Sugar taxes and weight loss predictions

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.

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[1] http://www.foodnavigator.com/Legislation/Danish-government-scraps-fat-tax-cancels-planned-sugar-tax

[2] Hall et al (2012) Am J Clin Nutr 12, 989-994

[3] Hall et al (2011) Lancet 378,826-837

[4] Biing-Hwan et al (2011) Econ Hum Biol 9, 329-341