Why he's hot when she's chilly
We may be warm-blooded creatures but you'd barely know it some days. Your fingers are freezing and your toes are like 10 little ice blocks but while you complain that the temperature is set to arctic, everyone else is acting like it's the Sahara.
Each person has a different sensitivity to the cold.
Les Toop, a professor of general practice and head of the department of health and general practice at Otago University, says the way a human feels the cold comes down to the individual's ability to generate and regulate temperature, how it is conserved or spent, and how that individual perceives temperature.
For example, take this Auckland couple: Australian Dani Allen-McAuliffe and Kiwi boyfriend Mike Revell.
In winter, Allen-McAuliffe wears a beanie around the office and runs her hands under the hot tap a few times a day to bring feeling back to her fingertips. She even gets goosebumps in summer.
When she was in London, she wore stockings under leggings under jeans and bought boots that were a size too big so she could fit three pairs of socks underneath.
Now living in New Zealand, in the evenings she watches TV snuggled under a blanket while her partner wears shorts and a T-shirt on the side of the couch nearest the window, hoping for a draught.
"I think there's something wrong with Mike," she says. "His feet are always hot. His dad's a doctor so I asked him once if he thought Mike had a problem.
"It's so painful, I end up having to put on extra clothes and sit on the column heater. He always kicks his feet out of the doona [duvet] because they're so hot. I swear to God, he's got a medical condition."
Allen-McAuliffe often wonders if she feels the cold more because she comes from the hotter climate of Cairns - which is tropical all year round but summer temperatures sometimes climb to 41 degrees Celsius - however, she suspects it might have more to do with having little fat or muscle mass for insulation.
Conversely, Revell has a decent muscle mass and grew up in Rotorua in one of those cold New Zealand houses where, during winter, you could see your breath inside. His university years were spent studying in Dunedin "in a flat like an icebox".
Toop explains that the two most important organs are the heart and brain so our bodies have evolved to keep these two parts warm.
However, humans are also designed with four appendages, the arms and legs, which are no good for heat conservation.
Temperature regulation is about keeping warm when it's cold and losing heat when it's warm. We do this in three ways: heat generation, heat conservation or loss (we are usually doing one or the other), and energy use.
Almost all heat is generated by muscle activity, so immediately there is a variation between individuals. For example, if you are active and have a large body or muscle mass, you generate a lot of heat. By contrast, if you have little body mass and muscle and don't use them much, you will generate very little heat.
"The obvious example is a skinny old person sitting at home not moving very much," Toop says.
"But on the other hand, remember the test match of All Blacks versus Ireland with the steam coming off them in the scrums? Their large bodies were making huge amounts of heat."
The other thing related to muscles is energy consumption, or the metabolic rate.
If you imagine the body as a car, it's like the tick-over rate. The tick-over rate can be altered by what a person eats, so if you miss meals, the body thinks it's going into starvation mode and it turns down muscle activity to conserve energy.
Our clever bodies also come fitted with sensors and receptors designed to keep the core temperature around 37C. You can have a few degrees either side where the enzyme system continues to work but if the core temperature gets too hot or too cold you'll die.
Heat conservation is the way in which we keep ourselves warm in these colder months. Starting from the outside and moving inwards, clothing is relevant. Each layer traps its own temperature. Hair on the head keeps heat from escaping and body hair keeps a layer of warm air next to the skin in a way that a shaved leg will not.
Next up is the body's fat layer, which acts as both a store of energy and as insulation.
Then we have "homeostatic mechanisms" which maintain automatic processes such as breathing and heart rate. We have heat regulation mechanisms, which, for example, heat the body while it's fighting a virus to kill the bugs. Another homeostatic mechanism regulates blood flow to the skin.
The skin is the body's largest organ and the bloodflow to it can be shut down or opened up (much the same as water flow in a radiator).
Think about when you've just played sport and your face is bright red and sweaty - that's because a huge amount of blood is going through to your skin.
If you are cold, that mechanism shuts down, so initially you shut down the bit that's radiating the most heat - your feet and hands. If you think about it, hands are the perfect radiator - a large area with little fat and good blood supply because they have so many nerve endings.
Some people have frozen hands because their body is pulling heat into the middle to keep their core warm. So if you wear enough clothes on your trunk, you don't have to pull blood away from your hands and feet. Ergo, a woolly hat and one of those sleeveless puffer jackets should start to warm your extremities.
Sweat is also important in temperature regulation. That's how you can live in a place like Darwin without dying - because you sweat and have exposed skin. As the sweat evaporates, the skin cools.
Toop says the final thing that plays a part in feeling the cold is perception, which takes into account how you grew up.
"I was looking for a job in Edinburgh and went to Arthur's Seat. It was one of those days where it was bitterly cold and the wind comes off the North Sea. I was wearing everything I owned and it still cut through like a knife.
"Around the corner comes this man in his 70s wearing a kilt and a shirt, his legs and arms exposed. He gave me a hearty welcome and I wondered why he wasn't dead of hypothermia. It was because he'd grown up like that and just didn't feel it.
"You'll always find one cold soul who'll want the heater on full blast, so some people's ability to register the cold as being uncomfortable and wanting to do something about it is simply down to perception.
"You see kids wearing next to nothing and you don't know if it's just bravado or if they're really not feeling it.
"Or in workplaces, people complain. Sometimes the temperature will only be raised or lowered by a degree but it happens so quickly people's homeostatic mechanisms don't have time to adjust.
"Sometimes, it just comes down to genetics. Or some medicines, either deliberately or by side effect, blunt the homeostatic mechanisms, and may drive blood into the skin."
So if you're one of those people who run cold, you could try eating regularly, moving your muscles and wearing warm clothing that keeps your core covered.
- © Fairfax NZ News
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