In an earlier investigation, the researchers had looked at the hormonal effects of diet cola ingestion on parathyroid hormone, calcium, phosphorus, insulin, alkaline phosphotase, and ghrelin.
The researchers thought that because of the phosphorus load, PTH would surge, but they found exactly the opposite, “which was that it comes down and sort of comes back to baseline; alkaline phosphatase increases also,” Larson said. “We thought, 'Well, that suggests there's some turnover of bone going on, and maybe there's some calcium being mobilized and it's going out in the urine and that might partially account for the fracture risk and decreased bone density that's being described.”
With results from that earlier study as the impetus, Larson and colleagues undertook the current study, for which they recruited 20 healthy women, ages 18 to 40.
Exclusion criteria were fracture within the prior six months, known bone disease or vitamin D deficiency, steroid or diuretic use, breast-feeding, and vitamin D supplementation above the current U.S. recommended daily allowance.
The participants were randomized to drink 24 ounces of either water or diet cola on two study days. Urine was collected for three hours after ingestion of the designated beverage and assayed for calcium, phosphorous, and creatinine using standard assays.
Data were analyzed on 16 participants; four were excluded because of lab error or failure to comply with the study protocol, the researchers said.
In addition to the higher calcium and phosphorus excretion, the investigators also found that normalized calcium and phosphorous excretion per gram of creatinine showed a trend in the same direction as total calcium and phosphorous per three hours. That figure did not achieve statistical significance, however.
Although the study was small, “it does look like there was a statistically significant rise in urine calcium,” said Larson. “The important part about that is that Diet Coke has no calcium content.”
Compared with milk, which also causes a rise in urine calcium but is replacing calcium at the same time, diet colas “would [create] an overall negative body calcium balance and that could partially explain why they appear to be bad for bones,” she said.
Although the study is too small to draw any firm conclusions, “certainly my personal practice among adolescent girls who tend to be concerned about their weight — and who drink diet beverages while they are in that critical period of bone formation — is to just try and counsel them to set habits of drinking calcium-containing beverages and maintaining adequate vitamin D,” said Larson.
Elizabeth Barrett-Connor, MD, of the University of California San Diego, called the study “fabulous.”
Barrett-Connor, who was not involved in the study, said that although it was a small and short-term trial, “it fits with all my preconceived ideas” about the nutritional problems with diet soda. “This [is new] but it just makes sense.”
The study was funded by the Walter Reed Department of Clinical Investigations.More