I'm now walking outside a fair bit every day (usually around 75-90 mins). Do I still need to take a vitamin D3 supplement? (I'm currently taking fish oil, algal oil, and a multivitamin that contains 800 IU of vitamin D.)

If you are young, and not taking cholesterol lowering medications, you should be making Vit D with sun exposure.

Age (elderly), and those statin drugs reduce the ability for sun exposure to work well.

I don't take my D3 in summer for that reason, as well.

If your skin is tanning, then longer exposures are needed perhaps, as the tan blocks some UV.

It also depends on your ethnicity and where you live. If you live in a northern climate, you probably do need to take a D supplement because while you may get adequate D from the sun in the summer, you will not in the winter.

If you have naturally darker skin, you may also need to take a D supplement as your skin may not allow enough natural production.

A blood test for vitamin D is not difficult to get, so you can always ask your doctor for one when you are in for something else.

I had been taking 2000 IU of D3 for about two years now because I live in a northern climate and don't spend a lot of time outside. When I had my blood tested at the Nedley Clinic, I was STILL low. He gave me a megadose of 50,000 IU and now I take 4000 IU per day in addition to my outdoor walking.