well, they're not alone:
http://press.endocrine.org/doi/abs/10.1210/jc.2006-2250 sun doesn't do the trick all by itself!
so if not sure about all nutrients that interact with d3, how about just for magnesium? if you haven't yet, and would care to do a dietary assessment, here's a list of healthy magnesium rich foods with amounts per serving. should get you started figuring out what you're getting each day
http://www.whfoods.com/genpage.php?tnam ... #foodchart
for any readers who don't know, daily recommended intakes for mag maintenance are around 300(f)-400(m) mg/d. varies with age:
http://www.hc-sc.gc.ca/fn-an/nutrition/ ... bl-eng.php
the WHO reports that in many developed countries worldwide, individuals fall short of dietary targets.
when taking supplemental vit d3, mag intake should be higher. i generally think of 600 as a good number to go for (without being able to cite a specific evidence-based recommendation as yet). the best i can do so far, is show the positive association between higher mag status and serum d3, in similar conditions of d3 intake from sun and diet:
Determinants of vitamin D status in patients with hip fracture and in elderly control subjects13
http://ajcn.nutrition.org/content/46/6/1005.full.pdf
Sunshine score and dietary and biochemical data from 125 patients with hip fracture and from 74 elderly control subjects
.........................................................control subjects....patients
Vitamin D intake (IU/d)..........................114 ± 44............116 ± 63
Serum Magnesium (mmol/L)......................0.82 ± 0.07........0.76 ± 0. 12
Serum 25-hydroxyvitamin D (nmol/L).........32.9 ± 13.6........18.5 ± 10.6
.....subgroup w low sunshine score.............24.3 ± 9.1.........13.3 ± 5.7
Serum l,25-dihydroxyvitamin D (pmal/L)....105 ± 31............79 ± 46
quite interesting. similar d3 intakes, different mag status, very different d3 status. with low sun exposure, controls with their better mag status have almost double the serum vit d3 levels. and if you dig into the full text, table 2, you can see that the pattern repeats in the medium and high sun exposure subgroups. all those mag levels are brutal, mind you. you want to be more like 0.95-1.1 mmol/L for serum mag

personal aside: correcting my mineral deficits *more than tripled* my serum response to a similar dose of oral d3.
a few years ago now, a randomly encountered pharmacist gave me some life-saving advice re d3 supplementation and mag timing - if supplementing d3, take half your daily mag *with* the d3, and the other half of the daily mag well away, on its own.
whether from oral intakes, sun exposure, or a sunlamp, it does seem that optimal mag status has a lot do do with making sure one's d3 regimen is successful.