Heat extremes should be avoided, remember only last year someone died and 250 were hospitalised in the Chicago Marathon. The danger of suffering a heat stroke is all too real.

Ok, a few things for people that use HR monitors and exercise in hot environments. When you are not properly hydrated your blood's viscosity alters; less blood plasma means that your heart needs to work harder to be able to provide your working muscles with much needed oxygen. A faster HR represents more calories burned according to your HR monitor, since the formula that it's using is based on how fast your heart beats. In reality, although your heart pumps blood faster, your work rate is much lower, your HR does not represent the normal pace/work rate/calories used, that is associated with under normal conditions. So, make sure you stay well hydrated and don't worry much about calories, run early in the morning or in the evening, always choose the best options when exercising

i dont know if someone said this yet but your body is able to keep a remarkable constant body temp with that being said you wouldnt any more calories at least anything to be significant because the heat is external, only heat generated internally would make a differnence. Also i dont agree with the heart monitor thing yes if your dehydrated your blood volume lowers but not significantly your body has numerous defenses against this happening for instance your pituitary and adrenal cortex produce aldosterone and ADH (antidiuretic hormone), not to mention your veins themselves hold an extra 1L of fluid. Therefore your blood volume stays relatively constant when it doesnt thats what we call heat stroke so unless your havin a heat stroke everytime you excercise your blood volume is not being compromised.

haha good on you. But yeah, i'm sure you'll find that you'll burn more in hotter situations.

It is, however the body is only so good and at a certain point, heat will override the bodie ability to keep constant temperature.

Furthermore! blood volume will decrease. The hormones you mentioned doesn't prevent sweat does it, and your vein theory is wrong. And no, a decrease in blood volume does not diagnose heat stroke. There's a common term (funninly enough i can't remember it, something shift) used to describe the common increase in heart rate after a while of exercising. Try it out fo yourself, exercise with the same load and after 20 minutes or so you will have a higher heart rate.

Interesting points & observations about the HR-Monitors.....I'd further suppose caffiene and other stimulants would also throw off the equation as well?

I've often wondered.....as I continued to ride my same typical route of 14.5 miles on my mountain bike (a year ago), I was doing it in faster time, in higher gears and it was less taxing: this because I was getting in better shape and losing some weight. As the weeks progressed, I noticed I was doing the ride using fewer and fewer calories. The HR-Monitor used to report some 1,100 calories and at one point it was as low as 770 calories (after several months).

Part of me figured it's not right: to move a set amount of weight "X" from 1,200 feet altitue to 2,000 feet altitude should require a set amount of energy. But then I thought, that's not right...because some means of moving the weight are more efficeint then others. What did I do? Develop more muscle fibers? Develop muscles that generate power using less glucose? Or did I develop more mitochondria to produce more power and a more efficient rate? I dunno, it's all quite fascinating and in the end all we have are our theories and what science tries to tell us.

Back on subject. If a person stays well hydrated....I don't think they'd burn more calories exercising in 84 degree weather then if they were in 72-degree weather. The higher temps just make it more difficult for the body to cool itself...so you sweat more. Intensity changes calories burnt, but not temperature (barring extremes).

Exactly. Think of all the people that leave kids/dogs in cars and then find em dead? Unless you are deliberately looking for heat, I would also say run in the morning with a water bottle.

It would probably be the duration BSL. Any of those othere theories you mentioned would be associated with more energy.

I'd like to see some thoughts on training in extreme cold. While its not really training when I go out ice fishing and walk a few miles pulling 100 plus pounds of weight there have been times I felt like I was going to have a heart attack. LOL

I read something somewhere that extreme cold prevents your lungs from getting the oxygen you require. Is this true?

aren't a lot of places that get extremely cold higher elevations? That would explain the oxygen issue....

Come to Michigan in January. I'll show you extreme cold. I think it was 10 for a high all month long. In early morning when I would be getting on the ice it would be below zero. Thats not including the wind chill.

I can't live like that....lol

The rain here is enough to have my DH complain regularly. This year has been particularly bad as we have had no summer. So far only more rain...well we did have 3 days of sun....

He wants to move to the south west....I am not fond of the desert....

not me I love the cold just as much as I love the summer. Theres nothing like walking on water before sunrise to catch some fish. Fire up the camp stove in the middle of the lake to cook a hot lunch, Crown Royal to keep you warm etc.
I think I do more outdoor activities in the winter than summer. With all the hunting seasons, ice fishing etc. Theres so much to do I can't make up my mind LOL

Better than sitting on the couch indoors all winter long.

So back on topic is it true your lungs can't draw air as efficiently when its cold?

In cold weather, there is more oxygen demand by the heart because it is working harder to do the work and maintain body heat.

Our body can cope with cold weather better than in a hot environment, and that's only because we have the right clothing that we can use in such a case. But, we can't keep training for a long period of time in hot environments cause, after a certain point, our body fails to thermoregulate. Your heart starts pumping blood harder because it needs to redirect it to the skin in an attempt to cool our temperature down, but, the act itself is very demanding due to the constant need for oxygen supply to the working muscles. The sun is not helping, our body gets warmer by the minute and a vicious circle begins which results to heart strokes in the long run. Now, when we are talking about running >10km as a training session, there shouldn't be a problem, as long as we are well hydrated and our energy stores are full.