Insulin induced salt retention is a phenomenon by which carbohydrate ingestion affects the ability of the kidney to excrete salt. Early reports of insulin induced salt retention were serendipitous and were made from scientific studies investigating the effects of fasting. These studies, and subsequent animal experiments have shown that during the initial phase of fasting there is a rapid excretion of sodium. Further, induction of a hyperinsulinaemic condition is able to enhance renal sodium reabsorption and cause a retention of sodium. Therefore it appears that insulin can regulate the salt stores within the body and may produce blood pressure and water retention effects. In terms of blood pressure, this is interesting because insulin induced salt retention may explain the association between body weight and blood pressure, the former being associated with a hyperinsulinaemic state. Secondly, insulin induced salt retention might explain much of the initial fall in body weight seen in energy restriction diets.
Insulin induced salt retention has been investigated in healthy subjects with normal blood pressure1. Subjects consumed a diet containing either 13 % carbohydrate or 52 % carbohydrate randomly while researcher monitored their physiological parameters. Subjects who were switched from the low carbohydrate diet to the high carbohydrate diet experienced a significant decrease in salt retention that lasted one week After this time the sodium retention normalised to baseline levels. In contrast, those who switched from a high to a low carbohydrate diet had a rapid increase in sodium retention which also lasted around 1 week before normalising. The high carbohydrate phases of the study were reflected in a two-fold increase in insulin levels which remained elevated into the second week of the study. In addition, the insulin levels were higher in the obese and overweight versus the normal weight subjects. During the high insulin phase, the subjects also experienced a fall in aldosterone, which fell even further during the second week of the diet.
Therefore increasing carbohydrate intake in healthy subjects causes a reciprocal increase in insulin levels and salt retention. These results support the insulin induced salt retention hypothesis but suggests that in healthy subjects, this effect is quickly neutralized by counterregulatory mechanisms. Because aldosterone is a hormone that causes retention of water and salt by the kidney, the lower plasma levels seen in the subjects might have been such counterregulatory mechanisms. This explains why the authors found no effect on the subjects blood pressure. But are such counterregulatory mechanisms sustainable in the long term obese? Well associations between fasting insulin levels and blood pressure have been found in overweight subjects2. In fact the association between insulin and blood pressure remains even after the body weight of the individual is taken into consideration. Therefore it is likely that the raised insulin levels in overweight individuals are the cause of elevated blood pressure and not their body weight.
Insulin induced salt retention therefore may have significant implications in the disease process, because elevated blood pressure is a risk factor for cardiovascular disease. But what are the implications for insulin induced salt retention on weight loss? Well most energy restrictive diets cause reductions in fasting insulin levels either because they restrict carbohydrates purposefully or through general energy restriction. As a result, energy restrictive diets cause initial reductions in plasma insulin levels. It would therefore be expected that such diets would cause increased rates of salt excretion initially, with a concomitant loss of water. This weight loss from this water could be considerable, and could certainly be able to explain the weight loss seen in many clinical trials that are conducted over a short duration. In fact, obesity may be a water retentive condition, such that considerable water is stored in the extracellular fluid of tissues. Energy restriction releases that water and thus rapid weight loss is possible through energy restriction in the short term.
