It’s based on my experience. And logic. What causes sleep apnea is a narrow and/or collapsable airway.
I have a female friend, 5’7” and 400#. Her doctor was CERTAIN she would have DM2 or at least be pre-diabetic. To the doctor’s shock, She didn’t, and wasn’t.
Why?
My friend additionally reported “sleeping like a bear”. (She also didn't have sleep apnea.) But her dentist had an interesting finding. The interior of her throat was so wide, he was afraid of losing his instruments down her throat.
The first order, root cause of sleep apnea is a narrow or collapsable airway, which either anatomically of functionally collapses during sleep.
So, let’s take it to the next step: what are the second order causes of
A narrow airway or collapsable airway?
The second order causes are several, among them:
1) a narrow face and/or high arched palate will result in a narrow airway in the left to right dimension;
2) a square jaw and/or receding chin will result in a narrow airway, top to bottom.
Add the loss of muscle tone that occurs during REM and you have an airway that will occlude.
Take thirty or forty years of undiagnosed (and hence untreated sleep apnea) and even the the most stalwart and self-disciplined will be hard put to avoid weight gain. And, of course, that will often further narrow the airway, making the SA worse.
The heritable anatomical causes are the reason SA runs in families.
Medicine has no idea how many people had the sleep apnea before the weight gain, because they aren't looking for it.
Besides, it is easier to blame the victim-than to deal with the costs and implementation strategies for societal-wide mass screening and testing for a condition for which many patients will not accept the gold standard treatment, PAP, anyway.
But the rich who have SA are routinely testing and treating their children, because they are on to this. The first line of treatment in children is to remove the tonsils and adenoids, followed by palate expansion and orthodonture, if necessary. Those measures alone sometimes result in “a cure”, although it may not be life long.
But even if the “cure” doesn't hold, it will moderate the AHI, allowing PAP treatment in patients who might otherwise require higher pressures than an ordinary APAP can provide. If you have SA, have your family members tested young, and then retested throughout life.
Can obesity, alone, cause SA? At the margin, certainly. Fatty deposits can narrow the airway. But the fundamental problem is the airway dimension and patency. (There are also neurological causes of loss of muscle tone in the airway.)
But I had severe SA at a very young age with a BMI of 19 and I know a lot of other people with the same experience. It’s just next to impossible to get diagnosed if you are some combination of young, thin, and female.