March 17, 2015

expert reaction to study on diet soda consumption and abdominal fat in older adults

A study published in the journal Journal of the American Geriatrics Society has attempted to establish a link between waist circumference and intake of “diet” soft drinks in a population Americans over the age of 65. They report that those who consumed higher levels of these drinks were more likely to gain more weight over the follow up period.

Ms Catherine Collins, Principal Dietitian at St George’s Hospital NHS Trust, said:

“The authors of this paper ambitiously state that a ‘striking dose-response relationship’ existed between abdominal fat (defined by a larger waist size), body mass index (as a marker of obesity) and diet-drink consumption in a group of Texan over-65 year olds initially recruited to the San Antonio Longitudinal Study of Aging (SALSA) in the early 1990’s.

“The authors justify their statement based on 10 year follow-up of a mere half of the original subject group (375 of 749) able to provide information at follow up on diet, lifestyle and body weight and shape. From this group researchers predicted an elevated cardio-metabolic risk in the mere 40 subjects taking more than 1.5 standardised ‘diet drinks’ daily, given their marginally significant higher BMI and a waist circumference 3 inches greater than their slimmer, non-diet soft-drinking counterparts – although were unable to provide information on non-fatal or fatal cardiac events across the study group.

“Superficially this research is bound to generate concern in those with middle-aged spread who don’t want to spread any more, and adds confusion to the debate about what soft-drink to choose if trying to keep your weight under control.

“However, the devil is in the missing detail, and as a dietitian I’m cynical of any study that demonstrates a ‘higher BMI/ larger waist size’ in diet-drink consumers whilst carefully avoiding the main reason why people of middle age or older usually take low calorie soft drinks compared to the sugar-full versions – namely that they are already overweight and don’t want to weigh more than they already do, or that they’re avoiding high-GI sugary drinks to control diabetes.

“The research merely confirms this premise given that of the 40 subjects consuming more than one diet drink a day, 24% of ‘diet drink consumers’ had confirmed diabetes. This suggests many subjects were taking positive steps to control weight or blood sugar level – not necessarily that diet drinks were playing weird games with their metabolism to keep them fatter than they need to be, as the public may infer from the headline grabbing statement.

“My cynicism is backed up by the group demographics provided. Whilst being unable to provide any information on daily calorie intake, the researchers were able to confirm that diet-drink consumers were more affluent, more likely to exercise, less likely to smoke, and be almost twice as likely to have confirmed diabetes than their sugary-drinking counterparts (24% vs 13%). A simpler explanation is that of a more well fed, affluent and health-scare aware group attempting to control (perhaps unsuccessfully) their body weight. From current evidence, I don’t consider that this study confirms that ‘diet drinks make you fat’, but simply confuses correlation with causation.

“It also appears that weekly energy expenditure from exercise averaged 500kcal more than the non-diet drink consumers, whilst BMI and waist-size were higher. Also of interest was the differences in BMI and waist size across study duration. Starting BMI averaged between 27-28 (i.e. overweight) in all subjects in their mid-60s at time of recruitment. BMI peaked in both groups around the age of 75, then gradually declined in both sexes towards their 80’s. In contrast, waist circumference continued to increase with increasing age, the rate of gain greatest for women. This interesting data complements other data on body shape and size as we age, but raises the thorny issue of just what is our ‘tape measure’ goal in someone who has successfully reached 80+ years of age without incident disease?

“When it comes to body weight and storage of surplus calories, all foods (and drinks) count. It would be worrying if the take-home message for dieters would be that you may as well have the additional 200kcal or so a day from sugar-sweetened drinks instead of choosing zero calorie drinks instead.”

Dr Nita Forouhi, Programme Leader, MRC Epidemiology Unit, University of Cambridge, said:

“While interesting for stimulating further research into the inter-relationships between types of beverages and obesity, these findings are not yet ready for translation into public health messages as this study does not establish that diet soda leads to accumulation of abdominal fat, due to current study limitations – these include lack of accounting for other dietary habits or total calorie intake and issues of ‘reverse causation’, as the consumption of diet sodas was relatively higher among those who were overweight or obese.

“Though the authors speculate that the higher waist girth is likely to represent the metabolically active deep visceral fat more so than the superficially located subcutaneous fat, they did not directly measure these, so it is unclear what type of “belly fat” would be associated with diet soda intakes”

‘Diet Soda Intake Is Associated with Long-Term Increases in Waist Circumference in a Biethnic Cohort of Older Adults: The San Antonio Longitudinal Study of Aging’ by Fowler et al. published in Journal of the American Geriatrics Society on Tuesday 17th March. 

Declared interests

None declared