What effect will a fat tax have?

From The Guardian:

Imposing a 10% "fat tax" on sugary drinks would help tackle soaring rates of obesity, according to new research by international experts.

Would it, by God? And your evidence?

"In testing taxation as an option for shifting beverage purchase patterns, we calculate that a 10% increase in the price of SSBs could potentially result in a decrease of 7.5ml per capita per day." A similar 10% hike in the cost of full-fat milk would also reduce consumption of it by 5ml per person per day and increased intake of reduced fat milk by 7ml per head every day, it adds.

Okay. So a 10% tax—essentially a tithe to be given to the Gods of public health—would reduce full-fat milk intake by 5ml a day, or 1,825 ml a year. That works out at 3.2 pints a year.

Doesn't sound like very much, does it? And indeed it's not. There are 380 calories in a pint of full-fat milk and 265 calories in a pint of semi-skimmed. If a 10% tax on full-fat milk makes people replace 3.2 pints of full-fat with 3.2 pints of semi-skimmed, they would reduce their calorie intake by 368 per year ((380-265 = 115) x 3.2 = 368). This works out at one calorie per day which is, to all intents and purposes, nothing.

But there is more. The fat-taxers seem to think that consumption of reduced fat milk will increase by 7ml a day—more than offsetting the 5ml fall in full-fat milk consumption. Why they believe this, I know not, but let's roll with it for a moment.

A 7ml increase is 2,555ml a year, ie. four and a half pints a year. If this is semi-skimmed, this is an extra 1,166 calories.

If it is skimmed milk (195 calories per pint), it would be an extra 858 calories.

Remember that people will supposedly lose 1,216 calories because the fat tax makes them reduce their full-fat milk consumption by 3.2 pints.

So if they switch from full-fat to semi-skimmed milk, they will lose fifty calories per year—a whopping 0.1 calorie per day! (1166 - 1216 = -50). Just watch that flab fly off!

Even if they switch to skimmed milk, they will lose only 358 calories per year, ie. one calorie a day (858 - 1216 = -358).

As for reducing consumption of "sugar-sweetened beverages" by 7.5ml a day, there are 400 calories in a litre of Coca-Cola, so a reduction of 7.5ml works out at 3 fewer calories per day. This, of course, assumes that people would replace their Coke with water or nothing at all. If they substituted something "healthy", like an Innocent Smoothie, they would wind up consuming more calories because a typical litre of smoothie contains more than 500 calories. (But we don't mention that because middle-class people like them.)

Since the average adult male needs 2,500 calories per day to maintain a healthy weight, the idea that a reduction of 0.1 to 3 calories would "help tackle soaring rates of obesity" must be described as extremely fanciful. It would, however, be an effective way of clawing more money out of the hands of the hapless electorate on the pretext of health. And that, dare I say, is the real appeal.