In rats, high fructose intake when pregnant can shift sex-ratio toward males (60:40)

What is it about?

The manuscript describes a direct effect of maternal nutrition on shifting of the secondary sex ratio in rat litters. We also describe how excess consumption of salt and fructose by pregnant rat dams has a marked impact on reproductive outcomes, reduced fertility for example. The diet used in this study recapitulates two of the most important aspects of a modern ‘Westernised diet’ – high fructose and salt intake. We suggest that continued consumption of higher than average intakes of these nutrients may have some hitherto unsuspected effects on reproductive function. In addition, we report for the first time in a litter-bearing animal, an effect of birth order on the sex of an implanted fetus – that is, a male is more likely to implant after a previous male (rather than after a female) and a female is more likely to implant after a previous female (rather than after a male). There is subjective observational data for this ‘birth order’ effect in humans, but we describe the effect here in an experimental animal model under controlled conditions.

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Why is it important?

We demonstrate for the first time a clear increase in the proportion of males born at term after feeding dams a diet high in fructose. We also show a marked impact of this diet on fertility (number of offspring at term) when combined with a diet high in salt. In general, these results are supportive of the sex-allocation hypothesis originally proposed by Trivers and Willard; that is, females in better body condition with the greatest food resource produce more male than female offspring. We also provide evidence that the shift in sex ratio likely has its origin in the ovary and subsequently ovulated oocyte since we saw 1) no greater selective developmental arrest of female embryos (the ratios of male:female were similar prior to implantation and at term and in large and small litters), 2) no effect of intrauterine position (i.e. secondary sex ratio was not affected by any given offspring being surrounded by two males, two females or a male/female) and 3) no effect of high glucose on the motility of sex-sorted sperm.

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