Fortification of salt with iron and iodine versus fortification of salt with iodine alone for improving iron and iodine status

This review sought to assess the effect of double‐fortified salt (DFS) compared to iodised salt (IS) on measures of iron and iodine status in all age groups.

The review authors included 18 studies (7 randomised controlled trials (RCTs), 7 cluster-RCTs, 4 controlled before and after (CBA) studies), involving over 8800 individuals from five countries. One study did not contribute to analyses.

Overall, five RCTs were rated at unclear risk of bias and two at high risk; six cluster-RCTs were rated at high risk of bias and one at unclear risk; and the four CBA studies were rated at high or unclear risk of bias.

All studies used IS as the comparator and measured and reported outcomes at study endpoint. 

The RCT evidence suggested that, compared to IS, DFS may:

  • slightly improve haemoglobin concentration (mean difference (MD) 0.43 g/dL, 95% confidence interval (CI) 0.23 to 0.63; 13 studies, 4564 participants; low-certainty evidence);
  • reduce the risk of anaemia by 21% (risk ratio (RR) 0.79, 95% CI 0.66 to 0.94; P = 0.007; 8 studies, 2593 participants; moderate-certainty evidence); and
  • reduce the risk of iron deficiency anaemia by 65% (RR 0.35, 95% CI 0.24 to 0.52; 5 studies, 1209 participants; low-certainty evidence). 

However, compared to IS, DFS may also:

  •  reduce urinary iodine concentration (MD −96.86 μg/L, 95% CI −164.99 to −28.73; 7 studies, 1594 participants; low-certainty evidence);
  • result in little to no difference in ferritin concentration (MD −3.94 µg/L, 95% CI −20.65 to 12.77; 5 studies, 1419 participants; low-certainty evidence); and
  •  result in little to no difference in transferrin receptor concentration (MD −4.68 mg/L, 95% CI −11.67 to 2.31; 5 studies, 1256 participants; low-certainty evidence).

Only one study reported adverse effects and no studies measured blood pressure.

The review authors concluded that DFS may have a small positive impact on the prevalence of anaemia compared to IS. However, they have relatively low confidence in the evidence for the outcomes of haemoglobin, urinary iodine, ferritin, and transferrin receptor concentration, and prevalence of iron deficiency anaemia. Not all studies provided data about all outcomes of interest; studies delivered the intervention differently; and studies were small, both in number and size.

The evidence is current to April 2021.

Read the full review here on the Cochrane Library.