Hot or cold drinks in winter?

In winter I normally take a flask of hot black currant drink out with me in the hills, whereas in summer I take a bottle of cold water.  Some people take water all year round claiming that the extra calorific value of a hot drink does not compensate for the extra weight.  What are the facts?

Nutritional value is measured in calories, as the dieters well know.  When you see nutritional information on a food packet it is given in kcal.  A kcal is what is popularly known as a calorie.

We need some basic information before it’s possible to do a calculation. 

My 600 ml stainless steel flask weighs 500g (empty)

A standard 1 litre Sigg bottle weighs 125 g.

If these are full of water/tea/black currant (no difference in weight) then the weights are:

600 ml in a flask     1100 g

1000 ml in bottle    1125 g

The difference in weight is 25 g, less than a wire gate karabiner, but the flask contains 400 ml less liquid, that’s about the same volume as 1½ mugs of tea.

Temperature.  Assuming it’s a cold day then the temperature of water in a Sigg bottle can get rather low.  I’ve certainly known it to start freezing (0ºC).  Assume you’ve got it in your rucksack which will insulate it, so lets assume it’s at 5º.  What about the drink in a flask.  Everyone knows water boils at 100ºC but you can’t drink it that hot without scalding yourself.  I measured the temperature of a cup of hot tea.  It was “sippable” but far too hot to just knock back.  This is therefore the maximum temperature of your drink in terms of additional calorific value because it’s hot.  And the temperature is… 54ºC.

It is not possible to define how much energy you get out of black currant drink (or tea/coffee/chocolate for that matter) as it varies so much from brand to brand (from virtually nothing to 100s of calories).  In any case, it is irrelevant as there’s nothing to stop you putting it in your water bottle, in which case the playing field is level and like is compared with like.

To calculate the calorific value of hot water you need something called the specific heat capacity.  This is a fundamental property of all materials and varies widely.  Water has a high specific heat capacity which means it requires a lot of energy to heat it up, the benefit is that this energy is stored in the water.  You need 4180 Joules of energy to raise the temperature of 1 kg of water by 1ºC.  To put this in context, if water falls 418 m then it’s temperature will rise by only 1ºC.

Based on this, the extra energy in 600 ml of drink in your flask at 54ºC over the same drink at 5ºC is 29 kcal. 

A standard size Mars bar has 299 kcal, a Nutrigrain bar 160 kcal.  In fact 29 kcal is equivalent to 1½ teaspoons of sugar.  So in pure energy terms, put 1½ extra teaspoons of sugar into the drink in your water bottle and you’ve got the same amount of energy as a hot flask but more liquid for the same weight.   Case closed?  don’t take a flask?  Well no actually.

In terms of volume of liquid, you can top up en-route.  I regularly drink straight out from mountain streams and lakes without filtering/boiling/sterilising.   In 20 years I’ve never been ill.  Even in sub-zero temperatures with snow and ice everywhere streams are still flowing.  On a long walk-in in winter I’ll half fill my cup with water  from a stream and then top up with black currant.  I then get a warm drink (the drink in the flask is much hotter than 54ºC) but double the quantity.  There are two exceptions to this: the first is the Cuillin Ridge where there is no water source, the second is in the Alps where no one in their right mind will drink anything that’s come out of a glacier.

Another reason for taking a flask is the instantaneous energy balance.  Assume you’ve added extra sugar to your Sigg bottle so the calorific values of the hot and cold drinks are the same.  If you drink hot liquid then the energy goes into your body instantly.  It feels warming.  Now drink the cold water.  Your body has to ingest the sugar and then burn it.  It’s a bit like taking an aspirin for a head ache.  The pill is not instant, it takes time to work.  The cold drink is the same, you’ll get the same amount of energy as the hot drink, eventually.  In fact the situation is even worse as your body must supply instant energy to raise the temperature of the cold liquid to 37ºC, your internal body temperature.  Hence the initial effect is to cool your body. 

And finally, perhaps the best reason for taking a hot drink up in winter is the psychological value it provides, which unfortunately is totally unquantifiable.  You’re cold, wet, it’s getting dark and you’re miles from the car.  Which would you prefer, a hot drink or an ice cold drink?

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