Thread by @kyleplanckton

Does taking fiber at the same time as PrEP for HIV affect the absorption of PrEP into the bloodstream?

It’s a question that comes up on this app in a cyclical fashion and nobody ever has a good answer. That’s because there’s never been a study looking at it!

Plus, fiber supplements usually tell you to wait 2 hours before or after taking any medication. Does this have merit? Studies have shown various interactions like this, but what about for PrEP? 🤔

As the saying goes, “absence of evidence is not evidence of absence.” In other words, just because there are no data showing an interaction, that doesn’t necessarily mean an interaction doesn’t exist. In this case, it’s just never been tested.

I set out to address this question in a laboratory environment, since I am a scientist but don’t have access to real human subjects.

In doing so, I tried to simulate, as authentically as possible, the conditions that happen in the body when PrEP and fiber are taken concurrently.

Basically, I used a liter of water at acidic pH (pH 2) and 37°C (98.6°F) to simulate the human stomach. I split the experiment into two arms for comparison: one was just PrEP, and the other was PrEP + a standard serving size of psyllium husk fiber (5 capsules). twitter.com/i/web/status/1…

FYI, I used generic Truvada for these experiments since that’s what I’m prescribed. I used one pill in each experimental arm, since that’s the usual dose if you take PrEP daily.

I let the mixtures (PrEP only, left; PrEP + fiber, right) stir for 90 minutes, as that’s the average amount of time it takes for stomach contents to get mixed and pass into the small intestine. The pills and the fiber capsules dissolved pretty fast.

After 90 minutes, I neutralized the acid with sodium bicarbonate—something our small intestine usually does for us. The small intestine is where most drugs are absorbed into the bloodstrram, which then transports them around the body to wherever they need to go.

*Here’s a crucial point: the prevailing hypothesis about WHY fiber might affect absorption is because fiber is a binder. It forms a gel-like substance that could, in theory, trap drugs inside of it and prevent them from being absorbed into the bloodstream.*

Therefore, what we care about is the amount of PrEP that remains dissolved in liquid and is NOT trapped in the gel. To tease this out, I spun the mixtures really fast in a centrifuge, which separates solids from liquids. They weren’t lying when they said fiber forms a gel! 😮😳 twitter.com/i/web/status/1…

Then, I took 3 samples of the liquid from each of the conditions (PrEP alone, PrEP + fiber) and put them on a mass spectrometer. The MS is an instrument in my lab that can quantify the number of molecules in a sample, which correlates to how much drug(s) remained soluble. twitter.com/i/web/status/1…

As a reminder, PrEP consists of two drugs combined into one pill, so I was monitoring two distinct masses. I’m talking about them jointly when I talk about drug levels.

To my honest surprise,

*the amount of drug molecules remaining in liquid was practically the same in both conditions.* ‼️

(The height of the bar represents the average of each group, and the error bars represent the standard deviation due to experimental handling.)

I ran a statistical test called an ANOVA on the data, which showed that there is no significant difference (ns) between the mean value of the groups. Apparently, all of that gelatinous fiber doesn’t trap a substantial amount of drug. That’s important to know!

But there was one caveat that kept bugging me: this is simulation in a lab. What if my system doesn’t accurately represent what actually happens in the body?

To test this, I repeated the experiment with digoxin, a heart medication that’s KNOWN to interact negatively with fiber.

Encouragingly, under the same experimental conditions, we see a significantly reduced amount of digoxin in the liquid phase. I made sure to use a clinically relevant dose: 250 mcg.

My positive control—a situation where I *expected* to see a difference—worked beautifully!

The P-value of 0.0018 means that there’s only a 0.18% chance we’d see this difference if the fiber didn’t actually affect digoxin solubility. So it’s pretty convincing that the results we see reflect reality!

In other words, fiber seems to trap a significant amount of digoxin, which would prevent it from being absorbed. This could help explain the clinical interaction we observe, where eating fiber at the same time as digoxin results in lower levels of the drug in the blood.

The digoxin results give more credibility to my earlier data. It seems that my model system might do a decent job of replicating fiber absorption dynamics, and testing PrEP in that model system shows no fiber-dependent differences in solubility.

➡️The takeaway? Taking PrEP and fiber together probably doesn’t affect the absorption of PrEP into the bloodstream because the presence of fiber does not affect the PrEP’s solubility. ⬅️

There are some important limitations to consider, however. First, as I mentioned before, this is just a simulation of what happens in the body. The only way to know *for sure* if PrEP interacts with fiber is to test this in a clinical trial and measure PrEP levels in the blood.

Furthermore, I only tested this with generic Truvada for PrEP (emtricitabine 200 mg + tenofovir disoproxil fumarate 300 mg). The conclusions may or may not be similar for Descovy, which has a different formulation and dosage of tenofovir.

Lastly, there are a lot of chemical and biological processes between a drug dissolving in the stomach and that drug ending up in the bloodstream. My study only looked at solubility and fiber binding, so we don’t know how much those other processes contribute to PrEP blood levels.

If it were me, I’d probably avoid taking fiber and any medication at the same time, since we don’t know if a fiber interaction exists for most meds. But if you currently take PrEP and fiber together, it’s likely not an issue.

All in all, I’m proud of my little gay science experiment, and I’m proud of you all for reading. Feel free to share and let me know your thoughts!! Happy to share any literature references + more details about my methods. 🤓

this was troye sivan’s ig story btw lol