Brown Fat (Brown Adipose Tissue): benefits, activation

In this Article (Index)

• What is "Brown Fat"?
• Brown Fat Weight Loss
• The "Browning of White adipose tissue
• Cold exposure and BAT activation
• Circadian rhythm and browning
• Exercise and browning
• Melatonin and BAT
• Intermittent Fasting and BAT
• Gut Microbiome's role in browning
• Caloric restriction
• Food and BAT
• Food components and BAT activation

What is "Brown Fat"?

Brown adipose tissue (BAT) or "Brown Fat", is a type of fatty tissue that is different from the regular white adipose tissue or WAT.

It is found in all mammals, but in small mammals it plays an important role in waking them up after hibernation by generating body heat. The mass of BAT in these creatures increases before winter and it helps burn energy during hibernation -to keep alive because hibernating animals can't shiver to heat their bodies- and to re-awaken at the end of the cycle (1).

It is also found in human babies, who can't shiver. The BAT is located on their back, between their shoulder blades. Brown fat generates heat to keep them warm.

Unlike WAT, the brown fat has a different structure. In shares the same embryonic origin as muscle cells unlike regular white fat.

Like all cells, BAT has mitochondria, a cellular organelle that generates energy to keep the cell alive. BAT has many more mitochondria than WAT and these have a higher iron content that gives the cell its brown color. BAT mitochondria also contains a protein called UCP1. This protein makes the mitochondria produce heat instead of ATP (adenosine triphosphate), a molecule used to store and provide energy.

Another difference between white and brown adipose tissue is that BAT has many lipid droplets (multilocular), while WAT has a single large lipid droplet (unilocular).

BAT is also innervated has many nerves and blood vessels that carry oxygen and nutrients to be burned to generate heat, and carry it to the rest of the body. Its role is, like in small mammals and babies, to burn calories to warm the body, a process called thermogenesis, when the temperature drops, the fat burned by BAT in the absence of shivering can account for almost 12% of the Basal Metabolic Rate (BMR), also called Resting Metabolic Rate or RMR (try our our online BMR calculator and learn how to use it as a starting point for your weight loss program) (2)

Prevalence and effects in adults

Infant BAT gradually disappears, but it is found in almost all adults though it represents a very small fraction of total body mass (0.1%)

In adults it is found in certain areas of the body, like the cervical area on the back of our neck, certain backbone areas, and above the collarbone (supraclavicular BAT), also around the heart, kidneys and armpits.

Individuals with more mass of BAT suffer less from cardiovascular disease than those with low detectable BAT mass. Less BAT mass leads to obesity and metabolic disorders, including hepatic fat or fatty liver.

Brown Fat Weight Loss

Different studies in animals and humans have shown that there is an inverse relationship between BAT activity and obesity, meaning that a more active brown adipose tissue is linked to a lower body weight, and the opposite: people with high body fat have lower BAT activity.

A negative side effect of obesity is chronic inflammation that leads to insulin resistance. Trials with mice show that brown fat produces a bioactive compound called maresin 2, that helps reduce inflammation caused by obesity by acting inside the liver (12).

The "Browning of White adipose tissue

Certain stimuli can provoke an increase in the number of a special kind of cell that resides in the adipose tissue and resembles BAT called beige or brite adipose tissue (brite comes from brown-in-white). These cells have multilocular lipids, many mitochondria and the expression of UCP1, suggesting that they too play a role in thermogenesis.

This opens the door to therapies to activate brown adipose tissue or promote the browning of white adipose tissue to prevent or reverse obesity and fight obesity-related diseases.

In the following sections we will review the different ways to activate BAT and increase energy output and, hopefully, lose weight.

Cold exposure and BAT activation

Since low temperature activates BAT's heat-generating activity, some clinical trials have studied the effects of a cold environment on animals and humans. One such study by Smith et al., (2014) (3) followed five healthy men for four months. They led a normal life during daytime but spent each night in a private room whose temperature was adjustable. They were exposed for at least 10 hours every nitght to the controlled temperature. BAT volume and biomarkers were taken to evaluate the impact of temperature on their metabolism.

Temperature was kept at 75° F (24°C) during the first month, 66°F (19°C) during the second, back to 75° F (24°C) during the third and increased to 81°F (27°C) for the final month.

As expected, BAT volume increased, by 42% and the fat metabolic activity rose by 10% during the coldest month, but it returned to its normal value during the 75° F (24°C) baseline month. The warmest month saw the figures completely reversed.

The subjects maintained the same caloric intake, and reported "an increase in desire to eat and reduction in satiety" during the cold month.

Metabolic markers improved with low temperatures, such as insulin sensitivity which improves glucose metabolism.

The authors pointed out that bedroom temperature has gradually increased from 66°F (19°C) to 70.7°F (21.5°C) over the last three decades in the U.S. and suggest that lowering the thermostats in winter could help control obesity.

Risks of temperature therapy

Cold as a BTA activator or browning inducer should take into account the possible side effects. Despite its positive anti-inflammatory and immunosupressive potential in healthy subjects, people with weakened immune systems or those suffering from cancer should avoid further suppression of their immune systems as it may promot cancer grwoth (4)

A study by Kanazawa (2020) (5) pointed out that the current global climate change with higher temperatures will increase obesity rates by about 12%, adding 2.2 lb (1 kg) to the average person's weight between 1961 and 2081.

Exercise and BAT activation

Exercise induces browning, causing deep changes to subcutaneous white adipose tissue or scWAT, increasing its mitochondrial activity however since most of these promising studies involved rodents, more research is needed to validate the effect of physical activity on WAT browning in humans (4)(6).

Circadian rhythm and browning

The circadian rythm is the body's master clock that synchronizes on a daily and seasonal basis many biological processes including blood sugar, hormones, sleep-wake cycles, body temperature and energy balance.

The circadian rythm can be disrupted by factors such as aging, poor sleeping habits, insomnia, exxposure to light during night hours or shift working. This leads to a higher risk of obesity, diabetes type 2 and cardiovascular diseases. Adipose tissue and BAT activity are linked to the circadian rythm (4).

Melatonin and BAT

Melatonin is a hormone that is secreted by the pineal gland at night. It regulates the circadian rythm and energy output in animals including humans.

A study (Halpern et al., 2019)(7) showed that when people with melatonin deficiency supplemented daily with 3 mg of melatonin for 3 months, their BAT volume and activity increased, their blood levels of cholesterol and triglycerides improved as well as insulin levels. This suggests that melatonin is a possible BAT activator.

Trials in mice have corroborated this melatonin role as BAT activator (8).

Food: diets, fasting and bioactive components

Intermittent Fasting and BAT

Tests with mice show that an every other day fasting (EODF) stimulated browning within WAT and "dramatically ameliorates obesity, insulin resistance and hepatic steatosis."

Gut Microbiome's role in browning

The study proved that the bacteria living in the gut play a role in the browning of white adipose tissue: Mice with a depleted gut microbiome showed resistance to beiging induced by fasting, but when they received a transplant of microbiota from EODF-treated mice, they activated browning of their WAT.

Intermittent fasting also altered the composition of the gut microbiome, increasing the abundance of Firmicutes bacteria and decreasing Bacteroidetes (9).

Caloric restriction

Reducing the amount of calories ingested can promote functional beige fat and lead to metabolic improvements. Caloric restriction causes browning in subcutaneous and visceral fatty tissue (10).

Food and BAT

Brown fat can also be activated by eating. This is known as diet-induced BAT activation or diet-induced thermogenesis or DIT. The brown adipose tissue is activated by nervous system inputs (vagal nerve and taste receptors), hormones (ghrelin, noradrenaline and secretin), bile acids and specific food ingredients (11).

Food components and BAT activation

The energy burn caused by BAT is also promoted by certain compounds found in food (11):

• Polyunsaturated fatty acids (PUFAs), increases thermogenesis.
• Fish oils such rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) activate UCP1 in BAT.
• Capsaicin, a spicy component of chili peppers and capsinoids found in red peppers activate receptors that increase the activity of nerves that innervate BAT causing it to increase energy expenditure.
• Other food compounds including thyme, curcumin (from turmeric), allicin (garlic), resveratrols (grapes and wine), retinoid acid (derived from vitamin A), quercetin (antioxidant found in apples, grapes, onions), and green tea's catechines and caffeine activate BAT thermogenesis.