The Links Between Water and Salt Intake, Body Weight, Hypertension and Kidney Stones: a Difficult Puzzle

Nephrolithiasis is a disease that strikes roughly 10% of the Italian population and its incidence in industrialized countries is on the increase. The most common form of the disease (80%) is Idiopathic Calcium Nephrolithiasis (ICN) with calcium-oxalate (CaOx) and/or calcium-phosphate (CaP) stones. The etiopathogenesis involves both genetic and acquired factors, the interplay of which leads to urinary biochemical anomalies at the root of stone formation. The elements and urinary compounds involved are known as "urinary stone risk factors". The risk factors for CaOx stones consist of low urine volume, hypercalciuria, hyperoxaluria, hyperuricosuria, hypocitraturia and hypomagnesuria. In the case of CaP stones, the hyperphosphaturia and pH parameters are of particular importance; a pH>7 promotes the formation of stones prevalently composed of phosphates, while a pH of between 6 and 7, associated with a volume <1l/day, can raise CaP supersaturation to a dangerously high level and lead to the formation of mixed CaOx and CaP stones. For uric acid stones, the elements involved are hyperuricosuria and pH<5.5. In general, the most prevalent alteration in ICN is hypercalciuria (50%). Hypertension and obesity are also social diseases with important epidemiological similarities to nephrolithiasis. These affinities have led to the search for a common pathogenic moment. As far as hypertension is concerned, various studies have demonstrated high calciuria in hypertensives with a linear relationship between 24-h calciuria and arterial blood pressure. The incidence of stone disease is greater in hypertensives than in normotensives and, by the same token, the incidence of hypertension is greater in stone formers than in non stone formers, but it is not clear whether nephrolithiasis is a risk factor for hypertension or vice versa. Moreover, a linear relationship exists between calciuria and natriuria, where the calcium is the dependent variable, with a much steeper slope of the straight line in stone formers and hypertensives compared to controls. It has, in fact, been demonstrated that to reduce calcium, it is more efficacious to reduce sodium intake as opposed to calcium intake. Finally, BMI and body weight are independently associated with an increase in stone risk even though, due to a number of bias (limited weight categories, low number of obese persons in the study populations, no control group, no recording of food intake) the studies published failed to be conclusive. In the final analysis, stone disease, arterial hypertension and excess weight/obesity prove to be closely interconnected and it is possible to intervene with targeted diets aimed at reducing the risk of illness and death from these diseases. Among such dietary approaches, the reduction of sodium chloride in food, increased hydration and an increased intake of foods with an alkaline potential seem to play an important role.

For many years now, the investigators research unit has been involved in projects, partially financed by the Italian Ministry of University and Research (MIUR), geared towards studying the effects induced by dietary changes in patients with calcium stone disease. The aim of the present project is to analyse in depth the relationship between stone disease, hypertension, body weight and water and salt intake both in the general population of the area of Parma (where historically and by gastronomic tradition, the usual diet tends to have a high salt content) and in a selected population of stone formers and hypertensives not under treatment. A representative sample of the population of the area of Parma will be studied, divided on the basis of weight category, in order to assess water and salt intake and relationships with the presence of hypertension, and a sample of normal and hypertensive stone formers randomized to receive for one year either water therapy+low salt diet or water therapy alone.