Both patients and medical experts have known for a while that XLHers tend to be overweight, which isn’t surprising, given that it hurts when we move, and we have an elevated risk of falling or otherwise seriously injure ourselves with physical activity, so we have extra disincentives to get the recommended amounts of daily exercise. What is less well-known is that the extent of our obesity can’t entirely be explained by reduced activity levels, so the experts have theorized for a while that there is some other mystery factor at work, possibly biochemical.
Recently, as reported in Pediatric Nephrology, “X-Linked hypophosphatemia, obesity and arterial hypertension,” researchers looked into whether we might have a second fibroblast growth factor issue, in addition to the FGF23 that causes the phosphate wasting. They looked to see whether our FGF21 (not 23) levels might also be abnormal, and found that they were not, so it’s not the mystery factor that contributes to our excess weight.
You might think that was a disappointing outcome, but it’s actually good news. Now we’ve got one less possibility for that mystery factor, and researchers can move on to investigate other possibilities, ruling them out, one by one, until the find the correct one. That’s how science works! It’s slow, but methodical.
Plus, it’s possible to look into a couple of different issues simultaneously, so in this study, the scientists, in addition to looking for a correlation between FGF21 and obesity, also looked at blood pressure readings in forty teens with XLH and compared them to their treatment regimens. What they found was that patients on burosumab had lower blood pressure (a good thing, and within normal limits, not excessively low) than patients on the old phosphorus/calcitriol regimen. We’ve all heard about how important it is to keep our blood pressure low, so this data suggests that burosumab is better than phosphorus/calcitriol for the long-term heart health of XLHers. (The article doesn’t address why blood pressure is better on burosumab than the old treatment, but it may be at least in part because of the salt in phosphorus supplements.)
There has long been scientific evidence (and patient anecdotes) of several secondary effects from the old treatment, including nephrocalcinosis, hyperparathyroidism, and gastrointestinal issues, and now this study offers some preliminary evidence of an additional risk, that of elevating our blood pressure. There’s no direct evidence, as far as I know, that the old treatment adversely affects the heart health of all XLHers, but for those of us who tend toward high blood pressure, (like me!) the effects of the old treatment on blood pressure is a risk we (and our clinicians) need to be aware of.
And, yes, I’m irritated by the article’s use of “standard of care” to describe phosphorus/calcitriol, since it implies that it actually works well, when we know it doesn’t, and the study itself reflects that burosumab is clearly better. But for now, it’s good to have this article available for 1) clinicians who berate you for not losing weight — permanent weight loss is widely known to be difficult enough when you can exercise easily and you don’t have a mystery factor working against you! — and 2) evidence of an additional risk to discuss with your doctor if you are taking phosphorus/calcitriol, especially as an adult, and even more especially if you have high blood pressure or a family history of high blood pressure.
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Please note that the author is a well-read patient, not a doctor, and is not offering medical or legal advice.
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