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STONE
DISEASE
Effect
of low-carbohydrate high-protein diets on acid-base balance, stone-forming
propensity, and calcium metabolism
Reddy ST, Wang CY, Sakhaee K, Brinkley L, Pak CY
Department of Internal Medicine, Section of General Internal Medicine,
The University of Chicago, IL, USA
Am J Kidney Dis. 2002; 40:265-74
- Background:
Low-carbohydrate high-protein (LCHP) diets are used commonly for
weight reduction. This study explores the relationship between such
diets and acid-base balance, kidney-stone risk, and calcium and bone
metabolism.
- Methods:
Ten healthy subjects participated in a metabolic study. Subjects initially
consumed their usual non-weight-reducing diet, then a severely carbohydrate-restricted
induction diet for 2 weeks, followed by a moderately carbohydrate-restricted
maintenance diet for 4 weeks.
- Results:
Urine pH decreased from 6.09 (Usual) to 5.56 (Induction; P < 0.01)
to 5.67 (Maintenance; P < 0.05). Net acid excretion increased by
56 mEq/d (Induction; P < 0.001) and 51 mEq/d (Maintenance; P <
0.001) from a baseline of 61 mEq/d. Urinary citrate levels decreased
from 763 mg/d (3.98 mmol/d) to 449 mg/d (2.34 mmol/d; P < 0.01) to
581 mg/d (3.03 mmol/d; P < 0.05). Urinary saturation of undissociated
uric acid increased more than twofold. Urinary calcium levels increased
from 160 mg/d (3.99 mmol/d) to 258 mg/d (6.44 mmol/d; P < 0.001)
to 248 mg/d (6.19 mmol/d; P < 0.01). This increase in urinary calcium
levels was not compensated by a commensurate increase in fractional
intestinal calcium absorption. Therefore, estimated calcium balance
decreased by 130 mg/d (3.24 mmol/d; P < 0.001) and 90 mg/d (2.25
mmol/d; P < 0.05). Urinary deoxypyridinoline and N-telopeptide levels
trended upward, whereas serum osteocalcin concentrations decreased significantly
(P < 0.01).
- Conclusion:
Consumption of an LCHP diet for 6 weeks delivers a marked acid load
to the kidney, increases the risk for stone formation, decreases estimated
calcium balance, and may increase the risk for bone loss.
- Editorial
Comment
The popularity of low carbohydrate-high protein diets prompted the authors
to explore the potential effect of these diets on stone-forming risk
and calcium balance. They enrolled 10 healthy volunteers to participate
in a 3-phase trial mimicking the 2 phases of the Atkins’ diet,
including induction (2 weeks) and maintenance (4 weeks), preceded by
a baseline usual diet phase (2 weeks). During the last week of each
phase of study, the subjects consumed constant metabolic diets corresponding
to the appropriate phase of the Atkins’diet, at home for 3 days
and as in inpatient in the General Clinical Research Unit for the last
4 days. On the last 2 days of each phase, two 24-hour urine samples
were collected for stone risk factors, and blood was collected for serum
electrolytes, markers of bone turnover, PTH and vitamin D. Additionally,
fractional intestinal calcium absorption was measured. Urine pH decreased
significantly during the diet, from 6.09 to 5.56 to 5.67, and net acid
excretion increased by 56 mEq/d and 51 mEq/d from baseline during the
2 study phases, respectively. Accordingly, urinary citrate decreased
from 763 mg/d to 449 mg/d and to 581 mg/d during the 3 phases. Urinary
calcium increased significantly from 160 mg/d to 258 mg/d to 248 mg/d,
respectively, despite no change in fractional intestinal calcium absorption.
Consequently, estimated calcium balance decreased by 130 mg/d and 90
mg/d from baseline during the 2 study phases, respectively.
Low carbohydrate-high protein diets have enjoyed increasing popularity
in recent years, because of the effectiveness of the diet in weight
reduction. However, the stone-forming propensity of a high protein diet
has been well established in the literature based on the increased acid
load, which results in an increase in urinary calcium and uric acid
and a decrease in urinary citrate and pH. Likewise high protein consumption
has been associated with a negative calcium balance and bone loss. This
carefully executed study clearly demonstrates that a low carbohydrate
high protein diet confers a marked acid load that increases stone risk
and decreases calcium balance, potentially threatening bone health.
Clearly, further study with a long-term trial is necessary; however,
stone formers, and those with compromised bone mineral density, should
be cautioned before embarking on low-carbohydrate, high-protein weight
reduction plans. The authors additionally plan to investigate whether
alkali therapy can counter the negative effects of the acid load, potentially
reducing the risk of the diet.
Dr.
Margaret S. Pearle
Associate Professor of Urology
University of Texas Southwestern Med Ctr
Dallas, Texas, USA
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