Although there is much evidence for dietary risk factors for hypertension in adults, few studies of children have been reported [14–21]. Even the association between salt intake and blood pressure has been inconsistent among children [14, 15]. Several studies have demonstrated the negative associations of blood pressure with calcium and magnesium among children [16–19], but very few researchers have examined whether the other dietary factors might affect blood pressure in childhood [19–21]. To our knowledge, this study is the first to demonstrate an association between seaweed intake and blood pressure in healthy children. Seaweed intake was negatively related to DBP in boys and to SBP in girls. Although we cannot prove a causal relationship because of a cross-sectional design of this study, the finding suggests that seaweed might have beneficial effects on blood pressure among children.
Our study of 3- to 6-year-old Japanese children showed that the difference between the highest and lowest tertiles was 3.5 mmHg in DBP among boys and 5.5 mmHg in SBP among girls. Decreasing blood pressure in healthy children would be potentially beneficial for blood pressure control in the future. Among Japanese adults aged 40 to 49 years, the multivariate-adjusted hazard ratio of all-cause mortality for each 10-mmHg SBP increase was reported to be 1.37 times in men and 1.19 in women during 9.8 years of follow-up . That for each 10-mm Hg DBP was 1.46 times in men and 1.40 times in women.
The negative association between seaweed and blood pressure in our study is supported by the results of several experimental studies [23–27]. A diet containing powdered brown seaweed lowered blood pressure and reduced the incidence of stroke in salt-loaded, stroke-prone spontaneously hypertensive rats [SHRs] . Hydrolysates of wakame (Undaria pinnatifida), a kind of seaweed, were reported to decrease systolic blood pressure after oral administration in SHRs . Peptides isolated from wakame also had inhibitory activity for angiotensin 1-conversing enzyme, causing an antihypertensive effect [25–27]. In addition, Ikeda et al. found that the administration of wakame delayed the development of stroke signs and improved the survival rate of salt-loaded, stroke-prone SHRs, although there was no significant difference in blood pressure changes compared with the control group . These findings suggest that seaweed may have preventive effects on hypertension and cerebrovascular diseases.
However, only a few epidemiological studies have reported an association between seaweed intake and blood pressure, and the results have been inconsistent [28–31]. Among 62 middle-aged patients with mild hypertension in Sweden, Krotkiewski et al. observed a significant decrease in mean blood pressure after the patients were given 12 and 24 g/day seaweed fiber for 4 weeks . In hypertensive elderly Japanese patients, systolic and diastolic blood pressure decreased after the patients received daily doses of 5 g of dried seaweed powder for 8 weeks . In a cross-sectional study of 190 hypertensive elderly Japanese patients, the patients treated with a low dose of a single drug ate more fruits and seaweed than the patients treated with a high dose of a single drug or multiple drugs, which suggested that the habitual intake of these food might help the control of blood pressure . However, among 7,081 Korean men 30 years of age and older, participants with metabolic syndrome, including hypertension, showed a higher intake of seaweed and oily foods than did participants without metabolic syndrome . These studies were conducted among adults, and, in this study, the relationship of seaweed intake to blood pressure has been first demonstrated among children.
In our study, seaweed seemed to have greater relationships with DBP than SBP among boys. Meanwhile, it seemed to have greater relationships with SBP than DBP among girls. Although the effects of seaweed intake on blood pressure might be different in mechanism between boys and girls, the reason for the discrepancy is unclear. In the studies among adults, one study  included only men and the others [28–30] analyzed the association in mixed group of men and women. More reports are needed to determine the association by sex.
The underlying mechanism responsible for the association between seaweed and blood pressure remains to be clarified. Seaweed contains large quantities of minerals and alginate, which is a kind of dietary fiber. Alginate has been reported to reduce blood pressure , and Yamori et al. presumed that alginic acid in seaweed may cause the inhibition of intentional sodium absorption . Potassium, calcium and magnesium also have been reported to reduce blood pressure in observational studies [13, 16, 17] and intervention trials [9–11, 18]. However, the results among our subjects did not show associations between dietary potassium, calcium and magnesium intake and blood pressure. Nonetheless, we cannot deny the possibility that simultaneous intake of several minerals through seaweed may be effective for blood pressure. Alternatively, other ingredients may play a role in the control of blood pressure since alginate or each mineral in seaweed is lower than the effective dose needed to lower blood pressure. The whole diet pattern including seaweed intake is also possible to be responsible for the lower blood pressure.
Although a dietary record would be more accurate or better if it had been used for a longer time or repeated over different seasons, using a dietary record was one of merits in this study. One limitation is that blood pressure was measured only once, which may have caused a large measurement error. However, it is unlikely that such a measurement error was directly dependent on seaweed intake. Nonetheless, repeated measurements are best practice and necessary in future studies. Another limitation was that height and weight were reported by the parents. However, the correlation coefficient between parents' reports and measured ones ranged from 0.90 to 0.96 for height and 0.95 to 0.99 for weight among 170 first-grade and 206 fourth-grade Japanese children . In our supplementary analysis among 103 children, the intra-class correlation between parent-reported and measured height and weight was high. Furthermore, the negative associations between seaweed intake and blood pressure were observed among them after the measured BMI was used as a confounder. Therefore, these differences would not greatly change the associations observed in our study. We also must note that BMI might not be necessarily a very good maker of fatness in growing children. Finally, the generalizability of our study is limited by the fact that our subjects were ethnically homogeneous Japanese children, whose diets differ from those of Western children.