Can salivary phosphate levels be an early biomarker to monitor the evolvement of obesity?

Abstract

Phosphate is an essential nutrient required for important biological reactions that maintain the normal homoeostatic control of the cell. The adverse effects of phosphate metabolism in obesity have not been studied in detail, chiefly because such an association is thought to be uncommon. However, in some animal models of obesity, serum phosphate levels were noted to be higher than the nonobese controls. For example, leptin-deficient (ob/ob) mice become severely obese and have high serum phosphate levels. In this study, we analyzed the phosphate content in saliva collected from children (n = 77; 10.5 ± 1.8) to evaluate association with body mass index; there is a significant increase of salivary phosphate content in obese compared to normal-weight children (ANOVA p < 0.001). The correlation coefficient (r) between BMI and phosphate was 0.33 (p = 0.0032). Our results suggest that the human salivary phosphate level may be an early biomarker of the genesis of obesity in children. The diagnostic importance lies in the fact that the salivary phosphate level could provide a noninvasive predictive marker in the development of obesity. Further studies will be required to understand the underlying mechanism of increased salivary phosphate accumulation in obese and overweight children. Nevertheless, its occurrence without systemic changes could be of diagnostic value, particularly in monitoring evolvement of obesity.

Fig. 1

Upper left panel: The body weight chart of wild-type and ob/ob animals. Note that compared to the wild-type control animals, the ob/ob mice are significantly heavier due to an excessive accumulation of fat tissues. Right panel: In accord with the relatively increased liver weight in ob/ob mice over wild-type controls, the histological analysis of the liver shows increased fat tissue accumulation in ob/ob mice. Lower left panel: Biochemical analysis of serum phosphate levels. Note that the serum phosphate levels are significantly higher in ob/ob mice compared with the wild-type (WT) mice at 3, 6 and 9 weeks of age. * p < 0.05 vs. WT. Serum samples collected from at least 4 or more mice were used for the measurements.

Fig. 2

Relative phosphate levels in saliva and plasma by mass spectroscopy. Central bars are the median, rectangles are the intraquartile range, whiskers are ×1.5 intraquartile range, asterisks are outside values and p is computed using the Mann-Whitney test. Relative phosphate values were obtained by dividing the spectrograph response by the median of all values. This comparison indicates that salivary phosphate levels in obese children were significantly greater than in normal healthy weight children whereas plasma levels were not. Approximate concentrations (mg/dl) were computed from colorometric determinations of aliquots.

Fig. 3

Least-squares fit linear function and 95% CIs. The relationship has an r² of 0.72 and a p < 0.0001 for prediction of phosphate from the spectrographic response by the function phosphate (mg/dl) = 5.03 MSR + 4.92. Thus, the median mass spectrographic value for saliva of 1.0 would be 9.95 mg/dl.