Effect Of Low-Carbohydrate High-Protein Diets On Acid-Base Balance, Stone-Forming Propensity, And Calcium Metabolism, American Journal of Kidney Diseases, 2002 (Free PDF)
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.
Conclusion: Consumption of a [low-carbohydrate, high-protein] 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.
Look at that. Calcium losses were over 100 mg a day, out in the urine. That’s not a pittance. It would be a pittance if they made up for it by increased calcium absorption. But they didn’t:
This increase in urinary calcium levels was not compensated by a commensurate increase in fractional intestinal calcium absorption.
They were eating a mere 19 grams of carbohydrate a day during induction (the first 2 weeks), more than what you’d get in a small apple. They bumped up to 33 mg during maintenance (following 4 weeks). This was a true Atkins-style low-carb diet.
While looking at that study, I saw this recent meta-analysis that cited it. It reviewed 30 studies:
Conclusion: High-protein diets were associated with increased GFR,* serum urea, urinary calcium excretion, and serum concentrations of uric acid. In the light of the high risk of kidney disease among obese, weight reduction programs recommending high-protein diets especially from animal sources should be handled with caution.
* GFR is glomerular filtration rate. High chronic filtration rates cause kidney damage. And high-protein diets cause high filtration rates.
All of these studies looked at kidney effects in apparently healthy people. If healthy people suffer kidney stones, calcium losses, and renal stress, what about people who aren’t healthy? The extra work requested of the kidney from high protein diets is a lot to ask of someone with renal insufficiency, a condition of diminished kidney function often seen in people with diabetes (even if they aren’t aware of it). Diabetes is probably the most common cause of kidney disease in this country, and high-protein diets have been shown to accelerate kidney disease.
The American Diabetes Association says people with diabetes shouldn’t be getting more than 20% of their calories from protein, to protect their kidneys. Yet there are doctors telling people with diabetes to eat a low-carb, high-protein diet. Do they inform them of the risks?
The PLOS ONE analysis said:
In addition, due to the National Health and Nutrition Examination Survey III (NHANES) data, approximately 30% of the US population feature characteristics of reduced kidney function (GFR = 60–89 ml/min/1.73 m2) increasing with age greater than 40 years .
I didn’t know that. 30% and higher. Wow. Low-carb diets, which by default have one eating more protein, may assist weight loss. But the cost for this weight loss is profound.