Sex differences and effects of dietary phosphorus source on intestinal phosphorus absorption and phosphorus balance in healthy rats

Sex differences related to phosphorus (P) homeostasis have been observed, including greater FGF-23 responses and urinary P excretion after oral P loading in females compared with males. However, sex differences in intestinal P absorption remain largely unknown. Here, our aim was to determine sex differences by dietary source on intestinal P absorption and balance in a pilot study of healthy rats. We hypothesized a sex*diet interaction, where females would have lower intestinal P absorption and P balance compared with males, but these sex difference would be greater with a high bioaccessible P diet (HBP) than with a low bioaccessible P diet (LBP). Male and female Sprague-Dawley rats were randomized by sex to either LBP or HBP diet groups (N=24 rats total, n=6 rats/sex*diet group) at 8-9 weeks of age and fed the experimental diets for 2 weeks. Both diets contained 0.7% P and 1.0% calcium, but the LBP diet was grain-based and the HBP diet was a casein-based. We analyzed the diets for P bioaccessibility using an in vitro simulated digestion method, which showed P bioaccessibility of 35% and 80% for the LBP and HBP diets, respectively. Metabolic balance was conducted on the final 4 days of the 2-week experiment to assess P balance. Intestinal fractional P absorption was determined by in vivo oral gavage using 33P radioisotope and blood sampling over 2 hours, followed by a 48-hour recovery period, followed by an IV injection of 33P and blood sampling over 2 hours prior to sacrifice. Fractional P absorption was determined from oral and IV 33P AUC of serum 33P activity as percent of dose. P balance was calculated as the average dietary P intake minus fecal and urine P over the 4-day balance period. P content of diet, feces, and urine was measured using microwave plasma-atomic emission spectroscopy. Statistical analysis was performed using two-way ANOVA with Tukey’s post-hoc comparisons in SAS. Results showed no significant main effect or interaction of sex or diet on intestinal fractional P absorption. P balance outcomes (available in subset of n=2-3 per sex*diet group) showed no significant effects for sex, diet, or their interaction on net P absorption or overall P balance. However, a non-significant trend was observed for a potential diet*sex interaction (p=0.06, overall model p=0.052) for net P absorption with higher values for males on the HBP diet (39±5%, lsmean±SE) compared with males on the LBP diet and females on either diet (15-17%±5-6%). There were significant main effects for diet (HBP >LBP) and sex (M >F) and an interaction effect on urine P where males on the HBP diet had higher urinary P than males on the LBP diet or females on either diet (all comparisons, p< 0.01); adjusting for net P absorption partially explained these effects. In conclusion, results of this pilot study suggest potential sex differences in components of P balance in healthy rats of emerging skeletal and sexual maturation age. Small sample size may have limited our ability to detect significant differences: from our variance estimates, n=8/group would have been needed to detect the sex*diet interaction on P balance as significant, and n=5/group for sex*diet on net P absorption. While our pilot results did not provide significant results in support of our hypothesis, the trends we observed with limited sample size and power still suggest the possibility of source-dependent sex differences in intestinal P absorption and balance, but this requires further investigation in rat models as well as humans, and at various life stages in health and disease. This study was funded by a seed grant from the University of Minnesota College of Food, Agricultural and Natural Resource Sciences. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.