Study Review: Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men
Necessary Background
- Brown fat is a type of fat in the human (and other animals like mice) body that has a chief function of heating our body up.
- Instead of making ATP from fatty acids, it actually burns the fat and makes heat instead.
- The mechanism of turning food into heat rather than energy (calories) has been explored for weight loss for a while. Dinitrophenol (DNP) has an incredibly fascinating story about it worth looking in to. Back in the 1900s in the mining insdustry, people that dealt with TNT actually found that they were losing weight and sweating. They found out it was from a molecule called DNP and started taking it to lose weight. The TLDR of it was that people actually lost a ton of weight but some died from becoming hyperthermic.
- Brown fat is concentrated in an area right above your clavicle called the supraclavicular fossa, so the researchers measure skin temperature here as a proxy for brown fat activity.
- The Gold standard of brown fat activity measurement is a PET scan. If you give a person glucose that is labeled with a tracer, the PET scan can measure the brown fat uptake that glucose directly.
What they did and how they did it
- They took two groups: winter-swimmers (people who swam in the cold Nordic waters regularly) and non-winter-swimmers. They measured a couple things:
- Metabolic Markers after Glucose Tolerance Test
- Their sympathetic response (pulse and blood pressure) to sticking their hand in cold water
- Their brown fat activity during a comfortable state and a cold state.
- They used three measures for Brown fat activity:
- PET scan during the end of cold exposure
- Infrared thermography (over supraclavicular fossa) all other times during sleep
- A little stick on thermometer (over supraclavicular fossa) called an iButton during sleep
- They used three measures for Brown fat activity:
What they found
- Winter swimmers have a colder core temperature at rest. (They conclude this might actually be the effect of sauna, which most winter swimmers use)
- Winter swimmers had no BAT glucose uptake at a thermal comfort state.
- Winter swimmers have higher cold-induced thermogenesis than control subjects, suggesting promise for cold exposure as a tool to improve metabolic health
- Human supraclavicular skin temperature varies with a diurnal rhythm. Winter swimmers have a unique peak in the morning, which needs to be studied more to understand if it matters.
- Overall Resting energy expenditure is greater in the winter swimmers than control.
Questions left unanswered
- It was very important that this study matched subjects by VO2 max. Their BMIs were matched but the control group had greater percentages of body fat, suggesting that the winter swimmers could simply have more muscular builds and greater fitness. That could explain the differences in energy expenditure and the improved metabolic markers in winter swimmers, however matching VO2 max makes that less likely.
- Are these metabolic changes transient? Do they disappear after stopping cold exposure?
- How much cold exposure is necessary to see these metabolic effects?
Takeaway
- Cold exposure affects brown fat in a somewhat confusing way. However, it seems to clearly overall have a positive effect on metabolism (Increased insulin sensitivity and increased metabolic rate). Exactly what is causing this effect is unknown. It is also unknown how much of this is simply due to cold exposure.
- With further studies, I would like to know exactly why metabolic rate is increasing, how much of that is due to cold (and not increased exercise), and how much these metabolic effects matter?
