[thedoc (OP)]

Linda Scully asked the Naked Scientists:
   
Aloha Chris,

Your discussion of calories burned from drinking cold water got me thinking.  If the average person burns 60cal/hour just maintaining body temperature, how many calories are burned while swimming or diving when heat transfer away from the body would be very high?  As a diver, we are told during training that a diver will lose heat 25 times faster in water than on land, but that can't be right, or everyone could lose weight rapidly just by sitting in a slightly cool bathtub.  I would appreciate your thoughts on this, especially since Victoria is an avid diver.

Thanks,

Lynn Scully
What do you think?

[chris]

This topic explores much the same question:

https://www.thenakedscientists.com/forum/index.php?topic=69561.0

[Colin2B]

Linda Scully asked the Naked Scientists:
   
Aloha Chris,

Your discussion of calories burned from drinking cold water got me thinking.  If the average person burns 60cal/hour just maintaining body temperature, how many calories are burned while swimming or diving when heat transfer away from the body would be very high?  As a diver, we are told during training that a diver will lose heat 25 times faster in water than on land, but that can't be right, or everyone could lose weight rapidly just by sitting in a slightly cool bathtub.  I would appreciate your thoughts on this, especially since Victoria is an avid diver.

Thanks,

Lynn Scully
What do you think?

Bjoern Brembs  asked the Naked Scientists:

....Every spring they go scuba diving in cold water - their motto: freeze yourself slim. They protect themselves against dangerous cooling, of course, but keeping the body at normal temp as well as the exercise of diving burns enough calories to get the waistline perfect for summer beach time :-)
So while cold water taken orally may not shed any pounds, external application may work :-)

I think the 25 times figure is a misunderstanding by your trainer. The conductivity of water is 25x that of air, so if we heat 2 equal pieces of metal and put one into still air and one into still water at the same temperature, the one in water will lose heat 25x faster than the one in air - ignoring convection and radiation. However, the human body is not passive and has defence mechanisms when in contact with cold water. Blood vessels near the surface constrict reducing heat loss, this also forces more blood to flow deeper in the limbs where larger veins and arteries run close together resulting in heat exchange between warmer blood going to the surface and cooler blood going to the core, warming the cooler blood and so core temperature is protected at the expense of surface temperature. The heat loss quoted would also assume direct skin contact, but most divers wear protective suits which again limits the heat loss.
Sitting in cool water will increase metabolic rate, but there is a limit to how long you can sit in the bathtub and, as with exercise, duration is a very important factor. Also, at tolerable temperatures the effect is not very great. In one study subjects were kept cool and still at 16C for 3hrs a day and the average increase in energy used was 76 kcal/hr. They were cooled to just above the point of shivering because that is another of the body's defence mechanisms, involuntary muscle contractions that can use up to 400 kcal/hr.
Muscle activity is a very effective means of keeping warm, far more than any increase in metabolic rate, because 70% of calories burned by exercise are heat. It also means that you cannot add the effect of being cold to that of any exercise you do while being cold, because as soon as you start to generate heat from muscle activity the metabolic heating turns down. Exercising, rather than sitting in a cold bath, has many other benefits and will burn more calories.
Interestingly, you can burn more calories from exercising in the warm because the body has to expend extra energy keeping cool, whereas exercising in the cold can eliminate sweating and other cooling mechanisms.
Of course, if you do exercise at temperatures where you feel cold you will burn more calories, but you will also need to take on more calories to keep warm, so you would need to be very disciplined to not take on extra calories. A study of cyclists on static bikes showed that they were most efficient at an air temperature of 11C which meant the exercise was still generating enough heat to keep warm.
Turning down the central heating has been shown to have an effect on metabolic rate, but only if you don't put on extra clothing and eat more to compensate.

There has been a study of pearl divers who freedive.  In winter the temperature is 10C and their metabolic rate goes up 30% compared to summer, but they only wear thin cotton suits. They also take on extra calories just to have the energy to function.  Because they dive all year round they become acclimatised to the cold water, whereas a recreational diver would be unlikely to function in these conditions. A scuba diver with protective suit is unlikely to be exposed to this degree of cold.

For scuba divers there is an extra issue. Burning calories either by metabolism or activity both use the same amount of oxygen and the diver only carries a limited supply in the air tank. So you can either be active and warm, or cold and rely on metabolism, but either way the calories used are limited by the air supply. This of course is not a limitation for a surface swimmer in cold water.
Having said that, my experience of cold water diving is that everyone goes off to the pub to indulge in their favourite menu, often fish & chips, steak & ale pie (with chips), mixed grill (with chips), all day breakfast (with chips), often followed by sticky toffee pud and at least 2 pints. I have never heard a diver ask for a light salad and mineral water!
At the end of the day, if you want to be slim you have to limit calories, particularly energy dense foods, and exercise is far more beneficial than just sitting around.

If anyone is tempted to try cold water exercise please make sure you are fit and acclimatised as there are dangers:
The vascular constriction response raises blood pressure which is a risk for stroke and heart attack.
Cooled blood is more likely to clot again raising risk of stroke and heart attack.
Exposure to cold can initiate heart arrhythmia.