Daniel M. Keller, PhD

November 19, 2012

SAN DIEGO, California — In patients with hypertension-associated kidney disease who reduced their intake of dietary acid by consuming base-inducing fruits and vegetables or bicarbonate, glomerular filtration rate (GFR) was preserved, according to a new study.

Adding base-inducing foods or bicarbonate to the diet also reduced the level of angiotensinogen, a marker of kidney angiotensin II activity, which can worsen kidney function, Nimrit Goraya, MD, assistant professor of medicine at the Texas A&M Health Science Center in Temple, reported during a news conference here at Kidney Week 2012: American Society of Nephrology 45th Annual Meeting.

She noted that diets in industrialized societies are largely acid-inducing. She and her colleagues studied patients with metabolic acidosis that was not severe enough to be treated according to current guidelines. Plasma total CO2 (PTCO2) levels above 24 mmol/L indicate a normal acid–base balance; lower levels indicate blood acidity. Guidelines recommend alkali therapy for metabolic acidosis in patients with chronic kidney disease and a PTCO2 level below 22 mmol/L, but not above that level.

The investigators tested the hypothesis that base-inducing fruits and vegetables or bicarbonate (NaHCO3) could slow the rate of decline of moderately reduced kidney function in patients with hypertension-associated kidney disease, high urine protein excretion, and moderate metabolic acidosis (a PTCO2 level of 22 to 24 mmol/L).

Participants' blood pressures were treated with angiotensin-converting-enzyme (ACE) inhibitors to a target of less than 130/80 mm Hg. Their dietary acid consumption was assessed using food diaries and net urinary acid excretion.

Patients were randomly assigned to receive fruits and vegetables (F+V) that reduce the potential renal acid load by 50% (n = 36), to receive oral NaHCO3 to reduce it by the same amount (n = 36), or to a time control group with no intervention (n = 36). The study supplied fruits and vegetables sufficient to feed all family members of patients randomized to the F+V group. Relevant clinical measurements were taken at study entry and annually for 3 years.

At 3 years, all 3 groups had experienced a decrease in systolic blood pressure from baseline (P < .05), from approximately 157 to 166 mm Hg to approximately 137 mm Hg in the control group, 136 mm Hg in the NaHCO3 group, and 130 mm Hg in the F+V group. Systolic pressure in the F+V group was the lowest, and was significantly lower than that in the control group (P < .05).

In addition, the ratio of urine angiotensinogen to creatinine had risen at 3 years from baseline in the control group (P < .05), but had fallen from baseline in the F+V and NaHCO3 groups, and was significantly lower than in the control group (P < .05 for both). There was no difference between the F+V and the NaHCO3 groups.

The estimated GFR was similar in the 3 groups at baseline — 39 mL/min per 1.73 m². At 3 years, estimated GFR had declined from baseline in all 3 groups (P < .05), but by a smaller amount in the F+V and NaHCO3 groups. The estimated GFRs of the F+V and NaHCO3 groups did not differ from each other, but they were higher than in the control group (P < .05).

In summary, Dr. Goraya said that the reduction in dietary acid preserved GFR in chronic kidney disease resulting from hypertensive nephropathy in patients with moderate metabolic acidosis. Urine angiotensinogen declined in the intervention groups designed to reduce dietary acid, suggesting that the preservation of GFR in these groups was mediated by reduced kidney angiotensin II activity.

She concluded that dietary acid reduction might protect the kidneys of patients with chronic kidney disease and moderate metabolic acidosis at levels below those for which current guidelines recommend therapy.

Kerri Cavanaugh, MD, MHS, assistant professor of medicine in the division of nephrology and medical director of the Vanderbilt Dialysis Clinic at the Vanderbilt University Medical Center in Nashville, Tennessee, told Medscape Medical News that the results are interesting, but warned that because they come from such a small study, they should be interpreted with caution and will probably not have a significant effect on clinical practice. She noted that after 3 years, both the F+V and NaHCO3 groups had better intermediate markers of kidney activity and less reduction in GFR.

"They did not differ from each other. More information about the serum bicarbonate levels over time would be useful to further interpret these results," she said.

Dr. Cavanaugh explained that many patients find it difficult to follow a diet high in fruits and vegetables, so might be more interested in a supplement. She added that there is general interest in this topic and in larger randomized controlled trials examining the impact of supplemental bicarbonate in moderate chronic kidney disease.

There was no commercial funding for the study. Dr. Goraya and Dr. Cavanaugh have disclosed no relevant financial relationships.

Kidney Week 2012: American Society of Nephrology 45th Annual Meeting. Abstract FR-OR117. Presented November 2, 2012.