polyphenolic compounds in energy expenditure enhancement. Isoflavones, for instance, which are found in leguminous plants (especially in soybean) are known to improve insulin sensitivity, reduce fat mass and promote the browning of white adipocytes. Flavonols, present in berries, onion, broccoli, kale and tomatoes, seem to also have WAT browning effects. Gallic acid, a phenolic acid found in high amounts in red wine, tea and berries has been related to increased expression of thermogenic-related genes in BAT of mice. Finally, curcumine, which is a flavor ingredient of curries, has been shown to stimulate EE, decreasing body weight and fat mass and improving cold tolerance in mice.
In light of the different studies compiled in this review, it is clear that, although the role of polyphenols in the enhancement of energy expenditure is supported by most of the available evidence, further studies are needed to better understand the mechanisms underlying the biological effects of these compounds. Moreover, the bioavailability in their food sources and, especially, their effects on energy expenditure and body weight in humans, still require thorough investigation. However, it is obvious that changes in dietary habits are crucial for preventing and managing obesity and thus, despite the several limitations and discrepancies in literature, the role of polyphenolic compounds on energy expenditure enhancement represents a promising target in the fighting against obesity and its related diseases.
So now you know what to do, at least in part, to prevent obesity and stay healthier through diet. Eat more vegetables and fruits (especially berries), drink tea and (responsibly) red wine…and you can even include some chocolate! It is not a secret that these foods are healthy…but now you know why!
Mele L. et al. 2017. Dietary (Poly)phenols, brown adipose tissue activation, and energy expenditure: A narrative review. Adv Nutr. 8(5):694-704