Nearly twice as many F/G-fed females died in OPAs compared with controls and a complex effect of the F/G diet on reproduction was also observed in OPAs. Specifically, F/G females exhibited higher reproductive success early in the study, but a decreasing rate of reproduction as the study progressed. Previous studies demonstrate a positive association between adiposity and reproductive success in females, which is thought to be due to the availability of energy reserves18. F/G feeding during the pre-OPA period may have increased adiposity (while not increasing body weight), which could underlie the early elevation in reproductive success. Subsequent reproductive decline and rise in mortality observed in the latter part of the OPA testing could be explained by the mounting physiological insults of the F/G diet interacting with the extreme energetic pressures of simultaneous gestation and lactation, which elevate metabolism by 18–25%19. Future studies designed to track these parameters may help in elucidating the relationship between the F/G diet, adiposity, reproductive success and mortality. 
 
 Overall, F/G-fed males were outcompeted by control animals as measured by competitive ability and reproduction. Though death rates of males were higher than females due to intra-sexual competition, they did not differ between dietary groups. Therefore, it is likely that the lower reproduction of F/G males was due to their decreased ability to defend territories, though other mechanisms may also have decreased reproductive output. Previous work in our system has indicated territorial males sire ≈80% of all pups within an enclosure, as F/G feeding decreased both territorial acquisition and reproduction by ≈25%, it is reasonable to assume that the majority of the reproductive decline is driven by decreased competitive ability, but that a portion of the difference could be due to other reproductive impairments15. Previous work demonstrates increased male adiposity due to added sugar at higher exposure levels (for example, Jurgens et al.8); therefore, a plausible explanation is that F/G-fed males could have increased adiposity, despite not having different body weights, leading to a disadvantage in the physical contests required to acquire territories and mates. 
 
 The sex-specific nature of our OPA findings is interesting though not unprecedented. OPAs revealed a similar mortality pattern, when females harbouring the selfish genetic element known as the t complex experienced an increased risk of mortality while males did not15. Likewise, at both cousin and sibling-levels of inbreeding reproductive output in males was impacted to a much higher degree than that of females13, 14. These findings are inline with the notion of a trade-off between reproduction and longevity (for review see Partridge et al.20). Given that the energetic allocation toward offspring is much higher for females than for males it is not surprising that females might incur increased mortality due to a nutritional manipulation, while males would be more likely to decrease reproductive output. 
 
 The only pre-OPA fasting plasma measure that was associated with the decreased reproduction and increased mortality exhibited by F/G-fed animals was total cholesterol, as no difference was seen in plasma glucose, insulin or triglyceride concentrations. These data suggest that the F/G-fed animals may be suffering from dysregulated lipid metabolism before OPA entrance and supports our hypothesis that differential adiposity may have existed between dietary treatments. In the future, tracking and fractionating cholesterol of animals within OPAs and determining if increased plasma values are predictive of OPA outcomes would be highly informative. 
 
 Impaired glucose clearance rates of F/G-fed females compared with controls before OPA entrance may reflect a yet to be identified physiological alteration, possibly loss of endocrine capacity, that underlies and predicts increased risk of death within OPA enclosures. However, this impairment in glucose clearance and the underlying physiological alterations that it biomarks is masked within 2 weeks of residing in OPAs, well before the majority of deaths have occurred. Nevertheless, disrupted glucose metabolism throughout the developmental period from weaning into adulthood could cause lasting physiological problems and the glucose disruption may persist in individuals within OPAs, but at levels below the detection limits of current assays. No diet induced differences were seen in regards to male glucose clearance (though both F/G and control-fed males exhibited slower clearance relative to control females) and differential mortality between groups did not occur indicating that perhaps differential glucose clearance is the basis of this sex-specific finding. That both dietary groups, as well as both sexes, markedly increased their rates of glucose clearance after entering OPAs is likely due to increased physical activity permitted by the larger living area as well as the increased energetic demands of reproductive and social activities21. As elevated activity leads to increased glucose disposal, metabolic impairments may be masked, making it difficult to establish connections between the pre-OPA alterations in metabolism to the adverse impacts observed in the OPA. 
 
 The increased rates of mortality and decreased reproduction observed in this study represent the lowest level of added sugar consumption shown to adversely affect mammalian health. These adverse organismal-level findings are detectable while standard clinical measures are either unaltered (body weight, glucose, insulin and triglycerides), or exhibit complicated patterns (cholesterol and glucose tolerance), indicating that the available clinical measures used are not of sufficient sensitivity to reflect the physiological impairments leading to early death in females and reproductive declines in males. 
 
 With complex organismal-level endpoints like survival, reproduction and competitive ability it is highly unlikely that a single molecular mechanism acting in isolation is responsible for producing the observed phenotype. It is more likely that a variety of mechanisms at the molecular, cellular and higher levels are acting in concert to alter phenotypes. Our glucose clearance and cholesterol results likely represent part of the mechanistic cascade that leads to the decreased survival, reproduction and competitive ability caused by F/G feeding; however, they do not represent its entirety. Clearly more mechanistic work is needed to elucidate the underpinnings of our findings, but current searches into the causes of added sugar-induced diseases are solely focused on extreme exposure levels and therefore may not aid in characterizing effects at human-relevant exposures. The characterization of mechanisms at relevant doses will be more difficult, as high doses are typically used to ‘push the system’ and decrease the amount of time and number of animals required, but the information garnered will be more readily translated to the human condition. Regardless of the mechanisms involved our study characterizes the ultimate impacts of F/G-feeding mice at human-relevant levels. 
 
 We detected substantial adverse outcomes due to an added sugar exposure consisting of a 1:1 ratio of fructose and glucose amounting to 25% kcal. Our results provide evidence that added sugar consumed at concentrations currently considered safe exerts dramatic adverse impacts on mammalian health22, 23. Many researchers have already made calls for reevaluation of these safe levels of consumption24. Although others have advocated for more drastic regulatory measures to curtail sugar consumption25. Our study provides experimental data directly supporting mammalian adversity at these ‘safe levels’ and provides a model system in which to investigate the molecular and physiological mechanisms causing this organismal adversity. 
 
 Though OPAs detected large differences in reproductive output and survival between F/G-fed and control animals, the results from our studies are likely conservative. First, OPAs were terminated at 34 weeks because of the common, diet independent, high rates of male attrition; it is likely that if the study continued for the entirety of the mouse lifespan that reproductive outputs between the treatments would have continued to diverge. Second, at the start of OPA assessment all animals were provided the same sugar enriched diet, because as yet we have no way to keep animals on their respective diets during OPAs. This means that all of the adverse effects of the F/G diet are a consequence of exposure before OPA entrance. Third, our F/G diet was based on a modified chow and not a refined diet, meaning that our F/G-fed animals showed impairment despite having the remainder of their diet being highly nutritive, whole grain-based, with optimum mineral and vitamin composition, analogous to human subjects consuming a healthy diet plus the equivalent of ≈3 cans (354 ml each) of soda per day. 
 
 By directly assessing the impacts of a treatment on the performance of house mice in OPAs, we are capable of bridging the environmental relevance and longitudinal nature of human studies with the controllability and experimental power of animal models. The data within this paper along with other similar successes using OPAs suggest that this and similar approaches will be an important tool in the detection and quantification of adversity caused by a wide array of treatments13, 14, 15. Once adversity is detected in OPAs and an adverse phenotype characterized, reductionist techniques can be applied to discover the mechanism and thus provide human translatability. As output measures are similar across experiments, OPAs allow for the direct comparison of disparate treatments. For example, our data indicate that the F/G diet is as detrimental to male reproduction as cousin-level inbreeding (Supplementary Fig. S2). 
 
 Currently, there is a great need for sensitive toxicity assessment methods that work across a broad range of experimental manipulations. This need is particularly strong for both pharmaceutical science where 73% of drugs that pass preclinical trials fail due to safety concerns and for toxicology, where shockingly few compounds receive critical or long-term toxicity testing26, 27. Likewise, with health-care costs spiralling out of control, partially due to an epidemic of diet-related non-communicable diseases, one could use this approach to identify other dietary components that may link to adverse outcomes (or benefits). Assuredly, the revelation that added sugar consumption at human-relevant concentrations increases death rates and decreases both competitive ability and reproductive output is evidence that OPAs have the potential to identify disease risks that escape notice by standard clinical measures when tested at relevant-human exposures. ||||| When mice ate a diet of 25 percent extra sugar -- the mouse equivalent of a healthy human diet plus three cans of soda daily -- females died at twice the normal rate and males were a quarter less likely to hold territory and reproduce, according to a toxicity test developed at the University of Utah. 
 
 "Our results provide evidence that added sugar consumed at concentrations currently considered safe exerts dramatic adverse impacts on mammalian health," the researchers say in a study set for online publication Tuesday, Aug. 13 in the journal Nature Communications. 
 
 "This demonstrates the adverse effects of added sugars at human-relevant levels," says University of Utah biology professor Wayne Potts, the study's senior author. He says previous studies using other tests fed mice large doses of sugar disproportionate to the amount people consume in sweetened beverages, baked goods and candy. 
 
 "I have reduced refined sugar intake and encouraged my family to do the same," he adds, noting that the new test showed that the 25 percent "added-sugar" diet -- 12.5 percent dextrose (the industrial name for glucose) and 12.5 percent fructose -- was just as harmful to the health of mice as being the inbred offspring of first cousins. 
 
 Even though the mice didn't become obese and showed few metabolic symptoms, the sensitive test showed "they died more often and tended to have fewer babies," says the study's first author, James Ruff, who recently earned his Ph.D. at the University of Utah. "We have shown that levels of sugar that people typically consume -- and that are considered safe by regulatory agencies -- impair the health of mice." 
 
 The new toxicity test placed groups of mice in room-sized pens nicknamed "mouse barns" with multiple nest boxes -- a much more realistic environment than small cages, allowing the mice to compete more naturally for mates and desirable territories, and thereby revealing subtle toxic effects on their performance, Potts says. 
 
 "This is a sensitive test for health and vigor declines," he says, noting that in a previous study, he used the same test to show how inbreeding hurt the health of mice. 
 
 "One advantage of this assay is we get the same readout no matter if we are testing inbreeding or added sugar," Potts says. "The mice tell us the level of health degradation is almost identical" from added-sugar and from cousin-level inbreeding. 
 
 The study says the need for a sensitive toxicity test exists not only for components of our diet, but "is particularly strong for both pharmaceutical science, where 73 percent of drugs that pass preclinical trials fail due to safety concerns, and for toxicology, where shockingly few compounds receive critical or long-term toxicity testing." 
 
 The study was funded by the National Institutes of Health and the National Science Foundation. 
 
 A Mouse Diet Equal to What a Quarter of Americans Eat 
 
 The experimental diet in the study provided 25 percent of calories from added sugar -- half fructose and half glucose -- no matter how many calories the mice ate. Both high-fructose corn syrup and table sugar (sucrose) are half fructose and half glucose. 
 
 Potts says the National Research Council recommends that for people, no more than 25 percent of calories should be from "added sugar," which means "they don't count what's naturally in an apple, banana, potato or other nonprocessed food. … The dose we selected is consumed by 13 percent to 25 percent of Americans." 
 
 The diet fed to the mice with the 25 percent sugar-added diet is equivalent to the diet of a person who drinks three cans daily of sweetened soda pop "plus a perfectly healthy, no-sugar-added diet," Potts says. 
 
 Ruff notes that sugar consumption in the American diet has increased 50 percent since the 1970s, accompanied by a dramatic increase in metabolic diseases such as diabetes, obesity, fatty liver and cardiovascular disease. 
 
 The researchers used a mouse supply company that makes specialized diets for research. Chow for the mice was a highly nutritious wheat-corn-soybean mix with vitamins and minerals. For experimental mice, glucose and fructose amounting to 25 percent of calories was included in the chow. For control mice, corn starch was used as a carbohydrate in place of the added sugars. 
 
 House Mice Behaving Naturally 
 
 Mice often live in homes with people, so "mice happen to be an excellent mammal to model human dietary issues because they've been living on the same diet as we have ever since the agricultural revolution 10,000 years ago," Potts says. 
 
 Mice typically used in labs come from strains bred in captivity for decades. They lack the territoriality shown by wild mice. So the study used mice descended from wild house mice that were "outbred" to prevent inbreeding typical of lab mice. 
 
 "They are highly competitive over food, nesting sites and territories," he says. "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." 
 
 So Potts' new test -- named the Organismal Performance Assay, or OPA -- uses mice "in a more natural ecological context" more likely to reveal toxic effects of whatever is being tested, he says. 
 
 "When you look at a mouse in a cage, it's like trying to evaluate the performance of a car by turning it on in a garage," Ruff says. "If it doesn't turn on, you've got a problem. But just because it does turn on, doesn't mean you don't have a problem. To really test it, you take it out on the road." 
 
 A big room was divided into 11 "mouse barns" used for the new test. Six were used in the study. Each "barn" was a 377-square-foot enclosure ringed by 3-foot walls. 
 
 Each mouse barn was divided by wire mesh fencing into six sections or "territories," but the mice could climb easily over the mesh. Within each of the six sections was a nest box, a feeding station and drinking water. 
 
 Four of the six sections in each barn were "optimal," more desirable territories because the nest boxes were opaque plastic storage bins, which mice entered via 2-inch holes at the bottom. Each bin had four nesting cages in it, and an enclosed feeder. 
 
 The two other sections were "suboptimal" territories with open planter trays instead of enclosed bins. Female mice had to nest communally in the trays. 
 
 Running the Experiment 
 
 The mice in the experiment began with 156 "founders" that were bred in Potts' colony, 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. 
 
 The mice stayed 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. They all 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. 
 
 The founder mice had implanted microchips, like those put in pets. Microchip readers were placed near the feeding stations to record which mice fed where and for how long. A male was considered dominant if he made more than 75 percent of the visits by males to a given feeding station. In reality, the dominant males made almost 100 percent of male visits to the feeder in the desirable territory they dominated. 
 
 With the 156 founder mice (58 male, 98 female), the researchers ran the experiment six times, with an average of 26 mice per experiment: eight to 10 males (competing for six territories, four desirable and two suboptimal) and 14 to 18 females. 
 
 The Findings: Added Sugar Impairs Mouse Lifespan and Reproduction 
 
 -- After 32 weeks in mouse barns, 35 percent of the females fed extra sugar died, twice the 17 percent death rate for female control mice. There was no difference in the 55 percent death among males who did and did not get added sugar. Ruff says males have much higher death rates than females in natural settings because they compete for territory, "but there's no relation to sugar." 
 
 -- Males on the added-sugar diet acquired and held 26 percent fewer territories than males on the control diet: control males occupied 47 percent of the territories while sugar-added mice controlled less than 36 percent. Male mice shared the remaining 17 percent of territories. 
 
 -- Males on the added-sugar diet produced 25 percent fewer offspring than control males, as determined by genetic analysis of the offspring. The 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. 
 
 The researchers studied another group of mice for metabolic changes. The only differences were minor: cholesterol was elevated in sugar-fed mice, and the ability to clear glucose from the blood was impaired in female sugar-fed mice. The study found no difference between mice on a regular diet and mice with the 25 percent sugar-added diet when it came to obesity, fasting insulin levels, fasting glucose or fasting triglycerides. 
 
 "Our test shows an adverse outcome from the added-sugar diet that couldn't be detected by conventional tests," Potts says. 
 
 Human-made toxic substances in the environment potentially affect all of us, and more are continually discovered, Potts says. 
 
 "You have to ask why we didn't discover them 20 years ago," he adds. "The answer is that until now, we haven't had a functional, broad and sensitive test to screen the potential toxic substances that are being released into the environment or in our drugs or our food supply." 
 
 Potts and Ruff conducted the study with University of Utah biology lab manager Linda Morrison and undergraduates Amanda Suchy, Sara Hugentobler, Mirtha Sosa and Bradley Schwartz, and with researchers Sin Gieng and Mark Shigenaga of Children's Hospital Oakland Research Institute in California.
– A steady diet of sodas could hinder your ability to reproduce or even shorten your life, if lab mice are any indication. Researchers at the University of Utah gave mice a diet of 25% sugar—equivalent to three extra sodas per day in people—and found the males less likely to reproduce or defend their territory, the Salt Lake Tribune reports. Females had it worse, dying at double their normal rate. And to think, the National Research Council tells us that this level of added sugar is considered safe in humans. "Added sugar consumed at concentrations currently considered safe exerts dramatic adverse impacts on mammalian health," the study says, according to Science Daily. Yet the mice weren't fatter and would have passed physicals, despite the negative health impacts—so the sugar's toxic damage is all "under the hood." Why rely on a study of mice? They're considered roughly equivalent to humans in dietary studies because they've had a similar diet to ours for about 10,000 years, and up to 80% of what's toxic to them is toxic to people, too.