In honour of the time-honoured New Year’s resolution of losing weight, my co-conspirator Dave calculated how to do so by doing no more than drinking cold beverages. 🙂 What to know more? Read on!

The sciencey bit

If you drink X kg of beer at Y deg C, and later pee it out at body temperature, then you have gained energy from digesting the beer but spent energy by warming it up. The change in energy should be something like:

X * 1800 – X * 4 * (37 – Y) = X * (1800 – 4 * (37 – Y))

Where the numbers are:

• Nutritional calorific value of beer: 430 (food) Cal / kg = 1800 kJ/kg
• Heat capacity of beer: 4.0 kJ/K/kg
• Body Temperature: 37 C

To make this calorie-neutral, you’d need:

Y = (4*37 – 1800)/4

which is -413 deg C, which is not just below the temperature where beer would freeze, but below absolute zero.

If you were willing to drink Kaliber All Natural Non-Alcoholic Brew Light Malt Beverage, which is about 140 Cal / kg = 586 kJ/kg, things would balance out at:

Y = (4*37 – 586)/4

which is “only” -110 deg C.

For something like Diet Coke, which seems to be about 4 Cal/kg = 16 kJ/kg:

Y = (4*37 – 16)/4

So, if you’re drinking it at cooler than 33 deg C, you’re probably losing calories.

If the coldest you’re willing to drink is about at about 4 deg C, then for your drink to result in you expending energy, then you need:

E – 4 (37 – Y) < = 0

where Y >= 4, and E is the energy content per kilo of your drink.

E < = 148 - 4Y <= 132 kJ/kg,

So, you’d be aiming for less than ~32 Cal/kg, or 9.5 Cal for a 300ml bottle.