What’s the latest evidence on this controversial question?
“I was looking for information on carbonated water and I found your article. I was very disappointed to see you say that the problem with soda is not something in the pop but the fact that calcium intake is lower in people who drink more soda. I disagree and so do the experts. For example:
[At this point in her email, Robyn had cut and pasted a paragraph from an unnamed “expert”]
[P]hosphoric acid [which is an ingredient in many sodas] interferes with calcium metabolism and the building of bone mass…A researcher from Harvard has found that soda pop consumption is closely associated with an increase in bone fractures in teenage girls.
Did I get it wrong, as Robyn is convinced? It’s certainly possible. Perhaps it’s time to revisit this topic and take a closer look at the evidence.
Nutrition Diva Endorsing Soda?
It appears that Robyn stumbled onto my episode on carbonated water from an internet search. If she were a regular reader, she’d know that I am no fan of soda. In fact, just last week, in a show dedicated to overhauling your diet, I called out soda and other sweetened beverages as the single most damaging thing in the typical American diet.
So it’s not as if I go around defending soda as a healthy, or even harmless, choice. But it is true that, in my article on whether carbonation is bad for you, I wrote the following:
Soda consumption—particularly cola consumption—has been linked to lower bone mineral density. However, the most recent research suggests that the reason people who drink a lot of cola have weaker bones is not because something in the soda is robbing calcium from their bones, but because they tend to have lower calcium intake.
So how does this square with the research cited by Robyn’s expert?
Does Phosphoric Acid in Soda cause Bone Loss?
Although Robyn didn’t cite her source, I found an internet article containing the paragraph she quoted, written by an Australian nutritionist named Owen Bond. In his article, Mr. Bond describes a study in which soda consumption was associated with an increase in bone fractures. Now, an association—also known as a correlation—only shows that two things coincide. It does not show that one thing is caused by the other. That's what we mean when we say "correlation is not causation."
Unfortunately, Mr. Bond didn’t cite his source, either. But I believe he is talking about a study done in 2000 by Grace Wyshak of Harvard University. And if you read the entire study, you'll come across a very interesting sentence: "[T]he mechanism by which cola drinks are associated with bone fractures...has neither been fully explored nor determined."
In other words, Dr. Wyshak explicitly states that although girls who drink more soda have more bone fractures, she cannot say why this is the case. It could be that the phosphorus interferes with bone metabolism. It could be that girls who drink more soda take in less calcium. It might be a combination of these two effects or any number of others.
Of course, Dr. Wyshak’s study was over ten years ago and her conclusions clearly invite researchers to explore the matter further to see if actual mechanisms can be identified. So what have we learned in the meantime? Quite honestly, not all that much.
A study in 2004 confirmed that kids who drank more cola were more likely to break their arms. However, these kids also logged significantly more hours watching TV, at the computer, and playing video games. In other words, they were a lot less active—and physical activity is one of the things that helps strengthen bone mass. In fact, when researchers adjusted for the differences in screen time between the arm-breakers and the non-arm-breakers, the association with cola disappeared. Interesting…
Do Soda Drinkers Take in Less Calcium?
In 2006, researchers examined data from the Framingham Health Study and found that adult women who drank cola every day had about 5% lower bone density than those who didn’t. Those who drank other kinds of soft drinks, which are usually lower in phosphoric acid, didn’t have reduced bone density. That’s pretty damning.
But hang on. What’s this? The researchers also note that the women who drank the most cola didn’t take in any more phosphorus than their non-cola drinking counterparts. They did, however take in less calcium. Let’s review: Their phosphorus intake was the same. Their calcium intake was lower. And their bone mineral density was lower. Hmm.
In any case, these particular researchers concluded that, “More research on the potential mechanisms by which phosphoric acid may affect bone is needed.”
One Thing We Can Agree on: Lots of Soda is Not Good For You
If you start reading the scientific literature on soda and bone density, you’ll see that there are a lot of suspects: Maybe it’s the phosphorus. Maybe it’s the acidity. Maybe it’s the caffeine. Each of these factors has been shown to increase calcium losses. (Although these effects are counteracted by sufficient calcium intake.)
See also: Diet for Healthy Bones
Maybe it’s other aspects of diet or lifestyle that are more common in heavy soda drinkers, like eating fewer fruits and vegetables, smoking more, or being less active. Just like a good episode of Law & Order, there are lots of probable suspects and, what’s worse, plenty of conflicting evidence. But just because experts don’t all agree on how or whether something in soda thins your bones, it’s pretty easy to find a consensus on the fact that drinking lots of sweetened or artificially sweetened soda isn’t a great habit.
Most experts (including the editors of the Harvard Health Letter) feel quite comfortable with unsweetened carbonated water or mineral water as a healthful alternative to soda. And that, actually, was the entire point of the article that Robyn came across in her search. In fact, drinking mineral water—either the fizzy kind or the kind without bubbles—can contribute to stronger teeth and bones!
Keep in Touch
Thanks, Robyn, for the opportunity to review and clarify the evidence on soda and bone health. If you have a suggestion for a future show topic or would like to find out about having me speak at your conference or event, send an email to firstname.lastname@example.org. You can also post comments and questions on my Nutrition Diva Facebook Page. I answer a lot of listener questions in my free weekly newsletter, so if you’ve sent a question my way, be sure you’re signed up to receive that.