How much harder is it to burn calories in hot weather?

Question

I make walks to try to keep a little fit and to burn calories/fat. At a temperature of about 16°C I can walk 10K in just above 2 hrs with almost no rest and not being excessively tired afterwards.

I often spend time in tropical countries. At a temperature of 25° and moderate humidity, my performance is severely limited. I would guess that I walk about 3K in 2 hrs and I get really tired. (I try to supplement fluid and electrolytes, of course.)

The actual temperature may also be higher than 25°, I think. I get the temperature readings from the Internet, and I think they usually indicate temperature in the shadow. However, taking walks you are most often in the sun.

I am a Western guy about 175 cm/83 kg.

My question is divided in two:

1. How many calories are expended in the two situations respectively? (Brisk 10K walk at 16°C or slow 3K with several breaks at 25°C, being much more tired after the latter. Both take about 2 hrs.) (The first would be about 600-700 kcal, but how much is the latter?)

I refer to EXTRA calories expended by the walking. I have not included the basal metabolism in the figure 600-700 kcal. One may also have to consider change in basal metabolism due to f.ex extra work like sweating needed for heat dissipation.

2. If the calorie consumption is much lower in the "hot" alternative, how should you then make your aerobic exercise in the tropics? Is the only alternative to buy a membership with an air conditioned gym and do treadmill etc?

The only thing I found on the Internet is that you burn more carbs (cp to fat) during exercise in hot climate. That may speak against exercising in hot climate, but my guess is that it is just a marginal factor in my situation.

My aim is to burn about 600 kcal extra. I do that in the "temperate" alternative with a 2 hr walk, but how do I do that in the tropical climate?

(OK. I know one answer is "Get up in the morning, you lazy guy." But early or late exercise is just not possible right now for me.)

Edit 1

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2375553/#!po=5.55556 This article mentions thermoregulatory vasodilation in skin as a main mechanism for getting tired more quickly in a hot climate. Cardiac output (f.ex monitored by heart rate) must be used to both muscle and skin. If this was the only mechanism limiting your capacity, I guess one would assume that cardiac exercise could be OK in hot climate but calory burning not necessarily the same when blood is shunted through skin for thermoregulatory purposes.

Edit 2

A useful, non-invasive method to investigate calory burn is indirect calorimetry (IC). (Measure O2 and CO2 in expired air to calculate calory burn pretty exactly.)

I haven't found any studies yet that do IC (and work load) in a hot environment and compare to a temperate environment. One may make the assumption that "blood is simply shunted through the skin to cool off" but to be sure you need experimental data. (Some cooling mechanisms may consume calories, f.ex.)

For example. If you feel "completely wasted" just walking slowly in a hot environment, would your pulse rate then be at the same level as it would be in a temperate environment, for the same level of pronounced "tiredness"? I am not sure the limiting factor is cardiac output, at least not for me. Last time making a walk in hot climate, feeling very weak in the legs, pulse was only around 108 and no real shortness of breath.

Many individual aspects also come into the picture. A short (having the same proportions) or a skinny person will dissipate heat more efficiently. A person with black skin is better protected from UV-rays, but may absorb more heat, under some conditions at least.

Edit 3

This year the L.A. Marathon was run in 31°C heat. Daniel Limo won on 2:10:35. His personal best is only slightly better than that: 2:08:39. As a Kenyan marathon runner he is probably extremely well adapted to heat (genetically and through training).

This good performance in hot weather is interesting. His body can not have wasted a lot of cardiac output by shunting blood through the skin for cooling. I wonder how he performs in cold weather? He maybe has to wear extra clothes when racing at lower temperature??

Answer 1

Your body needs to maintain temperature, when you walk in the sunlight, you have two factors warming you up, you also have two things cooling you, the contact with the relatively cold air, and the evaporative cooling from sweating.

When the temperature rises, the first cold factor becomes weaker, if the air is humid, the second factor will also be weaker since it gets harder to sweat effectively. Your body will then switch from one regulatory system (sweating), to the more critical one, making you stop walking, making you tired.

So eventhough you'll use far fewer calories, you will feel more tired.

I think that you get better at staying cool as your body adapt to the heat, you can also have some impact by chosing clothes that will reflect as much sunlight as possible (so white clothes.)

Is swimming an alternative? Even if the water is warm, it's still below 37 degrees centigrate and since it's water, it will cool you much faster.

Answer 2

I'm not a nutricionist so I can't answer with security question 1. Question 2, however, is a little easier for my knowledge. In theory, in hot weather you consume less calories because your body doesn't need to keep warm so hard. You know, temperature is an expression of heat, Energy = mass * (calorific constant of your body) * (temperature increase) or more in line on what happens to your body temperature: Energy = (area of your skin) * (global heat transmission coefficient) * (temperature difference between your body and exterior)

So, if you are in a hot place, temperature difference is smaller, so energy spent on heating your body is also smaller. Tips to burn more calories in hot weather: - Drink water as cold as you can. It's the best way to down you body temperature. Water is better than juice, sodas, cola... - Air condition.

Anyway, I don't thing there might be a huge difference, so don't get obsessed with it. You are probably getting tired because hot air is less dense and more humid than cold air, so you're taking less oxygen at each breath.

ADD EDIT 1: There is practically no energy consumption by sweating. ADD EDIT 2: I'd say the calories you burn because of moving isn't dependent on the temperature. You might burn less calories in warmer situation because you get exhausted faster, so you don't exercise as much than in a colder situation.

To sum up:

• Calories burned exclusively by exercise are independent of temperature.

• High temperature comes with higher humidity, so breathing becomes harder.

• Combining higher body temperature and harder breathing, you get exhausted faster.

• You burn less calories because your exercise volume and intensity are lower.

Solutions:

• Drink much cold water.

• Throw some extra water on your head and/or get clothed with transpirant clothes. Maybe wearing something like a cap would help too.

• Avoid running between 11:00 and 17:00. Coolest moment of the day is just before daybreak.

• Practice. No pain, no gain.