Update, 2026-08-25: This article originally flagged a gap in our verified ingredient list. That gap is now closed — this piece has been rebuilt against the current live product page and reflects the full, confirmed formula.

Leave-in hair oils promise a lot: shine, softness, "deep nourishment." But does the oil in a leave-in treatment actually get into your hair fiber, or does it mostly sit on the surface until your next wash? We looked at the actual published research, oil by oil, for every oil in our Prebiotic Nourishing Scalp Oil — castor, hemp, jojoba, sunflower, neem, radish seed, and carrot seed — to give you an honest, ingredient-by-ingredient answer.

The research consistently points to one main variable: the size and shape of an oil's dominant fatty acid molecule. Shorter, straighter, more saturated fatty acid chains diffuse into the hair fiber more readily than longer, unsaturated chains, which tend to spread out and stay closer to the surface (Keis et al., 2005; Determination of penetration and protection of fatty acids in bleached hair, 2023).

Castor Oil — 🟡 primarily a sealing oil, by broad consensus. Castor oil's main fatty acid, ricinoleic acid, is a large, viscous molecule — exactly the profile associated with staying on the surface rather than diffusing into the fiber. It's widely used in hair care as a sealant that coats the hair shaft and smooths the cuticle from the outside, though we didn't find one landmark study measuring its penetration depth the precise way coconut oil has been measured.

Hemp Seed Oil — 🟠 mixed, and mechanistically a little contradictory. Hemp seed oil is dominated by linoleic acid (roughly half its fatty acid profile) along with oleic and alpha-linolenic acids — a highly polyunsaturated oil. Going strictly by the chain-length-and-saturation principle above, that composition would predict hemp oil leans toward surface coating rather than deep penetration. At the same time, some sources describe hemp oil's small molecules moving past the cuticle into the cortex. A well-regarded, independently run hair-science review of penetrating-versus-coating oils (Science-y Hair Blog) categorizes hemp seed oil as non-penetrating and primarily lubricating — which fits the fatty-acid chemistry better than the "moves into the cortex" claim does. We're presenting all of it because picking the one that sounds better isn't how we want to talk about our own ingredients.

Jojoba Seed Oil — 🔵 a genuinely different category, not a triglyceride at all. Jojoba oil isn't chemically an oil in the same sense as castor or hemp — it's a liquid wax ester, structurally closer to human sebum than to a plant triglyceride. That structure is exactly why it's included in this formula: it conditions the scalp and smooths hair without the heaviness of a true oil, and its similarity to sebum is the basis for the "balances mixed oily-and-dry scalp types" positioning on our product page. But because it isn't a triglyceride, it doesn't fit neatly into the "penetrates vs. seals" framework built around fatty acid chain length — its role is closer to surface and near-surface conditioning than either classic penetration or classic sealing. This is actually the most consistently sourced ingredient in this piece: the same independent hair-science review referenced above groups jojoba with surface-coating oils like argan rather than penetrating ones, on the same wax-ester reasoning; we didn't find a dedicated hair-fiber penetration-depth study measuring jojoba the way coconut oil has been measured, and every secondary source we found agrees on why that's the expected result rather than disagreeing about it.

Sunflower Seed Oil — 🔴 direct negative evidence for penetration. This is the most important finding in this entire piece to say plainly: the landmark 2003 Rele & Mohile study that established coconut oil's penetration advantage tested sunflower oil directly against it — and found sunflower oil did not meaningfully reduce protein loss from hair the way coconut oil did (Rele & Mohile, 2003). Sunflower oil's high linoleic acid content is the same structural reason predicted by the chain-length principle above. We're not going to soften this: the direct evidence here points toward sunflower oil behaving as a surface/sealing oil in hair fiber, not a penetrating one.

Neem Oil (Melia Azadirachta Seed Oil), Radish Seed Oil, and Carrot Seed Oil — ⚪ genuine research gaps for penetration specifically, though not for everything. All three have documented traditional and cosmetic uses, and for neem in particular there's real peer-reviewed research — just not on the penetration question. A 2024 study found a rosemary-and-neem combination effective against dandruff and hair loss, and separate research supports neem's antifungal activity (Scientific Reports, 2024; antifungal activity research, PMC) — a legitimate, different claim (antifungal/anti-dandruff activity) from "neem's fatty acids penetrate the scalp," which remains unsupported beyond marketing copy. Radish seed oil is notable for an unusual fatty acid profile — supplier technical data describes a component structurally similar to 18-MEA, the natural lipid layer on healthy hair cuticle, which would explain its silicone-like slip and shine — though that specific comparison traces back to the raw-material supplier's own marketing material, not an independent study, so we're naming the source rather than presenting it as settled. Carrot seed oil has real fatty acid profiling research (roughly 59% petroselinic acid, 12% linoleic acid) and even an animal study on hair growth promotion, but animal-model results don't establish a human penetration mechanism, and we didn't find one for this oil either (carrot seed fatty acid composition; rat hair-growth study). These are honest gaps on the specific penetration question, not findings either way — we're naming them rather than quietly filling them in.

Our Prebiotic Nourishing Scalp Oil also includes rosemary, lemongrass, peppermint, lavender, and cedarwood essential oils, along with aloe, burdock, horsetail, and bamboo extracts. We're deliberately not scoring these against the penetration-vs-sealing framework above — essential oils and water-based extracts work through entirely different mechanisms (aromatic, antimicrobial, soothing) than carrier oils moving into or staying on the hair fiber, so applying a penetration score to them would be a category error, not useful information.

Across the seven oils in this formula, the honest picture is mixed by design, not by accident: jojoba is doing a genuinely different job than the others (surface conditioning, not penetration), castor is a well-established sealant, hemp's evidence pulls in two directions, sunflower has direct evidence against meaningful penetration, and neem, radish seed, and carrot seed simply haven't been studied for this specific question yet. We'd rather tell you which oils in our own formula have real penetration evidence, mixed evidence, or none at all than let "leave-in oil treatment" imply a uniform claim the research doesn't support.

• Rele, A.S. & Mohile, R.B. (2003). Effect of mineral oil, sunflower oil, and coconut oil on prevention of hair damage. Journal of Cosmetic Science, 54(2), 175–192. PubMed
• Keis, K., Persaud, D., Kamath, Y.K., & Rele, A.S. (2005). Investigation of penetration abilities of various oils into human hair fibers. Journal of Cosmetic Science, 56(5), 283–295. Journal record
• Determination of penetration and protection of fatty acids in bleached hair according to fatty acid chain length, and the application to understanding the protective effects of MCT oil and coconut oil (2023). Fashion and Textiles. Springer Nature

Ingredient list verified against the live Bea's Bayou product page (beasbayouskincare.com/products/good-biome-prebiotic-oil), fetched 2026-08-25. Claims note: no claim is made that this product treats seborrheic dermatitis, dandruff, or any diagnosed condition — see Product Bible Section 5, Product #2, and Section 8 Claims and Compliance Matrix. Ingredient-level penetration research is presented as background education, not as proof of finished-formula performance.