"Fizzy water could cause obesity by encouraging you to eat more," The Daily Telegraph reports.
Researchers aimed to see whether it could be the carbonation in soft drinks – rather than the sugar – that explains the link between soft drinks and obesity.
Overall, they found rats that drank diet or regular fizzy drinks ate more and gained more weight over six months than rats that drank flat soda or water.
The weight gain was associated with increased production of the appetite hormone ghrelin, which is produced by both rodents and humans.
The researchers then looked at the effects of carbonated drinks in 20 young men, and found they also had higher blood ghrelin levels after drinking fizzy drinks than after flat soda or water.
But we can't say from the results of this study alone that carbonation or ghrelin production is the full answer to the link between soft drink consumption and obesity.
It's likely that obesity is caused by multiple environmental, social and lifestyle factors, rather than carbonation on its own.
People who consume lots of fizzy drinks may also be more likely to have a less healthy diet and to be doing less exercise. The safest and cheapest bet for refreshment is plain old tap water.
Where did the story come from?
The study was carried out by researchers from Birzeit University in Palestine and was funded by grants from the same institution.
It was published in the peer-reviewed journal Obesity Research and Clinical Practice.
The coverage of the study in the UK media was accurate.
What kind of research was this?
This animal research aimed to see whether having fizzy drinks could contribute to weight gain.
The authors state that arguably there are many causes of obesity, including environmental, social and genetic factors.
They say numerous studies have observed links between obesity and soft drink consumption, mostly believed to be caused by the sugar content in these drinks.
But there's another element to both sugar sweetened and diet fizzy drinks: carbon dioxide. This study aimed to look at the effects of carbonation.
Animal research is a useful step to see how biological processes may work in humans, as we share many similarities with animals.
That said, we aren't identical to rodents, so any findings would always need to be validated in human trials.
Preliminary attempts at validation were made in this study. There are still likely to be many other issues involved with dietary intake and weight gain.
What did the research involve?
The study involved groups of male rats who were all fed a standard diet, but given one of four different drinks:
- tap water
- regular degassed (flat) soda
- regular carbonated soda
- diet carbonated soda
The researchers assessed food consumption, weighed the rats, and analysed blood sugar and cholesterol after six months on the diet.
They also looked at blood levels of the hormone ghrelin, which is released from the digestive system in response to hunger.
After death, the rats' stomachs were also examined to see how much ghrelin had been produced, and their liver was examined for fatty deposits.
In a second part of the study, 20 healthy human male students aged 18-23 were given a light breakfast followed one hour later by each of the four drinks.
The students repeated this experiment on different days so they were all trying the same drinks. They then had blood samples taken to measure ghrelin. Ghrelin is a hormone "used" by the digestive system to stimulate feelings of hunger.
What were the basic results?
Rats that drank tap water or flat soda weighed significantly less than those drinking the carbonated drinks. Rats that drank both the diet and sugary fizzy drinks gained a similar amount of weight.
Weight gain was slowest in the water-drinking rats compared with all three groups drinking soda.
Rats drinking the fizzy drinks ate significantly more food than those drinking water and flat soda. This was associated with increased blood levels of ghrelin, further supported by evidence of increased ghrelin secretion from the stomach.
There was no difference in blood sugar or cholesterol levels, but rats that drank fizzy drinks had more fat in the liver.
In the human volunteers, ghrelin levels were higher after drinking fizzy drinks one hour after food – three-folds higher than after flat soda, and six-folds higher than after water.
How did the researchers interpret the results?
The researchers concluded that, "This study clearly shows discernible effect of the carbon dioxide gas in carbonated drinks on increased food ingestion and heightened risk of weight gain, obesity and fatty liver disease by inducing ghrelin release."
There seemed to be a clear distinction in this study between fizzy and non-fizzy drink consumption in terms of weight gain, appetite and ghrelin production.
These findings were further supported by the study in healthy adult volunteers, which similarly showed that fizzy drinks increased ghrelin production.
It had been thought that the sugar content in soft drinks causes obesity, but this doesn't account for the link between weight gain and diet drinks that don't contain sugar. The researchers suggest carbonation could be the common link between the two.
But does this mean that carbonation and ghrelin production provide the whole answer as to why soft drink consumption is linked with obesity?
This is possible. But other unhealthy lifestyle factors, which this study didn't look at, could also be a common link between sugary and diet fizzy drinks.
In real life, people who drink lots of fizzy drinks may be more likely to have a less healthy diet and exercise less.
Another point to bear in mind is that this research was conducted primarily in rats. Human beings may not have identical biology.
Although the researchers did follow this up with a human study, they only looked at a very small sample of young men. We can't necessarily apply their results to women or other populations.
Even in the rats, they found that even though the rats had increased levels of the appetite hormone, there was no effect on the levels of another hormone that tells them when they're full. This means we can't be certain that ghrelin provides the whole answer.
Overall, this study raises the interesting possibility that fizzy drinks could stimulate the appetite and cause weight gain, which is definitely worthy of further research.
The best way to achieve a healthy weight is through eating a balanced diet and exercising regularly. And as unexciting as it may seem, water straight from the tap is the best option to quench your thirst.