Table 3.

Prevalence and unadjusted and adjusted prevalence ratio of low serum vitamin B₁₂ concentration and biochemically defined vitamin B₁₂ deficiency by B₁₂ supplement dose for adults aged >50 years in NHANES III (Third National Health and Nutrition Examination Survey; Phase 2, 1991–4)

	Low serum B12*					Biochemically defined B12 deficiency†
B12-containing supplement use	Prevalence (%)	95% CI	Unadj. PR	Adj. PR‡	Adj. 95% CI	Prevalence (%)	95% CI	Unadj. PR	Adj. PR‡	95% CI
Adults aged >50 years (n 3450)	4·4	3·1, 5·7	–	–	–	2·5	1·7, 3·4	–	–	–
Supplement non-users (n 2424)	5·9	4·1, 7·7	(referent)	–	–	3·7	2·5, 4·8	(referent)	–	–
Supplement users
Any amount (n 1026)	1·5§	0·0, 3·2	0·3	0·3	0·1, 1·7	0·4¶	0, 1·1	0·3	0·4	0·1, 1·7
>0 to 6 μg (n 623)	1·9 (5 cases)	0·0, 4·7	0·3	0·4	0·1, 1·6	0·1 (2 cases)	0·0, 0·3	0·2	0·5	0·1, 3·0
>6 to 25 μg (n 254)	1·2 (3 cases)	0·0, 3·4	0·2	0·3	0·0, 1·3	1·3 (2 cases)	0·0, 3·9	0·2	0·4	0·1, 2·4
>25 μg (n 149)	0·2 (1 case)	0·0, 0·5	0·0∥	0·0∥	0·0, 0·3	0·0** (no cases)	–	0·0** (no cases)	<0·001	–

Unadj., unadjusted; Adj., adjusted; PR, prevalence ratio.

The PROCSURVEY FREQ procedure in SAS version 9·1 (SAS Institute Inc., Cary, NC, USA) and the NHANES III, Phase 2 data sets were used to obtain these US population prevalence proportion estimates in adults aged >50 years.

^*Low serum B₁₂: serum B₁₂ <148 pmol/l.

^†Biochemically defined B₁₂ deficiency: serum B₁₂ <148 pmol/l with serum homocysteine >10 μmol/l.

^‡Adj. PR=prevalence ratio adjusted for age (continuous) gender, race (black race or other) and folate deficiency status.

^§Rao–Scott P value=0·001 compared with supplement non-users.

^∥Rao–Scott P value=0·0002 compared with supplement non-users.

^¶Point estimate for PR was 0·03, rounded to 0·0.

^**There were no cases of biochemical B₁₂ deficiency in the 149 survey participants over age 50 who were taking >25 μg of supplemental B₁₂.