Vitamin D deficiency represents a global health problem, and a recent study conducted in the United Kingdom suggests that the fortification of flour with the nutrient may offer the “optimal fortification” solution.

The research findings are contained in an article, “Does fortification of staple foods improve vitamin D intakes and status of groups at risk of deficiency? A United Kingdom modeling study.” The study was done by a research team led by Rachel E. Allen, Department of Health, Wellington House, London. Their paper was published in The American Journal of Clinical Nutrition.”

With an objective of reducing as much as possible the percentage of the public that is vitamin D deficient without exceeding the tolerable upper limit (U.L.), the researchers, conducting numerous different simulations, found that fortification of wheat flour with 10 micrograms of vitamin D per 100 grams of flour was more effective than the fortification of milk or the fortification of both flour and milk.

“This study provides new evidence that vitamin D fortification of wheat flour could be a viable option for safely improving vitamin D intakes and the status of United Kingdom population groups at risk of deficiency without increasing risk of exceeding current reference thresholds,” Dr. Allen said.

If flour were fortified with 10 micrograms of vitamin D, the percentage of at-risk individuals estimated to have intakes below the reference nutrient intakes (R.N.I.) would be 50%, down from 93%, the researchers said.

“Vitamin D deficiency leads to poor bone health and may play a role in risk of developing non-bone-related chronic diseases,” the researchers said. Young children, older adults, pregnant and breastfeeding women and certain ethnic minority groups are thought to be particularly at risk, they said. At the same time excessive intake of vitamin D may be toxic.

Because of mounting evidence linking vitamin D deficiency to chronic disease, interest in vitamin D fortification has been growing globally. In the United States, the addition of vitamin D to eligible foods is optional in most cases, with the exception of fortified milk. Fluid milk in the United States is not required to have vitamin D added unless the label declares that it is fortified.

In crafting their study, the U.K. researchers hypothesized that a universal fortification program would improve dietary intakes of vitamin D, so long as the foods selected as the fortification vehicle were consumed by those most at-risk for deficiency.

“The primary aim of this study was to identify the fortification vehicle and fortification concentration most likely to increase population vitamin D intakes and status above United Kingdom minimum reference thresholds for intake and status without exceeding maximum reference thresholds,” Dr. Allen said.

The researchers selected wheat flour and milk as the most widely consumed foods within the population. To simulate the impact of fortification, the researchers used consumption data from the U.K. National Diet and Nutrition Survey Rolling Program, which estimates individual dietary intake across the general U.K. population.

The impact of fortification on vitamin D intake was assessed for the entire population as well as for the at-risk groups.

With the plan to fortify all wheat flour, the researchers said vitamin D dietary sources would include all flour-based foods, including bread, pastries and pizza. Noodles and pasta were excluded.

The researchers found that fortification of flour and milk together resulted in less vitamin D deficiency but resulted in some proportion of the population exceeding the upper limit.

“Fortification of milk or milk and wheat flour combined was not as effective as fortification at 10 micrograms per 100 grams wheat flour,” the researchers said.

In all, the researchers looked at 15 different fortification scenarios/combinations for milk and flour.

While the flour alone option was most successful, the differences often were modest. For instance adding 5 milligrams of vitamin D per 100 liters of milk lowered the percentage of deficient consumers to 42% (versus 50% for the flour solution) but left 1% of consumers above the upper limit. Similarly combining 10 micrograms in flour and 2.5 milligrams in milk lowered the deficiency rate to 22% but left 0.5% of consumers above the upper limit.

“The effectiveness of selected vehicles and concentrations of vitamin D fortification to increase vitamin D intakes above the R.N.I. without exceeding the U.L. varied between at-risk groups,” the researchers said. “For example, fortification at 5 milligrams of vitamin D per 100 grams wheat flour and 1 milligram of vitamin D per 100 liters of milk was the most-effective scenario for young children and reduced the proportion with intakes below the R.N.I. from 93% to 50% with no individual exceeding the UL. However, in women of childbearing age and older adults, this fortification scenario was not as effective as fortification at 10 micrograms of vitamin D per 100 grams wheat flour. There was no difference in the impact of fortification on vitamin D intakes by socioeconomic group.”

Discussing their findings, the researchers noted that years earlier a similar modeling exercise was published by The Scientific Advisory Committee on Nutrition in assessing the potential impact of fortifying flour with folic acid on the health of the United Kingdom population. The subsequent fortification of flour with folic acid has been credited with significantly reducing the incidence of neural tube birth defects.

The researchers recommended the use of vitamin D2 as the source of the vitamin D. Even though it is less effective than D3, vitamin D2 ensures suitability to a vegan diet.

A rush to fortification was not advocated in the study.

“The requirement for additional research surrounding vitamin D is substantial,” Dr. Allen said. “There are gaps in the knowledge in relation to the biology of vitamin D, the relation between intake and the serum 25(OH)D (vitamin D levels in the body) concentration, and the potential impact of deficiency and excess on bone health and other chronic diseases, which suggests that additional research is required before fortification is implemented in the United Kingdom. Before the introduction of universal fortification, a detailed impact assessment including consumer research on its public acceptability would be required.”

The U.L. for vitamin D intake in the U.K. and in the study is about half the revised level proposed by the European Food Safety Authority.

“If adopted in the United Kingdom, much higher concentrations of fortification than used in the current analysis could be possible without risks of exceeding the U.L.,” Dr. Allen said. “The impact of any national fortification scheme on intakes, status, and health outcomes of individuals most at risk as well as in the general population should be closely monitored.”