Sugar-rich diets may have a negative impact on health independent of obesity, according to a new study in fruit flies which suggests excess consumption of the sweetener may also affect kidney function.

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The findings, published in the journal Cell Metabolism, noted that early death from excess sugar consumption is related to the build-up of a natural waste product, uric acid.

Researchers, including those from the MRC London Institute of Medical Sciences in the UK, discovered that the shortened survival of fruit flies fed a sugar-rich diet is not the result of their diabetic-like metabolic issues.

They said consuming too much sugar increases people's risk of developing metabolic disorders, such as obesity and diabetes, and can shorten people's life expectancy by several years.

While this reduction in lifespan is widely believed to be caused by metabolic defects, the new study noted that this may not always be the case.

"Just like humans, flies fed a high-sugar diet show many hallmarks of metabolic disease -- for instance, they become fat and insulin resistant," said Helena Cocheme, study co-author from the MRC London Institute of Medical Sciences.

"Obesity and diabetes are known to increase mortality in humans, and so people always assumed that this was how excess sugar is damaging for survival in flies," Cocheme said.

Like salt, the scientists said, sugar also causes dehydration, with thirst showing up as an early symptom of high blood sugar and diabetes.

"Water is vital for our health, yet its importance is often overlooked in metabolic studies. Therefore, we were surprised that flies fed a high-sugar diet did not show a reduced lifespan, simply by providing them with an extra source of water to drink," the UK-based scientist noted.

"Unexpectedly, we found that these flies still exhibited the typical metabolic defects associated with high dietary sugar," she added.

Based on this water effect, the researchers decided to focus on the fly renal system.

They showed that excess dietary sugar caused the flies to accumulate a molecule called uric acid, which is an end-product from the molecules that make up the DNA.

But, the study said, uric acid is also prone to crystallise, giving rise to kidney stones in the fly.

In the study, the researchers could prevent these stones, either by diluting their formation with drinking water or by blocking the production of uric acid with a drug.

They said this protected against the shortened survival associated with a sugar-rich diet.

"The sugar-fed flies may live longer when we give them access to water, but they are still unhealthy. And in humans, for instance, obesity increases the risk of heart disease," Cocheme said.

"But our study suggests that disruption of the purine pathway is the limiting factor for survival in high-sugar-fed flies. This means that early death by sugar is not necessarily a direct consequence of obesity itself," she added.

To study how dietary sugars impacted human health, the research team explored the influence of diet in healthy volunteers.

"Strikingly, just like flies, we found that dietary sugar intake in humans was associated with worse kidney function and higher purine levels in the blood", said Christoph Kaleta, co-author of the study.

Uric acid accumulation, according to the scientists, is a known direct cause of kidney stones in humans, as well as gout, a form of inflammatory arthritis.

Its levels also tend to increase with age, and can predict the onset of metabolic diseases such as diabetes, they said.

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"It will be very interesting to explore how our results from the fly translate to humans, and whether the purine pathway also contributes to regulating human survival," Cocheme said.

"There is substantial evidence that what we eat influences our life expectancy and our risk for age-related diseases. By focusing on the purine pathway, our group hopes to find new therapeutic targets and strategies that promote healthy ageing," she added.