Two doctors in this article argue that adults should be aiming at achieving bloods levels of 50ng/ml (equal to 125 nmol/l). Supplementation is the only feasible way to achieve this, they argue.
Current guidelines in the NHS indicate that there is no evidence that having greater than 50 nmol/l confers any additional benefit. Then again, the current 'recommended dosage' for an adult is 400 IU per day - this is an historical anachronism and seriously out of date.
Then again, NHS guidelines stress that there is no need for supplementation as moderate sun exposure and eating oily fish, eggs etc. should provide all you need!
Perhaps that is true - but who gets moderate sun exposure, on average, all year round? And how many of us eat the 3 or more portions of oily fish a week required to make a difference? As for eggs - they have to be free range (otherwise the chicken doesn't get any sun either), and probably two eggs a day at least?
This article makes the point that dietary sources are insufficient, and discusses how much sun exposure we ought to be aiming for. During the summer months, if we expose the whole torso, during the middle of the day, for about 25-30 minutes (or about half the time required to make your skin pinky), on both days of the weekend, that should be enough to produce enough for the whole week. And if you do this regularly, your liver will become 'topped up' with Vitamin D, and will release it slowly during times when you need it (and can't get access to sunlight). So, if you have followed this advice, during the early winter months you should be OK. But not after. And what if you haven't been able to get access to the sun during the summer months? For example, most people are scared of the sun and will automatically sun screen prior to exposing the body. This blocks UVB rays from producing Vitamin D!
The whole situation is a bit of a mess, and it is now very likely that low dosage (e.g. minimum of 1000 IU per day for an adult) supplementation is a smart move.
Update as of 29th October 2014 - a major newspaper in the UK headlines this - all parents should be aware of this as we enter the dark months.