June 01, 2019
Kalani L. Raphael
 

The Dietary Approaches to Stop Hypertension (DASH) diet lowers blood pressure and cardiovascular disease risk. In this issue, Banerjee et al. report that the DASH diet is associated with lower risk of end-stage kidney disease in individuals with stage 3 chronic kidney disease and hypertension. This association was particularly strong among those with diabetes. The DASH diet may have an important role in preventing end-stage kidney disease in select individuals with chronic kidney disease.

The DASH Trial is perhaps the mostwell-known dietary intervention trial ever conducted. The DASH Trial tested the effect of 2 diets on blood pressure (BP) over 8 weeks in 459 individuals with systolic BP < 160 and diastolic BP 80 to 95 mm Hg. Participants were randomly assigned to a control diet, a diet high in fruits and vegetables, or a combination diet high in fruits, vegetables, and low-fat dairy products and with reduced saturated and total fat (referred to as the DASH diet).

The sodium content was similar across the diets(3000 mg/d). When compared with the control diet, the DASH and fruit and vegetable diets lowered systolic/diastolic BP by 5.5/3.0 and 2.8/1.1 mm Hg, respectively. In the subset with hypertension, the DASH and fruit and vegetable diets had a remarkable effect, lowering systolic/diastolic BP by 11.4/5.5 and 7.2/2.8 mm Hg, respectively.1 The DASH diet is now widely recommended as a nutritional approach to prevent and manage hypertension and lower cardiovascular disease risk.

Hypertension is also a risk factor for chronic kidney disease (CKD) and CKD progression. It is plausible that the DASH diet might also preserve kidney function. Mounting evidence from epidemiological studies supports this hypothesis. In a study of nearly 15,000 Atherosclerosis Risk in Communities participants, Rebholz et al. found that those with poor accordance to a DASH diet had 16% higher risk of incident CKD over a median follow-up of 23 years.2 In the Nurses’ Health Study, those with greater accordance to a DASH diet had lower risk of rapid estimated glomerular filtration rate decline.3 These findings suggest that the DASH diet may prevent CKD.

But what about those with established CKD? Could the DASH diet preserve kidney function and prevent end-stage kidney disease (ESKD)? In this issue of Kidney International, Banerjee et al.4 evaluated the association between accordance to a DASH diet eating pattern and the risk of ESKD in adults with stage 3 CKD and hypertension.

The study consisted of 1110 participants in the Third National Health and Nutrition Examination Survey, which was conducted from 1988 to 1994 in the United States, before the DASH Trial was published. A DASH diet accordance score was calculated based on self-reported intake of 9 nutrients (saturated fat, total fat, protein, cholesterol, fiber, magnesium, calcium, potassium, and sodium). The maximum possible accordance score was 9, and scores ≥ 4.5 are considered to represent good accordance with a DASH dietary pattern.5

Notably, the median score in the cohort was 3.5 (interquartile range 2.0–4.0), indicating that ≥ 50% were not following a DASH-like diet. Compared with those with an accordance score in the highest quintile (score ≥ 5.0), those in the two lowest quintiles (score < 3.5) had approximately 2-fold higher risk of progressing to ESKD. Further analyses showed that the association between lower accordance and ESKD risk was particularly strong among those with diabetes.

Thus, consumption of a DASH-like diet may lower the risk of ESKD in patients with CKD. There was no association between accordance score and mortality, which is curious and disappointing, because the DASH diet lowers cardiovascular disease risk and many patients with stage 3 CKD die from cardiovascular disease before reaching ESKD.

Banerjee et al. conducted mediation analyses to identify individual factors that might explain the relationship between the DASH diet and ESKD risk. Potassium and magnesium intake stood out as strong mediators, dietary acid load and protein intake were partial mediators, and other nutritional factors (fiber, sodium, calcium, cholesterol, saturated fat) were nonmediators of this association.4

The observation that potassium was a strong mediator is notable and suggests that potassium-lite versions of the DASH diet, which might be tempting in patients with CKD, could be ineffective. This is the conundrum moving forward. Is the DASH diet safe in CKD?

One obvious concern is the effect on potassium balance. For an individual on a 2000 kcal/d diet, the DASH diet suggests 4 to 5 servings of fruits and 4 to 5 servings of vegetables each day (∼4700 mg/d of potassium). Studies of the DASH diet per se in CKD have not been conducted. In recent studies by Wesson and colleagues, supplementing the diet with fruits and vegetables to reduce the nonvolatile acid load by 50% (an increase of about 2–4 cups/d from the usual daily amount) did not cause significant hyperkalemia in patients with stage 3 or 4 CKD.

Nevertheless, it is not clear whether the net number of servings of fruits and vegetables approximated the number of servings suggested by the DASH diet. Participants in these studies also had serum potassium ≤ 4.5 meq/l at study entry, so the safety of supplementing the diet with fruits and vegetables in patients with CKD with higher serum potassium concentrations is uncertain.6 Another potential concern is the phosphate load from protein, low-fat dairy products, seeds, nuts, and legumes.

In a secondary analysis of the DASH Trial, the change in urinary phosphate excretion was similar between those on the DASH diet and the control diet; therefore, the difference in phosphate intake/absorption between those on the DASH and control diets was not likely to be substantial.7 This may be because some phosphate in the DASH diet is from plant sources, and bioavailability from plant sources is relatively low. Most phosphate is in the form of phytate, and mammals lack the degrading enzyme phytase.

The promising findings here tempt us to recommend the DASH diet in CKD despite uncertain safety. A short-term study evaluating the effect of the DASH diet in CKD on potassium and phosphate balance would have an immediate impact on the field. In the meantime, it is reasonable to restrict the DASH diet to those with serum potassium ≤ 4.5 meq/l and those with normal serum phosphate concentration. Even under these circumstances, the DASH diet is not worth the risk in stage 5 CKD and probably not in stage 4 CKD either. Potassium and phosphorus concentrations should be closely monitored if the DASH diet is recommended.

A few other points deserve mention. First, those in the highest quintile of DASH accordance score had mean caloric intake of 1200 kcal/d, and caloric intakes were generally low in the other quintiles (1500–1700 kcal/d). Because the nutrition data were collected from 24-hour dietary recalls, misclassification bias is a possibility. Second, the dietary acid load, reported here as potential renal acid load, was also quite low (mean 5.5 meq/d).

By comparison, the estimated potential renal acid load of the control and DASH diets in the DASH Trial were 31.8 and –25.4 meq/d, respectively. Hence, conclusions about dietary acid load as a partial mediator of the risk reduction associated with a DASH-like diet should be interpreted cautiously. Third, the National Health and Nutrition Examination Survey III cohort is 3 decades old, raising concerns that the results may not apply to more contemporary diets. It seems, though, that diets in the United States have actually gotten worse since the DASH diet was introduced.5

It is very possible that contemporary risks of ESKD in patients with CKD and poorer diet are worse than they were 30 years ago. Finally, the association between DASH diet accordance score and ESKD risk was independent of BP, suggesting that the DASH diet preserves kidney function through multiple pathways, not simply through an effect on BP. Other epidemiological studies have shown that the DASH diet eating pattern is associated with lower serum uric acid levels and lower risk of kidney stones (Figure 1).8,  9

Because dietary changes are challenging for many, and with worsening dietary patterns on a population level, gaining better insight into the underlying mechanisms between the DASH diet and preservation of kidney function is necessary and could lead to alternative and perhaps simpler dietary strategies to prevent ESKD in persons with CKD.