“Is sugar an invisible killer?” is the alarming question posed in the Daily Mail. The paper reports on a study which found that mice fed the human equivalent of three cans of fizzy drinks a day had worse health outcomes than mice on a regular diet.
Researchers fed one group of mice a normal diet, while the other group had a diet designed to mimic a diet rich in high fructose corn syrup, often added to human food during processing and preparation. The researchers say this equates to three cans of sugary drinks per day for a human.
After several weeks on the normal or sugary diet, the mice were then housed together. The researchers found the mice that had eaten the high added sugar diet fared worse. The female “sugar mice” had twice the risk of death, and all the “sugar mice” had increased cholesterol levels.
The researchers suggest that their results demonstrate the adverse effects of an added sugar diet, even when other environmental factors are not disruptive. Exactly why a sugary diet had these effects is still unclear, though similar effects of sugar have also been observed in previous rodent studies.
However, humans are not mice and similar effects may not occur in humans. Nevertheless we know that too much sugar in the diet is bad for us.
The study was carried out by researchers from University of Utah, Arizona State University and Children’s Hospital Oakland Research Institute, California, US. Funding was provided by the US National Institute of Health, with additional grants given to individual researchers.
The study was published in the peer-reviewed scientific journal Nature Communications.
Once past the exaggerated headlines labelling sugar an “invisible killer”, the media covered the research sensibly, making clear that it was conducted in mice.
This was an animal study aiming to look at the effects of a diet containing added sugar on mice. The sugary diet was specifically high in fructose and sucrose, mimicking the composition of high fructose corn syrup (HFCS). Devised in the 1950s, HFCS is a cheap way of sweetening food and drinks and is now widely used in food processing.
The researchers say that while previous studies in rodents have demonstrated the negative effects of a diet high in sugar (such as insulin resistance and high blood fats), these have usually studied sugar doses above the range people would normally be exposed to in their diet. Therefore, this study looked at whether human-relevant sugar concentrations have an effect on mouse health, including survival, competitive ability and reproduction.
In the added-sugar diet, 25% of the calories were derived from a mix of glucose and fructose sugars. This level of sugar is said to be consumed by up to a quarter of Americans and corresponds to drinking three cans of sugary drinks, such as cola, every day.
The study design involved feeding a group of mice two separate diets before housing them together in ‘semi-natural’ conditions – that is enclosed inside nest boxes and subjected to 12 hours of light and 12 hours of darkness with free food and water available. The mice were also able to freely interact with each other, so were in direct competition for territory, mates and food.
They did this so that they could detect “performance” differences between the two groups of animals.
Normal mice were split after weaning into two groups and fed a diet with normal amounts of food and water, but with or without added fructose/glucose. The mice fed the sugar diet derived 25% of their calories from equal amounts of fructose and glucose monosaccharides. This is about the same ratio as in high fructose corn syrup (HFCS) often added during food processing.
The mice followed these diets for 26 weeks, after which time they were all released into an enclosure where they were in direct competition. At this point, all mice were given the added-sugar diet, as it was no longer possible to keep them on separate diets.
The enclosure included various enclosed nest boxes in ‘optimal territory’ areas, and open nest boxes in suboptimal areas.
Competitive ability, reproductive success, survival and metabolic measures (such as body weight, and blood cholesterol levels) were assessed in the two groups.
The researchers found that the added-sugar diet had a negative effect on competitive ability, reproductive success, survival and some metabolic measures.
The added-sugar diet decreased survival in female mice. They had death rates almost twice as high as controls. There was no effect on male survival.
The added-sugar diet also decreased competitive ability of male mice, which acquired and defended fewer territories than males fed on the normal diet. After three weeks, the mice fed the added-sugar diet occupied 36% of territories compared to the normal diet mice who occupied 48%.
Compared with female mice fed the normal diet, the reproductive success of the mice fed the high-sugar diet was good initially, but gradually declined over time. Male mice fed the added-sugar diet sired around a quarter fewer offspring than males fed the normal diet.
The added-sugar diet had no effect on the body weight of either male or female mice. However, mice fed the added-sugar diet had higher levels of cholesterol and female mice fed the added-sugar diet had poorer glucose tolerance (tested using glucose tolerance tests).
The researchers conclude that when fed a sugar level meant to mimic that of the human diet, mice experience adverse effects upon survival, competiveness and reproductive capacity. They further conclude that the natural enclosures used in the study – which are meant to provide minimal environmental disruption to the animals – are a good technique for unmasking the negative effects of toxic substances.
This interesting animal research demonstrates the effects of feeding mice for 26 weeks with a diet supplemented by a mix of sugars mimicking high fructose corn syrup, then looking at their survival, reproduction and competitive performance when enclosed in a semi-natural environment with mice who had been fed a normal diet. The early diet of these animals seemed to have lasting effects on their survival, competiveness and reproductive capacity.
The researchers suggest that their results demonstrate the adverse effects of an added sugar diet, even when other environmental and other life conditions are not disruptive.
However, even though sugar levels were intended to model those consumed by humans, mice are not humans. It cannot be concluded that these mice are a direct model of human life with added dietary sugar.
It is also unclear why the researchers decided to give both groups of mice a high sugar diet once they had been enclosed in the same environment. If they had all been fed a natural diet once housed together in the enclosure, the results may have been different.
Nevertheless, the adverse health effects from a poor diet – including high sugar, high salt, and high saturated fat – are well documented. This study does not alter current diet and exercise recommendations.