So here you are, a woman in your late 50s, watching the bathroom scale inch up every week. You’re eating the same things you always have; following the same exercise regime—and you’re gaining weight anyway.

What’s up with that?

Aging. Menopause. As you age, your metabolism gradually slows, meaning that the calories you take in are burned more slowly than they used to be, leading to weight gain. And after menopause, that extra weight tends to collect in your belly, rather than distributing itself evenly around your body.

I’m not telling you anything you don’t already know, right? That post-menopausal belly-bulge is a challenge for most of us.

So we diet. And what’s the first, easiest thing we do to cut calories?

Switch from Coke to Diet Coke.

Am I right? Diet Coke is the top-selling diet beverage in America, #3 overall in carbonated beverage sales, surpassed only by regular Coke and regular Pepsi (diet Pepsi is #5, right after Mountain Dew). For most of us, it’s a no-brainer: lose 150 calories a 12-ounce pop by making the switch? Done.

But hold on. Since you’re post-menopausal, you’re not only gaining weight, you’re losing something, too: bone density. That’s right, the loss of estrogen that’s part of menopause not only causes hot flashes, mood swings, and painful sex, it means your bones are suffering as well. Estrogen protects your bones; less estrogen, less protection, more loss.

Big deal, you say? Bone loss leads to osteopenia, which can lead to osteoporosis, which can lead to fractures, which can lead to death. Did you know that 20% of women who fracture a hip eventually die from its associated after-effects? And that 50% never walk unaided again?

Sobering statistics. Not meant to scare you, but to make you take bone loss seriously.

Which brings us back to Diet Coke, the dieter’s supposed best friend.

Turns out Diet Coke (and all carbonated caffeine beverages) have not one, not two, but three strikes against them, when we’re talking bone health.

First, drinking any diet soft drink probably means you’re replacing a healthier drink—notably, milk, or calcium-fortified water or juice—with a nutritionally empty one. You’re only going to consume so much liquid in a day; if you drink five cans of soda, you’re probably not also going to drink the liquid equivalent of milk or juice—just shy of half a gallon. There goes a prime source of calcium, key for keeping bones strong.

Second, the phosphoric acid present in most sodas (not just diet sodas) leads to excessive calcium excretion—i.e., the calcium in your bloodstream is excreted via your urine at a faster rate. And, since your body maintains the calcium level in your bloodstream at all costs, your bones shed some of their calcium to bring the level in your blood back up. Leading to thinner bones.

And third, the caffeine in many soft drinks leaches calcium from your bones: you lose about 6 milligrams of calcium from your bones for every 100 milligrams of caffeine ingested. A typical 12-ounce can of diet cola soda contains about 45mg of caffeine; Mountain Dew is even higher, at 55mg. In all fairness, that 6mg of lost calcium from your daily recommendation of up to 1500mg is fairly insignificant. And to put things in perspective, coffee has about three times the caffeine of soft drinks. Still…