The sun – friend or foe? A critical look at sun protection, seed oils, and natural skin adaptation

The sun is the origin and source of all life on Earth. Its rays regulate the circadian rhythm, stimulate vitamin D synthesis, strengthen the immune system, and influence our mood. Yet it is increasingly portrayed as a threat—particularly with regard to skin aging and skin cancer. Sunscreen use is now considered standard advice. At the same time, other, often overlooked factors are being overlooked: including the role of seed oils, which lead to oxidative stress through UV radiation. This article critically examines how our modern sun protection practices, our diet rich in polyunsaturated fatty acids (PUFAs), and our natural use of light should be evaluated in a biologically and health-related context.

1. The biological significance of sunlight

UVB rays are essential for the synthesis of vitamin D3 from 7-dehydrocholesterol in the skin. This hormone is not only important for calcium metabolism, but also influences over 1,000 genes, the immune system, mood, and cell regeneration. A lack of sunlight—especially with the constant use of sunscreen—can therefore have far-reaching health consequences, such as vitamin D deficiency, depression, autoimmune diseases, and impaired cellular homeostasis.

2. Sunscreen: Protection or interference with natural processes?

Sunscreens block UV rays using chemical or physical filters. While this may provide short-term protection against sunburn, studies show that many chemical UV filters (e.g., oxybenzone, homosalate) have hormonal effects, promote oxidative processes, and enter the bloodstream. At the same time, they almost completely prevent the body's own vitamin D production. Paradoxically, long-term use may not reduce the risk of skin cancer in the long term, but rather shift it – for example, through chronically elevated inflammatory markers, vitamin deficiencies, and disruptions to the skin microbiome.

3. Oxidation by seed oils: The silent damage?

An often overlooked but biologically highly relevant aspect is the oxidation of polyunsaturated fatty acids from plant seed oils – both when applied externally to the skin and through food.

External use: Phototoxic reaction from natural cosmetics

Many cosmetic products rely on seed oils such as grape seed oil, argan oil, almond oil, hemp oil, or sunflower oil. These contain high concentrations of linoleic acid (omega-6), which is easily oxidized under sunlight. This produces reactive oxygen species (ROS) that attack cell membranes, promote inflammatory processes, and lead to lipid peroxidative damage in the epidermis. Dermatological studies discuss the role of these oxidized lipid fragments as pro-inflammatory, tissue-damaging, and potentially mutagenic. Despite their natural nature, such oils—especially when unprotected—can trigger phototoxic effects.

Oral intake: Systemic lipid peroxidation by seed oils

The systemic effect is even less well known. If the diet is regularly enriched with seed oils (e.g., safflower, corn germ, sunflower, or pumpkin seed oil), their unstable fatty acids penetrate the cell membranes, including those in the skin. When the body is exposed to UV radiation, these built-in PUFAs also oxidize under sunlight—with similar damage: inflammation, accelerated skin aging, tissue degeneration, and possibly an increased risk of cancer. This situation is particularly critical in cases of concurrent deficiencies in antioxidants such as vitamins E, C, or selenium.

Summary of risks from seed oils

• Very high linoleic acid content (~60–75%)
• High susceptibility to oxidation when exposed to UV light
• Formation of ROS, inflammatory mediators and mutagens
• Risk of topical use and internal administration
• Particularly harmful when there is a simultaneous lack of antioxidants

4. Fatty acids in comparison: stability and sun tolerance Oil / Fat	Main fatty acid(s)	Oxidative stability	Suitable for sun/heat?
Grape seed oil	Linoleic acid (Omega-6, ~70%)	Very low	No
Sunflower oil	Linoleic acid (Omega-6, ~60–70%)	Very low	No
safflower oil	Linoleic acid (Omega-6, ~75%)	Very low	No
Coconut oil	Saturated fatty acids (~90%)	Very high	Yes
Butter / Ghee	Saturated fatty acids (~65%)	High	Yes
Olive oil (virgin)	Oleic acid (Omega-9, ~70%)	Medium	⚠ In masses
Beef tallow / lard	Saturated + monounsaturated	High	Yes

5. Targeted sun habituation: Protection through regular stimuli

The development of natural skin protection against solar radiation does not occur through blockade, but through controlled irritation. A structured sun acclimatization is based on short, regular intervals of sun followed by regeneration phases in the shade. In the first one to two weeks, one to two sun phases of 10 to 15 minutes each day are recommended, preferably during the less light-intensive times of the morning or late afternoon. These phases should each be accompanied by a break in the shade for at least 30 to 60 minutes to reduce oxidative and thermal stress. From the third week onwards, exposure can be gradually increased to 20 to 30 minutes per interval , ideally divided into two to three blocks per day. After about four weeks of regular irritation, a total daily light exposure of 45 to 60 minutes can be achieved, depending on the individual skin type. The intermediate shade phases are essential: they allow the skin to cool down, activate cell repair processes, and normalize microvascular blood flow. The onset of skin pigmentation serves as an indicator of the activated self-protective mechanism. This form of gradual light adaptation not only promotes endogenous melanin production and epidermal thickening , but also the body's own production of antioxidant protective substances – an efficient, physiological sun protection that does not require a chemical barrier but instead occurs through rhythm and measure.

🌞 Table: Gradual sun habituation for healthy skin week	solar interval	Shadow break afterwards	Recommended time of day	Goal / Effect
1–2	1–2 × 10–15 minutes	30–60 minutes	Before 11:00 or after 16:00	Activation of vitamin D synthesis, setting the stimulus threshold
3–4	2–3 × 20–30 minutes	45–60 minutes	Also at lunchtime (with caution)	Melanin formation, epidermal adaptation
From 5	2–3 × 30–60 minutes	60 minutes or more	Individually depending on skin type	Stable self-protection, natural light tolerance

6. Sun as a biological vitality factor

Sunlight is not just an environmental phenomenon, but a central stimulus for the entire organism. It regulates hormone levels (serotonin, melatonin), has antidepressant, anti-inflammatory, and antimicrobial effects. Chronic light deficiency is linked to a variety of modern diseases, from osteoporosis to diabetes to depressive disorders.

Modern humans are increasingly living in conditions of lack of light, often out of fear of sunburn – a condition that permanently weakens the body’s natural rhythm and the immune system.

Conclusion

The sun is not an enemy—it is a biological stimulus with vital significance. Neither excessive sunbathing nor constant shielding are healthy. Rather, conscious, gradual exposure to sunlight, combined with carefully selected skincare and nutrition, is crucial. Seed oils—although "natural"—are an underestimated source of oxidative damage, both externally and internally, when exposed to direct UV light.

A natural lifestyle with moderate sun exposure, avoiding oils that are susceptible to oxidation, and instead rich in antioxidant protective substances, is the key to healthy skin, stable cell structures and a balanced immune system.

List of sources

1. Krutmann, J., et al. (2014). The role of oxidative stress in skin aging and photodamage . Free Radical Biology and Medicine, 84, 29–42.
2. Halliday, G.M. (2005). Inflammation, gene mutation and photoimmunosuppression in response to UVR-induced oxidative damage . Journal of Investigative Dermatology, 124(3), 99-109.
3. Ramsden, C.E., et al. (2013). Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study . BMJ, 346, e8707.
4. Holick, M.F. (2007). Vitamin D deficiency . New England Journal of Medicine, 357(3), 266-281.
5. Wang, Y., et al. (2015). Phototoxicity of vegetable oils under sunlight: implications for skin health . Journal of Dermatological Science, 78(2), 85-92.
6. Schwarz, A., et al. (2000). Increased oxidative stress in skin and blood of patients with actinic keratosis and skin cancer . Photodermatology, Photoimmunology & Photomedicine, 16(6), 233–237.