What is obesity?
Body mass index (BMI) is usually a useful measure to show whether people are a healthy weight for their height. A BMI of 18 to 25 shows a healthy weight, a BMI over 25 is considered overweight, and a BMI over 30 shows obesity.
Obesity is generally caused by consuming more energy (calories) than you burn off through physical activity or exercise. The body stores the excess energy as fat.
Obesity can lead to serious and life-threatening conditions, including type 2 diabetes, coronary heart disease, some types of cancer, and stroke. Obesity can also affect people's quality of life (well-being).
What are fats?
Fat is essential to a healthy, balanced diet. Fats help the body to absorb some vitamins, and are a source of some fatty acids that the body cannot make. The main types of fat in foods are:
- saturated (mostly from animal products, such as meat, cow's milk, cheese, cream, and butter); and
- unsaturated (mostly found in oils from plants and fish).
Health guidelines usually recommend that people should:
- reduce the overall amount of fat they eat;
- limit the amount of saturated fats they eat; and
- eat unsaturated fats instead of saturated fats.
Why we did this Cochrane Review
Rates of overweight and obesity are increasing around the world and are a major public health issue. We wanted to know whether increasing the prices of foods that have a lot of fats or saturated fats could put people off eating them, and reduce obesity.
What did we do?
We searched for studies that looked at the effects of a tax on the fat content of foods. We were interested in how a tax affected the:
- total amount of fat, and saturated fats eaten;
- total calories eaten;
- calories eaten from all fats and saturated fats; and
- rates of overweight and obesity.
Search date: we included evidence published up to September 2019.
What we found
We found two studies from Denmark, conducted during 2011 to 2012. One looked at how a tax on some high-fat foods affected household demand for them; the other looked at information on supermarket sales for certain high-fat foods (minced beef, cream, and sour cream). They compared their results with data from before the tax started.
Both studies looked at a small number of foods that people bought, but not what foods people ate. They didn't measure how much total fat or saturated fat were eaten.
What are the results of our review?
If the amount of foods bought reflected the amount of foods eaten, then taxing the fat content of certain foods:
- might reduce the total amount of fats eaten by 41.8 grams a week for each person in a household, in one study of 2000 households; and
- might reduce the amount of saturated fats eaten (in minced beef and cream), in one study of 1293 supermarkets.
No studies measured the effect of taxing the fat content of foods on calories eaten, on obesity or overweight, or on total food sales.
How reliable are these results?
We are not confident in the results because the evidence is only from two studies; and these studies only measured a small number of foods bought, and did not measure foods eaten. One study did not report statistics about the accuracy of its results.
The results were from observational studies, in which researchers observe the effect of a factor (such as taxation) without trying to change who does, or does not, experience it. Observational studies do not give as reliable evidence as randomized controlled studies, in which the treatments people receive are decided at random.
Conclusions
We did not find enough reliable evidence to find out whether a tax on the fat content of foods resulted in people eating less fat, or less saturated fat.
We did not find any evidence about how a tax on the fat content of foods affected obesity or overweight.
The results of our review will change when further evidence becomes available.
Given the very low quality of the evidence currently available, we are unable to reliably establish whether a tax on total fat or saturated fat is effective or ineffective in reducing consumption of total fat or saturated fat. There is currently no evidence on the effect of a tax on total fat or saturated fat on total energy intake or energy intake through saturated fat or total fat, or preventing the incidence or reducing the prevalence of overweight or obesity.
Overweight and obesity are increasing worldwide and are considered to be a major public health issue of the 21st century. Introducing taxation of the fat content in foods is considered a potentially powerful policy tool to reduce consumption of foods high in fat or saturated fat, or both.
To assess the effects of taxation of the fat content in food on consumption of total fat and saturated fat, energy intake, overweight, obesity, and other adverse health outcomes in the general population.
We searched CENTRAL, Cochrane Database of Systematic Reviews, MEDLINE, Embase, and 15 other databases and trial registers on 12 September 2019. We handsearched the reference lists of all records of included studies, searched websites of international organizations and institutions (14 October 2019), and contacted review advisory group members to identify planned, ongoing, or unpublished studies (26 February 2020).
In line with Cochrane Effective Practice and Organisation of Care Group (EPOC) criteria, we included the following study types: randomized controlled trials (RCTs), cluster-randomized controlled trials (cRCTs), non-randomized controlled trials (nRCTs), controlled before-after (CBA) studies, and interrupted time series studies. We included studies that evaluated the effects of taxes on the fat content in foods. Such a tax could be expressed as sales, excise, or special value added tax (VAT) on the final product or an intermediary product. Eligible interventions were taxation at any level, with no restriction on the duration or the implementation level (i.e. local, regional, national, or multinational). Eligible study populations were children (zero to 17 years) and adults (18 years or older) from any country and setting. We excluded studies that focused on specific subgroups only (e.g. people receiving pharmaceutical intervention; people undergoing a surgical intervention; ill people who are overweight or obese as a side effect, such as those with thyroiditis and depression; and people with chronic illness). Primary outcomes were total fat consumption, consumption of saturated fat, energy intake through fat, energy intake through saturated fat, total energy intake, and incidence/prevalence of overweight or obesity. We did not exclude studies based on country, setting, comparison, or population.
We used standard Cochrane methods for all phases of the review. Risk of bias of the included studies was assessed using the criteria of Cochrane’s ‘Risk of bias’ tool and the EPOC Group’s guidance. Results of the review are summarized narratively and the certainty of the evidence was assessed using the GRADE approach. These steps were done by two review authors, independently.
We identified 23,281 records from searching electronic databases and 1173 records from other sources, leading to a total of 24,454 records. Two studies met the criteria for inclusion in the review. Both included studies investigated the effect the Danish tax on saturated fat contained in selected food items between 2011 and 2012. Both studies used an interrupted time series design. Neither included study had a parallel control group from another geographic area. The included studies investigated an unbalanced panel of approximately 2000 households in Denmark and the sales data from a specific Danish supermarket chain (1293 stores). Therefore, the included studies did not address individual participants, and no restriction regarding age, sex, and socioeconomic characteristics were defined. We judged the overall risk of bias of the two included studies as unclear.
For the outcome total consumption of fat, a reduction of 41.8 grams per week per person in a household (P < 0.001) was estimated. For the consumption of saturated fat, one study reported a reduction of 4.2% from minced beef sales, a reduction of 5.8% from cream sales, and an increase of 0.5% to sour cream sales (no measures of statistical precision were reported for these estimates). These estimates are based on a restricted number of food types and derived from sales data; they do not measure individual intake. Moreover, these estimates do not account for other relevant sources of fat intake (e.g. packaged or processed food) or other food outlets (e.g. restaurants or cafeterias); hence, we judged the evidence on the effect of taxation on total fat consumption or saturated fat consumption to be very uncertain. We did not identify evidence on the effect of the intervention on energy intake or the incidence or prevalence of overweight or obesity.