Table 4.

Subgroup analysis for SMD of circulating vitamin C level in MetS vs. control subjects.

Stratification	Number of studies	Pooled SMD	95% CI	p-value	Heterogeneity
All studies	8	−0.82	−1.24, −0.40	P < 0.001	P < 0.001; I2 = 98%
Diagnostic criteria of MetSNCEP ATP III	5	−0.40	−0.68, −0.11	P = 0.006	P < 0.001; I2 = 96%
Other	3	−1.62	−2.59, −0.66	P = 0.001	P < 0.001; I2 = 96%
Geographical region
US	3	−0.12	−0.16, −0.07	P < 0.001	P = 0.18; I2 = 39%
Non-US	5	−1.31	−2.31, −0.31	P = 0.01	P < 0.001; I2 = 99%
Sample size
<1,000	5	−1.31	−2.31, −0.31	P = 0.23	P < 0.001; I2 = 99%
>1,000	3	−0.12	−0.16, −0.07	P < 0.001	P = 0.18; I2 = 39%
Study quality
High-quality	6	−0.46	−0.80, −0.13	P = 0.007	P < 0.001; I2 = 97%
Low-quality	2	−2.08	−2.31, −1.84	P < 0.001	P = 0.75; I2 = 0%