Should I supplement with vitamin D3 in summer?

This depends on your daily exposure to sunlight, especially UVB rays. Normally, our bodies produce enough vitamin D during the summer months, as we are more exposed to the sun at this time of year. However, if you work indoors, stay out of the sun or live in northern regions, your UVB exposure may not be sufficient to support adequate vitamin D synthesis. Protecting your skin with sunscreen and clothing also leads to low vitamin D synthesis.