Modern Summers: Rising Heat and UV Protection are Changing the Premise of Natural Vitamin D Synthesis

As 2026 brings forecasts of severe heat nationwide, the way Japanese people spend their summers is undergoing significant change. In recent years, more people are avoiding the outdoors due to extreme heat and the rising risk of heatstroke. Additionally, driven by growing beauty and health consciousness, UV protection through combinations of sunscreen, parasols, and UV-cut clothing has become standard practice.

Interest in Vitamin D deficiency is rising globally. Research from Jikei University School of Medicine reports that 98% of Japanese people suffer from Vitamin D deficiency. Vitamin D is a nutrient synthesized in the body upon exposure to ultraviolet (UV-B) rays. Dr. Koeda Ochi of the university points out the possibility that the premise of naturally synthesizing Vitamin D is changing due to recent heat waves and shifting lifestyles.

In response, Spic, the company behind the Vitamin C supplement 'Lypo-C,' conducted a survey on summer lifestyles and Vitamin D among 1,000 men and women in their 20s to 60s.

The survey revealed that 65% of respondents avoided going out last summer, while about 65% had few opportunities to be outdoors between 10 AM and 3 PM, the time considered crucial for Vitamin D synthesis. Furthermore, 85.2% of women stated they 'want to avoid UV rays as much as possible,' with 72.3% using multiple methods of protection like sunscreen, parasols, and UV-cut clothing, highlighting the reality of 'modern summer living.'

Additionally, only 33.7% were aware that sunscreen hinders Vitamin D synthesis, indicating that the relationship between UV protection and Vitamin D is not sufficiently understood.

Modern summers often involve a 'double structure'—staying indoors and blocking UV rays when outdoors—making it difficult for the body to produce Vitamin D. As extreme heat and indoor-centric lifestyles become the norm, we must rethink not just 'protecting' our health, but 'supplementing' it.

Changing Summer Habits: 'Avoiding the Outdoors' is Especially Pronounced Among Women Over 40 Last summer, which saw record heat, 65% of people avoided going out. This trend was particularly significant among women over 40: 79% of women in their 40s, 75% in their 50s, and 81% in their 60s reported avoiding outings, indicating a high level of avoidance behavior.

Reasons for avoiding going out included 'heat/extreme heat is difficult' and 'fear of heatstroke' for both genders. However, 48.3% of women cited concern about UV rays (sunburn, skin damage), significantly higher than the 22.5% of men. This highlights a growing awareness of skin damage like spots and wrinkles.

Moreover, approximately 65% of people reported spending almost no time outdoors, or only 1–2 days a week, during the prime hours for Vitamin D synthesis (10 AM–3 PM). Furthermore, about 80% reported spending most or all of their day indoors, confirming the expansion of an 'indoor-centric lifestyle.'

'Not Tanning' is the New Common Sense: UV Protection enters the Era of 'Layered Defense' Overall, 77.5% of people (85.2% of women) said they want to avoid UV rays as much as possible. Top reasons for avoiding UV rays included 'I don't want to get tanned' (61%), 'I'm worried about spots and wrinkles' (57.2%), and 'I want to prevent skin aging' (43.9%). This trend was especially pronounced in women, with 70% worried about spots and wrinkles—roughly 1.7 times the 41.6% reported by men.