Maintaining bone density into older age

I havent been to a gym in years but i use dumbells at home I have a 5kg pair. I've no idea if that is sufficient, I like to hope it is! 🤞
But await to be corrected by someone more knowledgeable!

I have a genetic disorder called NF1 which carries a high risk of osteopenia (48%) so I have been working hard to minimise bone loss.
It is a 360 lifestyle intervention around diet, supplements, sunshine and exercise.
Let's start with exercise since this is where you posted. You need to load your bones. You need to use weights and most people forget to load the spine, and the way to do that is weights over head. Do the exercises of your HIIT class with weights, the lunges, the squats, but also do farmer's walk, and overhead squats with a barbell or even walking around with a loaded barbell. As you stated cycling is great for heart and brain but since you are sitting your bottom, not much loading happening. Same with swimming .
Include jumps, rope skipping or jumping on a box or bench if your gym doesn't have a box.
Include soy in your diet, especially something called natto, it is fermented soy and highest source of vitamin K2 which puts calcium in your bones and not arteries
Or supplement with K2 in addition to calcium and vitamin D
Avoid sugar and also strangely saturated fats, that is fat from meat or cheese. It seems those fats will bind to calcium. So cheese is not a good source of calcium. So many are eating cheese for bones when it is actually damaging bones. Also we now know that the gut microbiome modulates bone remodelling especially through oestrogen regulation. One more reason to eat soy and vegetables, fruits, legumes and avoid gut damaging foods
Yes to sunshine but without sunscreen. Just enough to be pinkish, not burn.

And do a bone density scan so you know your baseline. Keep in mind when booking that results vary from machines sometimes so it is better to stick to the same imaging centre to compare future scans.

If you are really invested, pay a consultation with a physio specialising in osteoporosis .

Get lifting heavy weights. So many health benefits, especially for aging women.

Cycling is great exercise but does nothing for bone density. I'd add in a few weight training sessions per week.

I'm 64, postmenopausal for 10 years (no HRT) and have been weight training for 35+ years. No osteoporosis/osteopenia - weight lifting works! Along with keeping bones strong you definitely want to minimize the muscle loss that comes with aging. Keeping muscle mass/strength lessens your fracture risk.

It's Mel Robbins talking to an orthopedic surgeon with a special interest in ageing well -Dr Vonda Wright ( in case anyone wants to look her up)

https://open.spotify.com/episode/0KoSjFG6z8L6mVjScjPazN?si=FV9UWq_HQpu6x4AxA4W_NA

I listened to this only last week. You might appreciate it. I thought it was really good

Plenty of calcium and vitamin D. Go outside whenever you can. Sit in the shade, but walk in the sun - wear sunscreen! That will help with bone density too.