When does our bone density start declining and why? Is it possible to control this phenomena?

For most humans, our peak bone mass is achieved between the ages of 25 and 30. Between the ages of 30 and 40, bone mass plateaus and post 40 it begins to decline at the rate of 0.5% per year. Bone health relies on two specific cells: osteoblasts and osteoclasts. While the former lays down new bone mineral, the latter breaks down old or damaged bone. Post 40, the lifespan of osteoblasts begins to shorten while the activity of osteoclasts is steady or increases. This leads to a net loss of bone tissue.

The key reproductive hormones - Estrogen and Testosterone have a role to play here. These are not just sex hormones but also powerful bone protectors. They inhibit the osteoclasts. Secondly the growth hormone declines naturally with age, reducing the body's ability to stimulate the "builder" cells or the osteoblasts.

So, is there anything that can be done to prevent or even reverse this degeneration? A landmark study (the LIFTMOR trial) showed that High Intensity Resistance and Impact Training significantly improved bone mineral density in post menopausal women with low bone mass. As part of this trial, women were given two 30 minute sessions per week where they had to do resistance exercises in the form of: deadlifts (lifting a barbell from the ground to a standing position), back squats (squat with barbells positioned across the back of their upper back or shoulders) and overhead presses (taking a barbell from the chest to a fully locked out position over the head); doing 5 sets of 5 repetitions of each of these exercises.

They also did specific types of high impact jumping and landing exercises where they held on to an overhead bar and pulled themselves up and finally letting it go to land "hard" with impact. Ofcourse, the participants started with less weight and progressively added the load and all sessions were done under careful supervision. After 8 months, those who went through the training showed improvement in bone mineral density in their lumbar spine (lower back) by +2.9% compared to degeneration by -1.2% in those who did not. Likewise, even in femoral neck (hip joint), bone mineral density showed an improvement by +0.3% in those who participated compared to degeneration of -1.9% in those who didn't. 

Bones essentially grow only when there is a signal in the form of heavy load. Deep inside our bones are mature cells called osteocytes which control the functioning of osteoblasts and osteoclasts. When the sensors feel some pressure such as when you squat or jump or lift weights, they put osteoblasts into action to use the raw material in the form of protein, calcium and Vitamin D (which is essential for absorption of Calcium) to create bones.  So while walking or running is great for heart, the bone building cells only wake up when they get the desired signals. It is recommended to start slow with basic stability improvement moves just doing squats with your body weight alone and gradually increase the weights and the intensity. 

Sources: 

1. National Institute of Health https://pubmed.ncbi.nlm.nih.gov/28975661/