Fasting Phosphorus Levels Are a Stronger Mortality Predictor

Dietary phosphorus can throw off serum phosphorus test results.
Dietary phosphorus can throw off serum phosphorus test results.

ATLANTA—Fasting serum phosphorus levels have a stronger association with all-cause and cardiovascular mortality than non-fasting levels, researchers reported at the American Society of Nephrology's Kidney Week 2013.

Alex Chang, MD, MS of Geisinger Health System in Danville, Pa., and Morgan Grams, MD, PhD, of Johns Hopkins University in Baltimore, analyzed data from participants in the Third National Health and Nutrition Examination Survey (1988-1994). Fasting patients (those who went 12 hours or more without food or drink except for water) had significantly lower serum phosphorus levels than non-fasting participants (3.3 vs. 3.6 mg/dL). In fasting subjects, each 1 mg/dL increment in serum phosphorus was associated with a significant 48% increased adjusted risk of all-cause mortality and 67% increased adjusted risk of cardiovascular mortality. In non-fasting subjects, increases in serum phosphorus were not significantly associated with all-cause or cardiovascular mortality.

The researchers pointed out that most, but not all, studies have found associations between elevated serum phosphorus levels and increased mortality. Since serum phosphorus levels may be affected by dietary phosphorus intake, they hypothesized that the association between serum phosphorus and mortality would differ by fasting status.

In an interview with Renal & Urology News, Dr. Chang noted that it is known, but probably under-appreciated, that serum phosphorus levels vary throughout the day, with a nadir occurring in the morning, a small peak in the early afternoon followed by a drop, and then a larger nocturnal peak. “Higher phosphorus intake exaggerates the early afternoon rise in serum phosphorus,” Dr. Chang said. The effect of dietary phosphorus could introduce “noise” to randomly measured phosphorus and explain why some studies have not found an association between serum phosphorus and mortality.

“For this reason, we examined whether the association between serum phosphorus and mortality differed between those who fasted at least 12 hours and those who fasted less than 12 hours,” he said. “We found that fasting serum phosphorus is associated with mortality whereas non-fasting serum phosphorus was not associated with increased mortality.”

In clinical practice, “we need to recognize that serum phosphorus levels are affected by time of day and dietary phosphorus intake, Dr. Chang said. “We may want to consider basing management decisions on more than just a single, random serum phosphorus measurement. Unfortunately, it is hard to predict what will happen to serum phosphorus levels after a meal as this is probably affected by the amount of phosphorus consumed, the time of day, kidney function, and probably other factors.”

Considering the variation introduced by time of day and phosphorus intake, studies examining the relationship between serum phosphorus and outcomes should standardize data collection to account for these factors to enhance their power to detect associations. More research is needed to understand the effect that different levels of phosphorus intake have on serum phosphorus levels over the course of a day in patients with CKD, he said.

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