More is not always more
There is a belief that runs deep in the supplement and peptide world. If a little works, more must work better. Double the dose, double the result.
Biology rarely agrees, and this paper has a neat little example of why.
The Hainan team did not just test one dose of SS-31. They tested three: 2, 5 and 10 milligrams per kilogram of body weight, each given once, 30 minutes after the bleed. Then they looked at two outcomes to decide which dose to carry forward: how the rats behaved and how swollen their brains were.
Behaviour: only the middle dose clearly worked
Recall the scoring from Part 3. Each rat was rated on appetite, activity and walking, at 24 and 72 hours.
Here is what happened:
- Sham rats scored the same at both time points. The surgery alone did not cause lasting problems.
- Untreated bleed rats and vehicle rats were no different from each other. Injecting plain salt solution did nothing.
- 2 mg/kg SS-31 rats moved slightly better, but the difference from vehicle was not statistically significant at either time point.
- 5 mg/kg SS-31 rats scored significantly better than vehicle rats.
- 10 mg/kg SS-31 rats did not do better than the 5 mg/kg rats.
Read that last line again. Twice the dose, no extra benefit on behaviour.
Brain swelling: two doses worked, one worked best
Brain water content is a straightforward measure of swelling. You weigh the tissue fresh, dry it in an oven at 80 °C for three days, weigh it again, and the difference is water. A bleed makes the brain take on water, and more water means more swelling.
Here the picture was a little different:
- The bleed clearly increased brain water compared with sham rats.
- 2 mg/kg made no meaningful difference against vehicle.
- 5 mg/kg and 10 mg/kg both significantly reduced swelling.
- The 5 mg/kg dose did the best job of the three.
So on swelling, the high dose did help. It just did not beat the middle dose.
Putting both measures together, the authors concluded that 5 mg/kg gave the best results, and they used that dose for every experiment after this point.
Why would a bigger dose not help more?
The paper does not try to explain it, so I want to be careful not to put words in the authors' mouths. But this pattern is common enough in pharmacology that it is worth knowing the usual explanations.
The target fills up. If SS-31 works by settling on the inner membrane of mitochondria, there is only so much membrane. Once it is covered, extra peptide has nowhere useful to go.
Some damage is not reachable. A brain bleed does a lot of different things. A mitochondrial drug can only fix the mitochondrial part. Past a certain point, the injury left over is the kind SS-31 cannot touch.
Small groups, noisy answers. Each group in this experiment was small, six animals per group according to the figure legends. With numbers that small, "10 mg/kg was not better than 5" might partly reflect chance. It does not prove 10 is worse.
Any or all of these could be in play. What matters is the shape of the curve: going up from 2 to 5 made a real difference, going up from 5 to 10 did not add much.
The lesson beyond this paper
I find this one of the most useful findings in the study, and not just for SS-31.
When researchers find that a mid-range dose works as well as or better than a high one, they are telling you something about how the compound behaves. There is a sweet spot. Below it, not enough gets to the target. Above it, you are spending more for nothing, and in many compounds you start to pick up side effects instead.
This is why a dose found in a careful study is more than a number. It is a measured answer to a question about how much it takes to fill the target and no more.
A caution on translation
Do not take 5 mg/kg in a rat and multiply by your body weight. Rats are not small humans. They process drugs at very different rates, and there are established conversion methods researchers use precisely because simple scaling gets it wrong. The paper does not suggest a human dose, and neither will I.
The point of this part is the principle, not the number. In this model, the middle dose won. The high dose did not justify itself.
Next, we look at what that 5 mg/kg dose did to brain swelling and the leaky barrier between the blood and the brain.
This series discusses a peer-reviewed laboratory study in rats. The results have not been shown in people. Nothing here is medical advice, and SS-31 is discussed for research purposes only.
