Is there a nutritional connection to preeclampsia? That idea seems plausible at first, as when the blood samples of women have been analyzed, some researchers have found altered levels of various vitamins and minerals. Furthermore, preeclamptic women have altered patterns of weight gain during pregnancy; and obese women are more likely to develop preeclampsia.
Such considerations may lead one to speculate that certain diets may prevent or reverse the disease, in which case the appropriate diet becomes a therapeutic intervention. However the best research to date suggests this just isn't so.
Reviews of all the trials we could locate show that the major and well-designed trials mainly show no benefit. The very large trials show either no benefit to the generalized population (i.e. Calcium supplementation), or even potential harm (i.e. Vitamins C and E). Likewise, attempts to alter rate of maternal weight gain during pregnancy towards normal levels does not alter preeclampsia rates either.
In some instances, preeclampsia itself might be causing the alterations in nutritional status. The placenta, because it is shallowly implanted, needs to use other strategies to improve its ability to ferry nutrients to the fetus, strategies that include altering the maternal metabolism. In addition, obese women may be more likely to carry genes that both make them more likely to develop obesity in a modern environment and to develop preeclampsia in pregnancy.
The example of Vitamin D shows how one such supplementation research line has developed and been disproved. Sure, vitamin D levels are lower in early-onset preeclamptic women, but this happens once the symptoms of the disease have already appeared. And if we check women at the end of the first trimester, the women who will go on to develop preeclampsia cannot be picked out of the rest of the population. Some reviewers wonder if the researchers analyzed the correct form or metabolyte of Vitamin D when deciding if the woman is "deficient." Supplementing vitamin D doesn't seem to do much beyond raising vitamin D levels in blood work
That said, there is an important role for nutrition in pregnancy and it behooves everyone to check with a qualified health care provider to assess the appropriateness of their diet as soon as they learn they are pregnant.