Top of many people’s New Year resolutions list will be getting slimmer and a bit healthier by cutting down the calories — and, clearly, one particularly good way to do that is to cut back on the sweet stuff.
This includes soft drinks. Most of us know that fizzy drinks are bad for our waistlines as well as our teeth, with the equivalent of about seven teaspoons of sugar in a can of Coca-Cola. But is it just the sugar that is contributing to weight gain, or could it be the bubbles, too?
I love sparkling, carbonated water and when I first saw a study from the University of Birzeit, in the West Bank, showing that adding bubbles to sugary water made rats fatter and hungrier than a flat, sugary drink did, I thought it sounded a bit unlikely.
But in the BBC2 series Trust Me, I’m A Doctor, we like to examine even the most unlikely health claims — and as you’ll see in tomorrow’s programme, we put this finding to the test. We started by asking Dr James Brown, from Aston University in Birmingham, who has a particular interest in obesity and diabetes, to help us run our study.
Top of many people’s New Year resolutions list will be getting slimmer and a bit healthier by cutting down the calories — and, clearly, one particularly good way to do that is to cut back on the sweet stuff
We then recruited a group of healthy volunteers. The key thing was, we didn’t tell them what the experiment was really about because we didn’t want that knowledge to interfere with the results.
Instead of telling them it was mainly about bubbles, we said it was concerned with measuring the impact of sugary drinks on people’s appetite.
This includes soft drinks. Most of us know that fizzy drinks are bad for our waistlines as well as our teeth, with the equivalent of about seven teaspoons of sugar in a can of Coca-Cola
First, our volunteers fasted for ten hours. They were then given identical calorie-controlled cheese sandwiches to eat, to ensure as best we could that they all started off equally full.
An hour later, each volunteer was given one of four drinks, which were allocated at random: a glass of a fizzy sugary drink, a glass of the same drink but flat, a glass of fizzy water or a glass of flat water.
Ten minutes after they’d had their drink, James took a blood sample to measure their ghrelin levels. Ghrelin is one of the so-called ‘hunger hormones’ which is produced in your stomach and then goes to your brain to say: ‘I am hungry, I should eat.’
Our volunteers were then sent away with a food diary, so we could assess how many calories they consumed in the hours following their drink. We repeated the experiment three times over the next couple of weeks, but each time our volunteers were given a different drink, so by the end they’d had all four drinks.
On their final visit, we told them the true purpose of the study and James gave them the results.
‘What we found was really exciting,’ he told them. ‘When you had a fizzy drink your ghrelin levels were about 50 per cent higher than when you had a non-fizzy drink.’
In other words, drinking a fizzy, sugary drink makes you a lot hungrier an hour later than drinking the same drink, but flat.
This increase in ghrelin wasn’t only seen after drinking sugary fizzy drinks; there was also a slight effect when James and his team compared the impact of carbonated with still water.
And it wasn’t just their levels of the hunger hormone ghrelin: when we looked at the effect fizz had on how much food our volunteers ate later in the day, we found they consumed on average 120 calories more after they’d had a carbonated drink than after a non-carbonated one. ‘That is a really significant finding,’ James told me.
So, on top of the 140 calories in the fizzy drink, they consumed another 120 calories later in the day as a direct result of the extra ghrelin produced by having had a fizzy drink.
But why should bubbles have this effect?
In other words, drinking a fizzy, sugary drink makes you a lot hungrier an hour later than drinking the same drink, but flat
We don’t know for sure, but James thinks there are two possible reasons. The first is that when you knock back a fizzy drink, carbon dioxide — the gas used to make drinks fizzy — directly causes receptor cells in your stomach to release ghrelin.
Another possibility is that the stomach bloats and stretches a little bit from that extra gas, and it is this stretching that stimulates cells to release ghrelin.
So does that mean it’s ok to drink flat, sugary drinks?
No, says James. ‘I think the take-home message from this, sadly and boringly, is that the healthiest option is probably still water.’
This is early days, but our test provides another really good reason why you should steer clear of the fizzy sweet stuff.
As for me, I’ll still have the occasional glass of fizzy water. But in light of these findings — and to cut down my plastic consumption — I’ll probably go more for tap water in future.