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Dietary fat increases vitamin D-3 absorption.
J Acad Nutr Diet. 2015 Feb;115(2):225-30.

BACKGROUND: The plasma 25-hydroxyvitamin D response to supplementation with vitamin D varies widely, but vitamin D absorption differences based on diet composition is poorly understood.

OBJECTIVES: We tested the hypotheses that absorption of vitamin D-3 is greater when the supplement is taken with a meal containing fat than with a fat-free meal and that absorption is greater when the fat in the meal has a higher monounsaturated-to-polyunsaturated fatty acid ratio (MUFA:PUFA).

DESIGN: Open, three-group, single-dose vitamin D-3 comparative absorption experiment.

PARTICIPANTS/SETTING: Our 1-day study was conducted in 50 healthy older men and women who were randomly assigned to one of three meal groups: fat-free meal, and a meal with 30% of calories as fat with a low (1:4) and one with a high (4:1) MUFA:PUFA. After a 12-hour fast, all subjects took a single 50,000 IU vitamin D-3 supplement with their test breakfast meal.

MAIN OUTCOME MEASURES: Plasma vitamin D-3 was measured by liquid chromatography-mass spectrometry before and 10, 12 (the expected peak), and 14 hours after the dose.

STATISTICAL ANALYSES PERFORMED: Means were compared with two-tailed t tests for independent samples. Group differences in vitamin D-3 absorption across the measurement time points were examined by analysis of variance with the repeated measures subcommand of the general linear models procedure.

RESULTS: The mean peak (12-hour) plasma vitamin D-3 level after the dose was 32% (95% CI 11% to 52%) greater in subjects consuming fat-containing compared with fat-free meals (P=0.003). Absorption did not differ significantly at any time point in the high and low MUFA and PUFA groups.

CONCLUSIONS: The presence of fat in a meal with which a vitamin D-3 supplement is taken significantly enhances absorption of the supplement, but the MUFA:PUFA of the fat in that meal does not influence its absorption.

This may also help explain why some say MS'rs are D3 tolerant/insensitive, since they would also generally speaking, try for a low-fat diet. But, (I haven't read the original study) do not forget that Omega 3's are also a fat, and are also generally considered good for MS'rs.

CureOrBust wrote:This may also help explain why some say MS'rs are D3 tolerant/insensitive, since they would also generally speaking, try for a low-fat diet. But, (I haven't read the original study) do not forget that Omega 3's are also a fat, and are also generally considered good for MS'rs.

Yes, I've had trouble getting my D3 levels up despite taking 3000 IU/day. I've recently started taking my D3 with dinner so I hope that will help.

Might the "largest meal" be the most likely to contain fat?

Taking vitamin D with the largest meal improves absorption and results in higher serum levels of 25-hydroxyvitamin D.
http://www.ncbi.nlm.nih.gov/pubmed/20200983

Abstract
Many patients treated for vitamin D deficiency fail to achieve an adequate serum level of 25-hydroxyvitamin D [25(OH)D] despite high doses of ergo- or cholecalciferol. The objective of this study was to determine whether administration of vitamin D supplement with the largest meal of the day would improve absorption and increase serum levels of 25(OH)D. This was a prospective cohort study in an ambulatory tertiary-care referral center. Patients seen at the Cleveland Clinic Foundation Bone Clinic for the treatment of vitamin D deficiency who were not responding to treatment make up the study group. Subjects were instructed to take their usual vitamin D supplement with the largest meal of the day. The main outcome measure was the serum 259(OH)D level after 2 to 3 months. Seventeen patients were analyzed. The mean age (+/-SD) and sex (F/M) ratio were 64.5 +/- 11.0 years and 13 females and 4 males, respectively. The dose of 25(OH)D ranged from 1000 to 50,000 IU daily. The mean baseline serum 25(OH)D level (+/-SD) was 30.5 +/- 4.7 ng/mL (range 21.6 to 38.8 ng/mL). The mean serum 25(OH)D level after diet modification (+/-SD) was 47.2 +/- 10.9 ng/mL (range 34.7 to 74.0 ng/mL, p < .01). Overall, the average serum 25(OH)D level increased by 56.7% +/- 36.7%. A subgroup analysis based on the weekly dose of vitamin D was performed, and a similar trend was observed.Thus it is concluded that taking vitamin D with the largest meal improves absorption and results in about a 50% increase in serum levels of 25(OH)D levels achieved. Similar increases were observed in a wide range of vitamin D doses taken for a variety of medical conditions.

Copyright 2010 American Society for Bone and Mineral Research.