Do diet drinks really make you fatter?

"Does even Diet Coke make you fat?" asks the Mail Online. The question is prompted by the results of a large US study involving more than 23,000 US adults. It found that people who were overweight or obese drank more sugar-free drinks than people who were a healthy weight.

Researchers believe this could indicate that these people were more conscious of calorie counting.

However, the study also found that obese or overweight people who consumed sugar-free drinks tended to eat more food. This brought their total daily calorie intake up to the level of overweight and obese people who consumed sugar-sweetened drinks.  

Although the diet drinks reduced the calories consumed from drinks, this was counterbalanced by people eating more food than people of a similar weight who drank sugar-sweetened drinks.

The researchers offer a number of interesting hypotheses about why this may be the case. One suggestion is that although these drinks are sugar-free, they still activate the brain's "sugar reward" pathways, so the person still has a "sweet tooth" that causes them to snack more.

Another suggestion is that people may simply transfer the calorie intake they used to get from sugary drinks to eating more food.

However, because of the study design – a cross-sectional study taking data from one point in time – the results cannot conclusively answer the Mail's question about whether a Diet Coke will make you fat.

Despite this, the research serves as a reminder that people should take their total calorie intake from both food and drink into account when trying to lose weight. This should be part of a healthy lifestyle that includes regular physical activity.

For those confused about whether diet or regular soft drinks are better for your health, consider tap water as a cheap, safe and calorie-free alternative. Find out how to lose weight the healthy way as part of the NHS Choices weight loss plan.

Where did the story come from?

The study was carried out by researchers from the Johns Hopkins Bloomberg School of Public Health in the US, and was funded by the US National Heart, Lung, and Blood Institute.

It was published in the peer-reviewed American Journal of Public Health.

The Mail Online coverage generally stuck to the facts of the underlying study, but accompanying headlines that "sugar-free fizzy drinks make people eat more food" were incorrectly presented as fact when this was actually speculation.

What kind of research was this?

This was a cross-sectional study looking at national patterns in adult diet drink consumption and calorie intake in people of different weight categories.

A calorie is simply a measure of energy. It is measured in units called kilocalories (kcals). Calories are often used to describe the energy content of food and drinks, and can be found on most food and drink labels. One calorie is the same as one kcal.

Diet soft drinks have the sugar content replaced by artificial sweeteners. This maintains the sweet taste, but takes out many of the calories that some people are trying to avoid in a bid to either prevent weight gain or lose weight.

As this was a cross-sectional study, it cannot prove causation – that is, it cannot prove that diet drinks make people fat because they then eat more food.

What did the research involve?

Researchers looked at 24-hour dietary recall data recorded in the past as part of the National Health and Nutrition Examination Survey (NHANES) in the US between 1999 and 2010.

NHANES is a population-based survey designed to collect information on the health and nutrition of the US population. It included information on total calorie intake, caloric intake from drinks (sugar-sweetened and diet drinks) and calorie consumption from solid food (main meals and snacks).

The researchers looked for patterns in calorie intake over the years and across the "healthy weight", "overweight" and "obese" weight categories. 

The participants were all aged 20 years or over. Survey respondents were excluded if they were pregnant or had diabetes at the time of data collection, or if their dietary recall was incomplete or unreliable (as determined by the NHANES staff).

The study's analysis was appropriate and aimed to balance the results to be representative of the general US population. It also balanced results for the potentially influential factors (confounders) of race or ethnicity, gender, income, age, marital status, employment status and education.

The researchers did not perform separate analyses for individuals who consumed multiple drink types, as the overlap between categories was very small. For example, only 4.4% of the sample reported consuming both sugar-sweetened drinks and diet drinks. Separate analyses of this sort would be time consuming and would probably add little useful information to the end results.

What were the basic results?

A total of 23,965 people were analysed in the study. 

Drinking habits

Overall, 61% of adults consumed sugar-sweetened drinks and 15% of adults consumed diet drinks. Obese people were the most likely to drink diet drinks, followed by overweight people. Overall, 11% of healthy-weight, 19% of overweight and 22% of obese adults reported drinking diet drinks the previous day.

Overweight and obese adults were also significantly more likely to consume sugar-sweetened drinks compared with healthy-weight adults (63% versus 59%).

For all other drinks categories (alcohol, juice and milk), significantly fewer obese adults consumed those beverages than healthy-weight adults.

Calorie intake

Among diet beverage drinkers – who may be more conscious about calories – total calorie consumption increased significantly by body weight, with overweight and obese adults consuming more than healthy-weight adults, and obese adults consuming more than overweight adults (healthy weight: 2,095 kcal/day; overweight: 2,196 kcal/day; obese: 2,280 kcal/day). Despite drinking diet drinks, overweight and obese people still consumed more calories than those who were a healthy weight.

The added calories in the heavier weight categories appeared to be related to extra food consumption. Among diet beverage drinkers, consumption of solid food calories increased significantly with each body weight category (healthy weight: 1,841 kcal/day; overweight: 1,965 kcal/day; obese: 2,058 kcal/day).

The net increase in daily solid food consumption associated with diet drink consumption was 88 kcals for overweight and 194 kcals for obese adults. The calorie intake was 73 kcals less in healthy-weight people.

How did the researchers interpret the results?

The researchers concluded that: "Overweight and obese adults drink more diet beverages than healthy-weight adults, and consume significantly more solid food calories and a comparable total calories than overweight and obese adults who drink SSBs [sugar sweetened beverages]."

One of the researchers was quoted in the Mail Online as saying that, "The results of our study suggest that overweight and obese adults looking to lose or maintain their weight – who have already made the switch from sugary to diet beverages – may need to look carefully at other components of their solid food diet, particularly sweet snacks, to potentially identify areas for modification."

Conclusion

This large US-representative cross-sectional study indicates that overweight and obese people drink more diet drinks than healthy-weight people, but still consume the same amount of calories as those drinking sugar-sweetened drinks. The extra calories were made up by eating more food.

This suggests that people who are overweight or obese may be switching to diet drinks to reduce their calorie intake when trying to control or reduce their weight.

However, they consumed significantly more calories from food, which brought their total energy intake in line with those drinking sugar-sweetened drinks – effectively cancelling out any calorie reducing effect of the diet drink. The researchers noted that this may mean that, "When adults replace SSBs with non-calorie beverage alternatives, they make few other changes to their diet."

The study had a number of strengths, including its large sample size and the fact that it was broadly representative of the US population, which has some similarities to the UK.

However, there were some limitations that should be considered:

• The diet information used in the research relied on participants accurately and honestly recalling their food and drink intake in the previous 24-hour period. If any of the weight groups systematically under- or overestimated their food and drink intake, this would distort the results.
• The study was conducted in the US. The drinking habits of people in the UK may differ, which could result in different patterns being found.

The Mail Online mentioned the possibility that the reason overweight and obese people consumed more food may be because the artificial sweeteners in diet drinks disrupt appetite control. This was also discussed in the underlying study.

However, this theory was only mentioned in the discussion section, where authors speculate about the possible causes of their results.

This link between artificial sweeteners and appetite disruption was not examined in the research directly and is purely speculative. Assessing the evidence of the strength of such a link, if it does exist, would be an interesting avenue for future research.

This research reminds that we should take account of the calories from both food and drink when trying to lose weight.