So why don't we use cold to help us lose weight? I've never heard of ice baths as a weight loss solution, and you would think that somebody would've promoted it as the new fad weight loss craze if it worked as well as it seemed to during my swimming. I decided to do a proper academic investigation of the subject to see what real researchers have found. I actually found a ton of stuff, and almost none of it directly related to what I want. It's going to take me a while to sift through it all, so I'm going to put my actual report after the jump.
I started with learning about the body's ability to thermoregulate. Thermoregulation is the body's ability to maintain its core temperature when the environmental temperature is above or below the thermoneutral zone. The thermoneutral zone is the range of temperatures in which a person feels comfortable, i.e. they neither sweat nor shiver. According to Cannon & Needergaard, it is "operationally defined as the temparature zone at which the lowest metabolic rate is observed." (Side note, that is not quite true--the lowest metabolic rate is observed right before death. The thermoneutral zone is the lowest metabolic rate when you're not in danger of freezing or boiling to death).
The thermoneutral zone is different for every individual, and people can habituate to a new thermoneutral zone without any changes to their body. I.e., if you live in Canada long enough, you will be used to the cold, and feel extra hot in Texas. The normal thermoneutral zone for humans is around 30 degrees Celsius, and lets say it has a range of 26-34 degrees.
While your body is in its thermoneutral zone, your metabolism will act at its normal rate. If you are exposed to temperatures above or below the thermoneutral zone, within minutes your body will respond, and your metabolism will increase. While the studies I read didn't provide exact numbers, they did indicate that exposure to cold will increase your metabolism about twice as much as exposure to heat. I have illustrated this principal in the following graph.
Now, I'm going to be focusing on the application of cold, as it is more effective for increasing metabolism, but I want to stop and discuss briefly the application of heat. First, why does your metabolism increase in the heat? It's because sweating, our way to cool off, is an active process. You can think of all your sweat glands as little mechanical pumps that have to be powered, so our body does metabolic work to produce the energy for them to run. But it doesn't take much energy to sweat. Additionally, sweating is mostly an anaerobic process, which means it's fueled by glucose (carbohydrate) and not fat. The ironic thing is, we always think of sweating the pounds off and burning the fat--we talk in terms of heat. But applying heat to ourselves does not help us lose weight and fat as much as applying cold could (theoretically).
Back to the main point: when humans are exposed to cold, we have 2 responses:
- adiabatic: our body tries to reduce heat loss by peripheral vasoconstriction to centralize heat (i.e. blood leaves your skin and goes to your internal organs--this is why your fingers and toes get frostbite first)
- calorigenic: our bodies increase their metabolism to generate extra heat to keep us warm
Within the calorigenic response, we have 2 mechanisms: shivering and non-shivering thermogenesis. I don't think I need to define what shivering is, but I will note that it is mostly an anaerobic process, which means it's fueled mostly by carbohydrates and not fats. Non-shivering thermogenesis is a process controlled by brown adipose tissue, where fuels (thankfully mostly fats) are burned simply for the sake of generating heat. In skeletal muscle, fuels are burned for doing the work of movement, and a lot of heat is generated as a biproduct (in fact, more heat is generated than useful work). However, brown adipose tissue burns for the sake of producing heat without work. Interestingly, brown adipose tissue (which I'm going to abbreviate BAT as it comes up a lot from here on out) is considered an organ.
In recent years, I can remember hearing about BAT a decent bit, though I never paid much attention as it wasn't pertinent to what I was specifically studying. There is hope that it can be linked to helping reduce the incidence of obesity. Unfortunately, not everyone has BAT, or at least not enough BAT to be detected. Here, I would also like to point out that there is some confusion about whether the BAT did not exist or was just not active, though in the end the result is the same, so the distinction doesn't matter yet (until scientists figure out how to force inactive BAT to become active). Of people under the age of 50, only about 50% have BAT. After the age of 50, it drops to about 10%. There is no difference between genders. People who are more fit have more BAT than people who are less fit.
Most studies so far into the effects of cold exposure on humans have been looking at it from a basic science and survival stand point, rather than its applications. The basic design of these experiments has been: take a dozen young people of moderate fitness, expose them to the cold (ranging from 4-18 degrees Celsius) for 1-2 hours, and see what happens with their metabolism. After 2 hours of exposure to the cold, metabolism was 2.6-3x above resting levels (Hamm et al., 2002; Tikuisis et al., 2000). Additionally, lipid oxidation (the rate of fat burning in metabolism) increased 376% in one experiment (Hamm et al., 2002). Another study showed that after cold exposure, metabolism remained elevated by 2% for the next 24 hours (Buemann et al., 2006).
So far so good: a 300%+ increase in fat burning, and all you have to do is sit there in the cold? And there's a lasting effect? Yes please.
Those studies are a decade old. Lets look at more recent studies, after BAT became a popular phenomenon.
Yoneshiro et al (2011) exposed men to 19 degrees Celsius for 2 hours. Only those with BAT had a significantly increased metabolism. Those without saw no increase. This study was done in a sample of 20-28 year old who were normal weight and healthy.
Vijgen et al (2011) exposed 2 men and 13 women to an average of 14 degrees Celsius for 2 hours. Those who had BAT had an increase in metabolism. Those without saw no increase. This study was done in a sample of morbidly obese people, (BMI = 42) mean age 39.
What we learn from these 2 studies: BAT mediates non-shivering thermogenesis. Morbidly obese people are so well insulated against the cold that cold exposure is not effective.
One possible limitation of these two studies is that their cold condition wasn't all that cold. I mean, I guess it depends on who you're talking to, but for me, I'm still in shorts and a t shirt at 19 degrees. The older studies were much colder than that.
Some other limitations to consider: in the studies that were really effective (Hamm et al., 2002; Tikuisis et al., 2000), participants were either put into water up to their neck or put into a full body suit that could have cold water circulating through it. The suit is obviously not practical for consumers. In terms of the cold water, you could fill up a bath tub or large trashcan with ice water to achieve the same effect. The trash can method is commonly used amongst athletes for post game recovery. Additionally, you could go to the beach or a back yard pool when the water was cold. Using water instead of air is more effective because of the increased cold loss humans experience through conduction and convection in water. Trying to replicate these experiments with your air conditioner in your home would be a huge waste of energy and wouldn't work effectively.
Now, jumping in your backyard pool or the ocean would be feasible if you have access to either of those, but you would still be hard pressed to convince someone to voluntarily freeze their buns off, let alone for 2 hours like they did in these experiments. So, feasibility of cold exposure interventions so far is looking pretty poor.
I'd like to go back to the idea about insulation in the morbidly obese people. These are the people who need help the most from this kind of crazy intervention, but it's not very effective in them. You would be hard pressed to expose them to a temperature cold enough to penetrate all their insulation (i.e. stomach fat) that wouldn't also give their digits frost bite. Ironically, it turns out that fit lean people, who wouldn't need this kind of thing, would have the best response. In the end, it seems to me that it would be the people who are moderately overweight that could have the most benefit from using cold therapy. They would be able to respond to it, and have some weight they need to shed. The next thing to do is to figure out a feasible way to deliver cold therapy.
References (most of these are free to the public)
Beumann et al (2006) Effect of moderate cold exposure on 24 hour energy expenditure: similar responses in postobese and nonobese women. AJP Endoc and Metabl, 263, E1040-E1045
Cannon & Needergaard (2011) Nonshivering thermogenesis and its adequate measurement in metabolic studies. J Exp Biol, 214, 242-253
Castellani et al (1999) Thermoregulation during cold exposure: effects of prior exercise. J Appl Physiol, 87, 247-252
Hamm et al (2002) Effect of cold exposure on fuel utilization in humans: plasma glucose, muscle glycogen, and lipids. J Appl Physiol, 93, 77-84
Maeda et al (2005) Effects of lifestyle, body composition, and physical fitness on cold tolerance in humans. J Physiol Antrhopol Appl Human Sci, 24, 439-443
Maeda et al (2007) Involvement of basal metabolic rate in determination of cold tolerance. J Physiol Antrhopol, 26, 415-418
Tikuisis et al (2000) Comparison of thermoregulatory responses between men and women immersed in cold water. J Appl Physiol, 89, 1403-1411
Vijgen et al (2011) Brown adipose tissue in morbidly obese subjects, PLOS One, 24, e17247
Yoneshiro et al (2011) Age related decrease in cold activated brown adipose tissue and accumulation of body fat. Obesity, 19, 1755-1760
Yoneshiro et al (2011) Brown adipose tissue, whole-body energy expenditure, and thermogenesis in healthy adult men. Obesity, 19, 13-16
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