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Women and men may have different optimal times for burning fat. Randy Faris/Getty Images

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  • Unlike white fat, which stores energy, brown fat consumes calories when activated, increasing the body’s metabolism.
  • According to new research, early morning cold is more effective at activating brown fat for men, though not for women.
  • Experts are hoping that brown fat’s properties can be leveraged to lower the risk of type 2 diabetes and to maintain weight.

Brown fat, or brown adipose tissue, is one of the hot topics in metabolic research. Unlike white fat that people hoping to maintain a healthy weight would like to shed, brown fat increases the body’s metabolism, producing heat to burn calories and white fat.

Research has shown that short-term exposure to cold activates brown fat. New brown fat research in humans investigates the potential influence of the body’s own clock, its circadian rhythm, on the activation of the tissue.

It finds, for men at least, that exposure to cold in the morning is more effective at activating brown fat heat production, or “thermogenesis,” than any other time of day.

A similar effect was not clearly observed in women.

The research describes a random crossover trial in which 24 lean young adults — 12 men and 12 women — were exposed to cold water-filled mattresses randomly at 7:45 am or at 7:45 pm.

In each 2.5-hour session, the water in the mattress was gradually reduced to 9 °C, or until a participant began shivering. The water remained at that cold temperature for 90 minutes.

The researchers indirectly tracked participants’ expenditure of energy — and presumably brown fat activation — by measuring their oxygen consumption and carbon dioxide production. They did this before each cooling session, as the mattress was being cooled when it had reached its final temperature and after 90 minutes of chill.

In addition, participants’ skin temperatures were frequently assessed via infrared thermography of the skin just above the clavicle, and blood samples tracked their free fatty acid concentrations, triglycerides, and cholesterol levels.

The research was presented this month at the European Congress on Obesity in Dublin, Ireland.

Dr. Shingo Kajimura, principal investigator at Beth Israel Deaconess Medical Center and professor of medicine at Harvard Medical School, who was not involved in the study, explained the difference between white and brown fat.

“Brown fat is a very energetically active type of fat, which is completely different from energy-storing fat that everyone wants to get rid of. Both are fat cells, but their function is opposite,” he told Medical News Today.

He noted that brown fat’s hue comes from the color of its energy-burning mitochondria, which contain iron.

“Brown fat is really brown because it has a lot of mitochondria, meaning that it is energetically very active,” said Dr. Kajimura.

While noting that the metabolic benefits of brown fat remain “a lot of hypothesis and expectation,” he said “the idea is that if you can activate, or have more, brown fat, then presumably those fat cells consume more energy instead of storing it.”

In light of “compelling evidence in epidemiological studies showing that if you have brown fat, you are protected from type 2 diabetes by 50%,” Dr. Kajimura said the temptation to view brown fat as a promising target for increasing metabolism is powerful.

Dr. André Carpentier is unconvinced regarding brown fat’s impact on type 2 diabetes, based on his research.

“I do believe BAT [brown adipose tissue] is a fascinating tissue, but we cannot invoke this tissue as a plausible target to lower blood sugar (at least under currently tested stimuli) in humans,” he told MNT.

“It could still play a role in chronic energy expenditure, but it is likely to be a modest effect, ancillary to the other current approaches (lifestyle, GLP-1 agonists and the flurry of double- and triple-hormone receptor agonists that are under development),” he added.

Dr. Carpentier said he has found that brown fat in humans does not significantly affect clearing triglyceride, fatty acid concentrations, or glucose from the blood.

One of the study’s surprising findings is that women begin shivering at lower temperatures than men.

The study’s lead investigator, Dr. Mariëtte R. Boon, co-author with Professor Liesbeth van Rossum of Fat: The Secret Organ, said women “started to shiver later and had more constriction of the blood vessels in their hands and feet.”

This could possibly explain the smaller effect of morning cold on women.

“The question is why women were more cold-tolerant in the morning. That is something we should study further,” said said Dr. Boon.

Dr. Carpentier noted the importance of how one induces cooling in such trials, saying, “Personalized cooling was early established as an attempt to minimize shivering with the assumption that, if you can eliminate shivering, the increase in energy expenditure would be attributable to non-shivering thermogenesis.”

“We have shown that you cannot avoid deep central muscle energy metabolism even with careful avoidance of shivering” in humans, he said.

Previous research has found that the body’s circadian rhythm affects the activation of brown fat in rodents, said Dr. Kajimura. He added, however, that mice being nocturnal turns timing on its head when it comes to similar trials in humans.

“That’s one of the motivations for the authors to validate the observation in humans,” he said.

As for the difference in the apparent influence of circadian rhythm in men and women, there are numerous instances in which this is already known to be the case, beyond shivers.

“For instance, the circadian rhythms of melatonin (the hormone that is involved in sleeping) and body temperature are set to an earlier hour in women than in men, even when the women and men maintain nearly identical and consistent bedtimes and wake times,” Dr. Boon said.

Moreover, women tend to wake up earlier than men and exhibit a greater preference for morning activities than men,” Dr. Boon added.

Other research indicates that men benefit more from exercise in the evening than women.

Dr. Kajimura said observing such effects in humans will be crucial going forward.

“What is missing [in existing research] — and that’s why this paper is important — is observations in humans. You want to be cautious about the major conclusion from the paper. But, at least for the male, it seems like a pretty interesting study,” he said.

Asked what avenues of research in brown fat most interest him, Dr. Kajimura said he thinks “that more and more people want to know about the nutritional components that can activate brown fat.”

“The idea is to safely activate brown fat,” he said, noting that remaining too long in the cold is dangerous in its own way, being bad for the heart and promoting hypertension. Overusing thyroid drugs, which are metabolism activators, would also be problematic.

Dr. Carpentier would like to see a repeat of this study using more direct measurements of brown fat thermogenesis using PET 15O2 or 11C-acetate in vivo.

“Only with such a study would we have good evidence of diurnal variation of BAT thermogenesis,” he said.

Dr. Boon summarized:

“The clinical relevance of this study is that, at least in men, taking your cold shower in the morning may be more beneficial.”