The Danish
government has abandoned its tax on fat and and its plans for a sugar tax. A spokesperson for the tax ministry is quoted
thus: “The suggestions to tax foods for
public health reasons are misguided at best and may be counter-productive at
worst. Not only do such taxes not work,
especially when they choose the wrong food to tax, they can become expensive
liabilities for the businesses forced to become tax collectors on the
governments behalf”[1].
Shortly we will have our annual budget here in Ireland and notwithstanding
the volte-face of our Danish colleagues, the likelihood is that we will face
such a tax soon. In general, the
predicted weight changes associated with projected taxes on sugar sweetened
beverages are grossly overestimated.
A recent
consensus statement of the American Society of Nutrition (ASN) and the
International Life Sciences Institute (ILSI) has examined the topic: “Energy
balance and its components: Implications
for body weight regulation”[2]. One of the areas covered by this paper is the
popular and widely held belief that to lose 1lb of body weight, you need to
reduce caloric intake by 3,500 kcal.
This figure assumes that a loss of 1lb of body weight is made up
entirely of adipose tissue which is 86% fat and the fat has 9 kcal per
gram. This 3,500 kcal figure is widely
used in predicting the benefit of weight loss from a sugar sweetened beverage
tax. It has many flaws.
Firstly, a 1lb
weight loss will not be 100% fat but will also involve the loss of some lean
tissue (muscle and protein elements of adipose tissue and its metabolism). Whereas fat has an energy value of 9 kcal/g,
lean tissue has a value of 4 kcal/g. The
exact ratio of the loss of lean and fat in weight reduction depends largely on
the level of fat in the body at the outset.
The higher the intake level of fat, the higher the proportion of fat
lost. However, as a person sheds fat,
the ratio of fat to lean changes in favour of the latter, so subsequent weight
loss will have a lower ratio of fat to lean.
The blanket use of the 3,500kcal value ignores this.
The second
criticism of this rule is that it ignores time.
If you shed 3,500kcal per week every week, that would differ from a
deficit of 3,500 kcal per month every month.
The former leads to a daily deficit of 500 kcal while the latter is just
117 kcal. Even the most non-expert
dieter knows that such differences in daily energy deficits will lead to
radically different rates of weight loss.
Thirdly, the 3,500 kcal rule assumes complete linearity – in other words
the rule equally applies, pound after pound of weight loss. We saw above that
progressive weight loss will progressively increase the % of that weight loss
as lean tissue but more importantly, the 3,500kcal rule ignores a major
adaptation in energy expenditure.
Basically, our basal metabolic rate (BMR) falls as we restrict our
caloric intake. Since BMR accounts for
88% of energy expenditure in most sedentary persons, that means that a fall in
BMR represents a significant adaptive response through increased efficiency of
energy use making weight loss progressively more difficult.
Researchers at
the US National Institute of Health have developed a very detailed mathematical
model which predicts weight loss based on a wide variety of inputs[3]. The model has been validated against a number
of highly controlled weight loss programmes.
Together with researchers based at the USDA and the economics
departments of the universities of Florida and Minnesota, they have examined
the likely weight loss that would accrue from a tax of 20% (about 0.5 cents per
ounce) on sugar sweetened beverages in the US[4]. They concluded that the nutritional input
would be a reduction of energy intake of 34-47 kcal per day for adults. Using the 3,500 kcal rule, an average weight
loss of 1.60kg would be predicted for year 1 rising to 8kg in year 5 and to
16kg in year 10. However, when the
dynamic mathematical model is used, the corresponding figures for years 1, 5
and 10 are, respectively, 0.97, 1.78 and 1.84 kg loss. The % of US citizens that are over-weight is
predicted to fall from existing levels of 66.9% over-weight to 51.5%
over-weight in 5 years time using the 3,500 kcal rate but using the dynamic mathematical
model, the 5-year figure for the over-weight population in the US would be just
62.3%. Clearly, the continued use of the
3,500 kcal rule in predicting weight loss should cease and the recommendations
of the consensus statement of the ASN and ILSI should apply: “Every permanent 10 kcal change in energy
intake per day will lead to an eventual weight change of 1lb when the body
reaches a new steady state. It will take
nearly a year to achieve 50% and about 3 years to achieve 95%”.
My back of
envelope calculations based on the National Adult Nutrition Survey is that
extrapolating from the US model (footnote 4), a tax on sugar sweetened
beverages might lead to a weight loss of 0.6 lb at the end of year 1. That of
course is subject to an error estimate such that it might be higher but
equally, it might be lower. Many of the advocates of fat taxes might argue that
they will take that “thank you very much” as a start and then move to the next
food. But you cannot continue to add tax to the cost of food.
