The research shows that when mice consumed a diet containing 25% extra sugar - an amount considered safe and relevant in human consumption equivalents - females died at twice the rate of rats fed a standard diet, while males were 25% less likely to hold territory and reproduce.
Writing in Nature Communications, the research team describe the findings from an animal model 'toxicity test' developed at the University of Utah, USA - which gave the mice the equivalent level of sugar gained by humans from the consumption of three cans of soda.
"Our results provide evidence that added sugar consumed at concentrations currently considered safe exerts dramatic adverse impacts on mammalian health," said the study authors - led by seniour author Professor Wayne Potts.
"This demonstrates the adverse effects of added sugars at human-relevant levels," explained Potts, who noted that previous studies using other tests fed mice large doses of sugar disproportionate to the amount people consume in sweetened beverages, baked goods and candy.
The new research, however, suggests that a 25% 'added-sugar' diet (12.5 percent dextrose and 12.5 percent fructose) may be just as harmful to the health of mice as being the inbred offspring of first cousins, said the researchers.
Dr James Ruff, first author of the study, noted that while the mice did not become obese and showed few metabolic symptoms, the Utah study did show that mice fed the diet "died more often and tended to have fewer babies."
"We have shown that levels of sugar that people typically consume – and that are considered safe by regulatory agencies – impair the health of mice," said Ruff.
Commenting on the research, Catherine Collins, principal dietitian at St George’s Hospital NHS Trust, UK, noted that while the researchers state that this diet mimicked the high sugar diet (25% added sugar) commonly taken by some people in the US, a recent UK survey indicated an average sugar intake of around 11% of total calories – "far less than half the amount fed to the mice in this study."
“So what can we take from this? Certainly, for swaggeringly territorial wild mice, a high sugar diet made them weaker at defending their homestead, and influenced fertility," said Collins.
"Unfortunately the study doesn’t address whether this was due to micronutrient deficiencies, or that well-sugared mice didn’t feel in the mood to recreate."
Alison Boyd, director of Sugar Nutrition UK added: “This is very early research in animals which cannot be translated into humans."
"The scientific evidence on sugar has been reviewed on numerous occasions by independent expert committees including the World Health Organization and European Food Safety Authority. They have concluded that moderate levels of sugar consumption are not implicated in any of the major lifestyle diseases," she commented.
"Like all sources of calories, sugar can be consumed within a healthy, calorie-balanced diet and active lifestyle.”
The Utah study placed groups of mice in room-sized pens nicknamed 'mouse barns' with multiple nest boxes – something the researchers suggest is a much more realistic environment than small cages and allows the mice to compete more naturally for mates and desirable territories. This, Potts said, reveals any subtle effects on performance.
"This is a sensitive test for health and vigour declines," he explained, noting that in a previous study he used the same test to show how inbreeding hurt the health of mice."
The experimental diet in the study provided 25% of calories from added sugar – half fructose and half glucose – no matter how many calories the mice ate. This diet is equivalent to the diet of a person who drinks three cans daily of sweetened soda in a day "plus a perfectly healthy, no-sugar-added diet," said Potts.
He explained that the mice used in the trial, descended from wild house mice, are 'highly competitive' over food, nesting sites and territories.
"This competition demands high performance from their bodies, so if there is a defect in any physiological systems, they tend to do more poorly during high competition."
The team created two colonies with 156 'founders' that were weaned at four weeks, and then assigned either to the added-sugar diet or the control diet - with half the males and half the females on each diet.
Mice remained in cages with siblings of the same sex (to prevent reproduction) for 26 weeks while they were fed these diets.
Then the mice were placed in the mouse barns to live, compete with each other and breed for 32 more weeks. All mice received the same added-sugar diet while in the mouse barns, so the study only tested for differences caused by the mice eating different diets for the previous 26 weeks, said Potts.
The team revealed their key findings as:
- After 32 weeks in mouse barns, 35% of the females fed extra sugar died. This was twice the 17% death rate for female control mice.
- There was no difference in the 55% death among males who did and did not get added sugar.
- Males on the added-sugar diet acquired and held 26% fewer territories than males on the control diet: control males occupied 47% of the territories while sugar-added mice controlled less than 36%.
- Males on the added-sugar diet produced 25% fewer offspring than control males, as determined by genetic analysis of the offspring.
- Sugar-added females had higher reproduction rates than controls initially – likely because the sugar gave them extra energy to handle the burden of pregnancy – but then had lower reproductive rates as the study progressed, partly because they had higher death rates linked to sugar.
"Our test shows an adverse outcome from the added-sugar diet that couldn't be detected by conventional tests," suggested Potts.
The study was funded by the US National Institutes of Health and the National Science Foundation.
Source: Nature Communications
Published online ahead of print, doi: 10.1038/ncomms3245
"Human-relevant levels of added sugar consumption increase female mortality and lower male fitness in mice"
Authors: James S. Ruff, Amanda K. Suchy,Sara A. Hugentobler