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Milk Products and Kidney Stones

Synopsis

The mechanisms through which milk products may modulate the risk of kidney stones have been well characterized. The main mechanism involves the calcium/oxalate balance in urine, in that a lower intake of calcium can lead to a higher urinary oxalate concentration and thus the formation of kidney stones. The components contained in milk products that may play an important role include:

• Calcium,
• Potassium,
• Magnesium,
• Water.

The Evidence

In a 2008 systematic review, Robert P. Heaney summarizes the research to date regarding calcium supplementation and incident kidney stone risk as follows: 1

• Several reports indicate an increase in the prevalence of stone disease in women in the U.S. over the past 40 years;
• The lifetime risk of having a kidney stone is estimated at 12% in men and 7% in women;
• About 74% of kidney stones in North America are composed of calcium oxalate;
• Urinary oxalate has been reported to be a more powerful risk factor for stones than urinary calcium;
• Most population-based studies are based on food sources of calcium while most clinical trials are based on supplements;
• Increased calcium intake, from food or supplements, tends to reduce the risk of kidney stones;
• Data from 12 randomized controlled trials do not show an increased risk with a calcium supplement intake of up to 2000 mg/day;
• Several observational studies of both incidence and prevalence came to similar findings;
• Only one randomized controlled trial (the calcium arm of the Woman’s Health Initiative Study) showed an increased risk with higher calcium supplement intakes, but there are several criticisms regarding this observation, including the self-reporting of kidney stones by participants;
• Overall, the available evidence does not support a causal link between calcium and kidney stones in the case of both calcium supplements and calcium from food.

A five-year randomized controlled trial comparing the effects of two diets (normal and low calcium) in 120 men with recurrent calcium oxalate stones and hypercalciuria indicated a significant reduction in the risk of recurrence by 51% (RR = 0.49; 95% CI, 0.24-0.98, p = 0.04) with the normal calcium diet.2

An 8-year prospective cohort study on the dietary factors and risk of incident kidney stones among 96,245 female participants in the Nurses’ Health Study II (ages 27 to 44 years) indicated the following: 3

• The multivariate relative risk among women in the highest quintile of dietary calcium intake compared with those in the lowest quintile was 0.73, or a 27% reduction in the incidence of kidney stones (95% CI, 0.59-0.90, ptrend = 0.007);
• Calcium supplements were not associated with a risk of incident kidney stones;
• Phytate intake was associated with a decreased risk. The relative risk in the highest quintile of phytate intake was 0.63 (95% CI, 0.51-0.78) compared to the lowest quintile;
• Higher intakes of both animal protein and fluid were also protective, whereas higher intakes of sucrose were associated with an increased risk:
  • Animal protein = RR = 0.84 ( 95% CI: 0.68-1.04), ptrend = 0.05;
  • Fluid = RR = 0.68, (95% CI: 0.56-0.83), ptrend < 0.01;
  • Sucrose = RR = 1.31, (95% CI: 1.07-1.60), ptrend = 0.01.
• Sodium, potassium, and magnesium intakes were not independently associated with a risk of incident kidney stones after adjusting for other dietary factors.

Another prospective cohort study with 12 years of follow-up involving 91,731 women aged 34 to 59 years who were part of the Nurses’ Health Study I demonstrated that: 4

• After an adjustment for potential risk factors, dietary calcium intake was inversely associated with a risk for kidney stones, while calcium supplements were positively associated with this risk;
• The relative risk for stone formation in women in the highest quintile of dietary calcium intake compared with women in the lowest quintile was 0.65 (95% CI: 0.50-0.83);
• The relative risk in women who took calcium supplements compared with women who did not was 1.20 (95% CI: 1.02-1.41);
• Other dietary factors showed the following relative risks among women in the highest quintile of intake compared with those in the lowest quintile: Sucrose = 1.52 (95% CI : 1.18-1.96),
• Sodium = 1.30 (95% CI : 1.05-1.62),
• Fluid = 0.61 (95% CI : 0.48-0.78),
• Potassium = 0.65 (95% CI : 0.51-0.84).

Similarly, another prospective cohort study of 45,619 men aged 40 to 75 years after 14 years of follow-up indicated the following: 5

• The relative risk in the highest versus lowest quintile group of calcium intake was 0.69, or a 31% reduction in risk (95% CI: 0.56-0.87, ptrend = 0.01);
• This risk was attenuated to 0.83 after further adjustments for other potential confounding factors, such as BMI, thiazid diuretic use, calcium supplements, animal protein, potassium, sodium, vitamin C, magnesium, alcohol, and fluid.
• The relative risk associated with other dietary factors was as follows: potassium = 0.54; magnesium = 0.71; fluid = 0.71; and animal protein = 1.38. Sodium, phosphorus, sucrose, phytate, vitamin B6, vitamin D, and supplemental calcium were not independently associated with this risk.

Potential Mechanisms

Hypercalciuria is the most common metabolic abnormality associated with kidney stones. However, urinary oxalate has been reported to be a more powerful risk factor for stones than urinary calcium.1 The bulk of ingested calcium remains unabsorbed in the intestine, where it forms complexes with oxalate and prevents its absorption, thus lowering the renal oxalate load.2

Several other factors have been associated with an increased risk of kidney stones, such as high intakes of oxalate-containing foods, protein and salt and low intakes of fluid, fibre, potassium, magnesium and phosphate.1

Conclusions

There is good evidence indicating that higher intakes of milk products and calcium do not increase the risk of kidney stones and may in fact reduce the risk.

Randomized controlled trials of high versus low intakes of milk products and dietary calcium versus calcium supplements are needed to provide more definitive answers.

Keywords: randomized controlled trials, systematic review, urinary calcium, oxalate, calcium oxalate, kidney stones, calcium

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