A high salt intake results in higher blood pressure in children as well as adults. The higher the blood pressure in childhood, the higher it will be in adult-hood and high blood pressure increases the risk of heart disease and stroke. Reducing salt intake by around 2.5g a day can reduce the risk of having a stroke or a heart attack in later life by a quarter. Eating too much salt in childhood can also lead to a preference for salty foods and therefore an increased risk of heart disease or stroke as an adult.
Summary of studies linking salt intake to blood pressure in children
Epidemiological studies have shown that those individuals with a higher salt intake have a higher blood pressure. For example:
The NDNS showed that use of salt at the table for both boys and girls and the use of salt in cooking for girls was associated with increased systolic blood pressure .
In a Swedish study, diastolic blood pressure in a total of 738 children at 4, 8 and 13 years old were related positively to sodium intakes at 8 yrs (but not in the other two age groups) .
In a Dutch study blood pressures of over 200 children (mean age 13 years) were measured, then re-measured yearly over six years; also urinary sodium and potassium concentrations were measured. Children with a larger rise in blood pressure had lower urinary potassium, or higher urinary sodium/potassium ratios  . This suggests that both dietary potassium and dietary sodium are related to the rise in blood pressure in childhood and may be important in the early pathogenesis of primary hypertension.
Controlled trials demonstrate that a reduction in salt intake does lower blood pressure in children. For example, the first ever meta-analysis of controlled salt reduction trials in children, demonstrates that reducing children's salt intake by half results in immediate falls (1.2 mmHg systolic and 1.3 mmHg diastolic in children 8-16yrs of age and 2.5 mmHg systolic in babies) in blood pressure. The meta-analysis looks at the combined results of ten trials studying a total of 966 children and adolescents aged between 8 and 16, and three trials studying a total of 551 infants . These reductions are extremely significant at a population level. For example, previous studies have suggested that a 2-3mmHg reduction in systolic blood pressure across the whole UK population would confer more benefit than all of the blood pressure treatment tablets that are currently being prescribed .
There is also evidence from studies in both children and animals to suggest that we should be thinking about restricting salt intake very early on in the development of an infant. Research implies that exposure to increased dietary sodium in early life may programme the development of higher blood pressure later in life. One study examined the effect of dietary sodium on blood pressure in 476 newborns. The infants were assigned to receive either a low sodium diet or a normal sodium diet for the first 6 months of life. At 6 months infants fed the low sodium diet had a 2.1 mmHg lower systolic pressure than infants fed the normal sodium diet . Thirty-five percent of these individuals were followed up 15 years later. It was found that blood pressure was significantly lower in the individuals who had received the low sodium diet in the first 6 months of life .
Article - 'Importance of salt in determining blood pressure in children: meta-analysis of controlled trials' (2006). Feng J. He, Graham A. MacGregor. Hypertension 48 : 861-869.
'Hypertension editorial commentary: Salt reduction for preventing hypertension and cardiovascular disease - a population approach should include children' (2006). Jeffrey A. Cutler, Edward J. Roccella. Hypertension 48 : 818-819.