Many food industry observers will have noticed the frisson of interest generated by recent research on the role diet soft drinks can play in weight loss.
The study from the University of Colorado’s Anschutz Health and Wellness Center suggested that drinking diet beverages helps people lose weight even more effectively than water. It appeared in the June issue of the journal Obesity.
The 12-week clinical study put 303 participants on a diet and found that subjects who consumed beverages made with non-nutritive sweeteners (NNS) lost an average of 6kg – 44% more than the control group, that drank water and lost an average of 4kg. More than half of the participants in the diet beverage group – 64% – lost at least 5% of their body weight, compared with only 43% of the control group.
These sorts of results sound like good news for an industry that is increasingly under pressure from governments and health organisations to tackle the ballooning obesity epidemic.
Increasing sales (Return to top)
Sweetener sales in Europe are set to grow by almost 12%, from 720M (£570M) last year to 810M (£644M) by 2016, according to health food business Food For Thought.
“This study clearly demonstrates diet beverages can in fact help people lose weight, directly countering myths in recent years that suggest the opposite – effect weight gain,” says Dr James Hill, executive director of the Anschutz Health and Wellness Center, and a co-author of the study. “In fact, those who drank diet beverages lost more weight and reported feeling significantly less hungry than those who drank water alone.”
However, while the resulting headlines boasted that people lose more weight and feel less hungry when drinking diet beverages than they do when drinking water, the study has drawn criticism for its design, even within the pages of Obesity.
Dr Stephen Anton of the Department of Aging and Geriatric Research and the Department of Clinical and Health Psychology at the University of Florida was tasked with critiquing the study in the accompanying Obesity editorial.
Anton acknowledges that the findings “provide an important contribution to the literature and strongly suggest that individuals who consume NNS should not be discouraged from continuing to consume these beverages during weight loss efforts”. But he also sounds several notes of caution.
First, he says that 12 weeks is not long enough to confirm the long-term impact of consuming NNSs on weight and says only reports of at least a year will provide critical information and evidence to fully evaluate the effects of NNSs on weight loss.
Second, he criticises the control of the experiment, since the only restriction on the water group was not to drink NNS-sweetened beverages and the researchers didn't measure how much NNSs they may have consumed in other foods.
In fact, there is no way of knowing whether the overall consumption of NNS rose or fell for any individual in either group, since the study included no information on NNS intake in food, either before or during the study.
Anton also notes that the researchers haven’t managed to tease out a possible mechanism through which the NNS beverages might promote greater weight loss than water.
Participant information on caloric intake or adherence to the diet was not provided, he says. Without it, it is difficult to know if dietary intake differed between groups or if there was another mechanism responsible for the weight loss.
Scepticism (Return to top)
Elsewhere, some scientists are sceptical about the University of Colorado’s results. Susan Swithers is professor of psychological sciences at Purdue University and a behavioural neuroscientist. She believes the experiment design was fatally flawed and says conclusions couldn't be drawn on what contributed to the differences between the groups.
Beyond that, she is a stern critic of the role played by NNS-sweetened beverages in general. “Right now, the data provide little support for NNS as useful tools [for weight control], and long-term data indicate that they're associated with increased risk for health problems,” she says.
That said, Swithers claims it’s unfortunate people choose to focus on weight management when they should be more concerned about the health outcomes associated with excess weight.
The journal Trends in Endocrinology & Metabolism published her review of a range of recent work on the topic in July 2013. In it, Swithers looked at a variety of research, including epidemiological studies (which can spot correlations, but not cause-and-effect) and interventional studies on animals, which were designed to pinpoint causal relationships.
The paper was a review of studies that had been carried out in various approaches over the past five to seven years, she says. Long-term epidemiological studies identified increased risks of diseases, such as type 2 diabetes, heart disease, stroke and metabolic syndrome in regular consumers of diet soft drinks, compared with non-consumers.
“But, it also included results from recent intervention studies that indicated that diet soft drinks did not produce greater weight loss than water,” adds Swithers.
More studies needed (Return to top)
Although she concedes that the correlations highlighted by epidemiology cannot show that drinking diet drinks actually causes a higher risk of weight-related health problems, she stresses that it’s equally impossible to infer a causal connection the other way round.
In the past, the links were used as evidence that unhealthy people chose to consume diet drinks. But there was no way to draw that conclusion from correlation data, she adds.
“However, using preclinical approaches, we can actually determine causal relationships because we can randomly assign subjects to consume foods or beverages made with high-intensity sweeteners or with sugars.”
When such approaches are used in animals, excess weight gain is recorded, as well as changes in metabolic and hormonal responses from the groups receiving high-intensity sweeteners, “indicating that there is a plausible biological mechanism by which this effect could occur in people”, she adds.
Meanwhile, other observers believe evidence and logic support the use of NNS to tackle obesity-related health problems. Sara Stanner, science programme manager at the British Nutrition Foundation (BNF), is one of these. She says, if you’re trying to lose weight, then NNSs are a good route and help you to reduce sugar consumption. “The evidence is that many people should be eating less sugar,” she says.
Although she hasn’t looked at which studies were included in Swithers’s Purdue review, Stanner is sceptical about the applicability of many animal studies in humans. “Lots of animal studies look at dosing in very high levels which you can't apply at a human level,” she says.
On a related note, Stanner also says that the BNF has been sifting through research, trying to unearth any evidence that the increasing use of sweeteners might prevent people from retraining themselves to enjoy foods that are not as sweet.
But the team found that, while there was lots of evidence that a gradual reduction in salt can train people to enjoy less salty food, there was very little evidence to support a similar strategy for retraining a sweet tooth.