Determinants of serum 25-hydroxyvitamin D levels in healthy young adults living in the Western Cape, South Africa

Figures & data

Table 1: Interpretation of serum 25-hydroxyvitamin D (25(OH)D) levels

Interpretation	Endocrine Society^Citation15	Institute of Medicine^Citation16
Deficiency	< 20 ng/ml (< 50 nmol/l)	< 12 ng/ml (< 30 nmol/l)
Insufficiency	20–29 ng/ml (50–72.5 nmol/l)	12–20 ng/ml (30–50 mnol/l)
Sufficiency	30–60 ng/ml (75–150 nmol/l)	> 20 ng/ml (> 50 nmol/l)
Toxicity	> 150 ng/ml (> 374 nmol/l)	>50 ng/ml (> 125 nmol/l)

Table 2: Participant demographics and descriptive statistics: winter phase

	Total (n = 242)		Males (n = 121)		Females (n = 121)
Factor	% (n)		% (n)		% (n)
Race:
African/Black	23.6 (n = 57)		19.8 (n = 24)		27.3 (n = 33)
Mixed race	29.8 (n = 72)		29.8 (n = 36)		29.8 (n = 36)
Asian	2.5 (n = 6)		2.5 (n = 3)		2.5 (n = 3)
Indian	5.4 (n = 13)		6.6 (n = 8)		4.1 (n = 5)
White	38.4 (n = 93)		41.3 (n = 50)		35.5 (n = 43)
Province:
Eastern Cape	9.5 (n = 23)		9.9 (n = 12)		9.1 (n = 11)
Free State	2.1 (n = 5)		1.7 (n = 2)		2.5 (n = 3)
Gauteng	9.1 (n = 22)		6.6 (n = 8)		11.6 (n = 14)
KwaZulu-Natal	11.9 (n = 29)		11.6 (n = 14)		12.4 (n = 15)
Limpopo	3.3 (n = 8)		2.5 (n = 3)		4.1 (n = 5)
Mpumalanga	1.7 (n = 4)		0.8 (n = 1)		2.5 (n = 3)
Northern Cape	2.5 (n = 6)		3.3 (n = 4)		1.7 (n = 2)
Northwest Province	0.8 (n = 2)		1.7 (n = 2)		0
Western Cape	59.0 (n = 143)		62.0 (n = 75)		56.2 (n = 68)
Lifestyle:
Smoking	11.2 (n = 27)		14.2 (n = 17)		8.3 (n = 10)
Alcohol use	60.5 (n = 147)		68.6 (n = 83)		52.9 (n = 64)
Exercise	87.0 (n = 210)		90.1 (n = 109)		83.5 (n = 101)
	Mean	SD	Mean	SD	Mean	SD
Age (years)	20.4	2.3	20.4	1.7	20.4	2.8
Anthropometry
Average weight (kg)	71.1	14.0	77.6	14.0	64.6	11.8
Average height (cm)	170.0	9.5	176.6	6.8	163.4	6.8
Average BMI^† (kg/m^2)	24.5	4.0	24.9	3.8	24.2	4.2
Average WC (cm)	81.5	9.7	84.2	9.6	78.9	9.2
Skin reflectometry
Average FH EI^‡	17.1	3.2	18.2	2.7	16.1	3.2
Average FH MI^||	40.5	13.9	42.2	14.1	38.7	13.4
Average UIA EI^‡	11.6*	4.7	12.3	4.7	11.0	4.5
Average UIA MI^||	38.1*	13.6	37.9	13.6	38.3	13.6
Total EI Average	14.4*	3.7	15.3	3.4	13.5	3.7
Total MI Average	39.3*	13.3	40.1	13.5	38.5	13.2
* n = 241
Vitamin D (vit D) intake
FFQ§ Vit D (mcg)	5.0	4.1	6.3	4.8	3.8	2.9
Supplemental Vit D (mcg)	2.9	12.9	3.3	7.8	2.5	16.6
Total Vit D (mcg)	8.0	13.8	9.6	9.5	6.3	16.9
Serum Vit D (ng/mL)	63.8	41.4	69.0	38.3	58.6^a	± 43.7

^†BMI: body mass index; ^‡EI: erythema index; ^§FFQ: food frequency questionnaire; ^||MI: melanin index. ^aGender difference p < 0.01.

Figure 1: Serum 25(OH)D and Fitzpatrick skin type classifications. Participants with lighter skin tones (II and III) had significantly higher 25(OH)D levels than those with darker skin tones (IV and V) (p = 0.02).

Figure 2: Serum 25(OH)D and height.

Table 3: VEPAC paired analysis of participants who returned for phase 2 of study

	Phase 1 (Winter) (n = 213)		Phase 2 (Summer) (n = 213)
Factor	Mean	SD	Mean	SD	p-value
Anthropometry:
Average BMI* kg/m^2	24.6	4.1	24.5	4.1	0.36
Average weight (kg)	71.5	14.3	71.4	14.4	0.55
Average WC^† (cm)	81.8	9.8	74.1	11.4	< 0.001
Skin reflectometry:
Average FH EI^‡	17.2	3.1	17.6	3.4	0.01
Average FH MI^§	40.3	13.8	40.2	14.9	0.77
Average UIA EI^‡ (n = 212)	11.5	4.6	11.8	4.3	0.15
Average UIA MI^§ (n = 212)	37.8	13.4	35.5	12.7	< 0.001
Total EI Average (n = 212)	14.3	3.6	14.7	3.5	< 0.001
Total MI Average (n = 212)	39.1	13.2	37.8	13.4	< 0.001
Vitamin D intake
FFQ^|| vitamin D (mcg)	4.9	3.7	4.5	3.2	0.09
Supplemental vitamin D (mcg)	2.9	13.5	2.6	12.7	0.51
Tot vitamin D (mcg)	7.9	14.3	7.1	13.0	0.15
Serum Vit D (ng/ml)	64.1	41.2	53.4	32.3	< 0.001

*BMI = body mass index; ^†WC = waist circumference; ^‡EI = erythema index; ^§MI = melanin index; ^||FFQ = food frequency questionnaire. Numbers in bold indicate statistical significance.

Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930.

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Institute of Medicine, Food and Nutrition Board. Dietary reference intakes for calcium and vitamin D. Washington (DC): National Academy Press; 2010.

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