Why you shouldn’t take a cold juice on a hot day
The Sunday Nation
Nairobi,
30 December 2012
Does it make any
sense to drink a cold glass of juice on a hot day? That sounds like a
trivial question but let’s think about it: juice has a fairly high sugar
content – that’s why it’s sweet – and this is broken down in the body to
generate energy. This energy is presents itself as heat; it has a
warming effect on the body.
So the question is
whether the cooling effect of the cold juice takes away more heat than
that generated when its sugar is broken down. To find out, we simply
calculate the two quantities.
Now the human
temperature body is about 36 degrees celcius and a chilled drink may be
at about 5 degrees. Thus when you take the cold juice, your body loses
heat in warming the drink over a range of 31 degrees (36 minus 5).
A glass of juice is
about 250ml, which is approximately 250g (a quarter of a kilogram). The
juice is mainly made of water and therefore it heat capacity is about
one kilocalorie per kilogram – perhaps a little more. This is the amount
of energy required to raise its temperature by one degree celsius.
Therefore, warming the juice from 5 to 36 degrees takes away 7.75 kCal
(0.25 x 31).
The dietary energy
generated when the juice is broken down by the body varies depending on
the fruit from which it is extracted. Juice from a passion fruit has
about 140 kilocalories in every 250ml while that from oranges has 120
kcal.
These quantities are
much larger than the heat expended by the body in warming the cold
drink. Thus it turns out that drinking a cold glass of juice on a hot
day will only give temporary relief. Once the body breaks down the
drink, it will gain about twenty times more heat; that is you will feel
warmer that you were!
By now you must be
wondering about the effect of a cold soda. The nutritional information
label on a bottle of Coca-Cola states that it has 105kcal in every
250ml. Thus drinking a coke – or any other soda for that matter – is
also unlikely to yield lasting cooling.
The one drink that
you can be sure will cool you effectively is chilled water! This is
because it does not have any dietary energy. It has zero calories.
How about hot tea on
a cold day: does it work better than room-temperature juice? The answer
is equally surprising. A cup of tea with milk and one sugar has about
50kcal – this is just a third of the 140kcal contained in a glass of
passion juice!
And with about 90
dietary kilocalories, even a glass of beer will generate more heat in
the body than a hot cup of tea.
This leaves one final
question: if a cold juice heats the body more than it cools it, why do
we drink them? I suspect it has something to do with the instant
gratification compared to delayed lasting effect.
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