Solubility and Permeability: The Two Gates Every Ingredient Must Pass

Every active ingredient has to get through two gates before it can do anything useful. First, it has to dissolve. Then it has to cross a biological barrier to get where it works. Fail either one, and it doesn't matter how good the ingredient is.

Think of it this way: solubility unlocks the ingredient, turning it from something solid into something the body can use. Permeability opens the door, letting it pass through the gut wall or the skin. You need both.

Gate one: solubility

To be absorbed, an ingredient first has to dissolve in the fluids of the digestive tract. Ingredients that don't dissolve well tend to pass straight through the body. Curcumin and CBD are two well-known examples. Both are oily, water-resistant compounds, and both are famous for poor absorption.

This isn't just a supplement problem. The pharmaceutical industry wrestles with it constantly. According to a widely cited review, about 40% of approved drugs and nearly 90% of drug candidates in development are poorly water-soluble. Poor solubility is also one of the most common reasons promising compounds never make it to market.

Gate two: permeability

Dissolving is only half the job. Once in solution, an ingredient still has to cross cell membranes to reach the bloodstream. Those membranes are made largely of fats, so molecules that are too water-loving or too large struggle to get through.

Some ingredients face an extra obstacle: the gut actively pushes them back out. Berberine is a good example. Researchers have linked its very low absorption in part to transport proteins in the intestinal wall that pump it back into the gut before it can be absorbed.

How the industry sorts the problem

Regulators and drug developers use a simple framework called the Biopharmaceutics Classification System, which sorts compounds by these two gates:

• Class I: dissolves easily and crosses barriers easily. The easy ones.
• Class II: crosses barriers well but doesn't dissolve well. Solubility is the bottleneck.
• Class III: dissolves well but struggles to cross barriers. Permeability is the bottleneck.
• Class IV: struggles with both. The hardest to deliver.

Many of the botanicals and nutraceuticals brands care about most behave like Class II or Class IV compounds, which is exactly why they so often underdeliver.

The same two gates apply to skin

Topical products face their own version of this. An active has to be released from the cream or serum and move into the stratum corneum, the skin's outer layer, which is rich in lipids. Ingredients that are very water-loving have trouble entering that layer. Ingredients that are very oily can enter it but tend to stay put instead of moving deeper. The actives that penetrate best sit in a narrow middle ground, which most don't naturally occupy.

That's why so many skincare and scalp care actives stay on the surface, no matter how high the concentration on the label.

What getting through both gates delivers

When an ingredient dissolves properly and crosses its barrier efficiently, three things improve:

• Faster onset. Important for categories like energy, focus, mood support, and pain relief, where people expect to feel something.
• Lower effective dose. Less ingredient needed for the same result, which helps both cost and tolerability.
• More consistent results. Less variation from dose to dose and person to person, which builds trust and repeat purchase.

How formulators get past the gates

There's no single fix, but the most effective approaches work on both gates at once. Reducing particle size helps ingredients dissolve faster. Carrier systems built from lipids, similar to those found in cell membranes and skin, can hold water-soluble and oil-soluble actives together, keep them dispersed, and help them move across barriers the body would otherwise block.

The takeaway for any brand: before asking whether an ingredient works, ask whether it can get through both gates. At NuVessl, that's the question we help brands answer.