It began with what Harriet Kelsall describes as ‘a dramatic scene’. The mother-of-two, then 46, was out for a run when she stumbled and hit the pavement hard. ‘I grazed my knee and there was blood everywhere,’ she says. ‘Instinctively I’d thrown out my left hand to break the fall – and that was what really hurt.’

But finding that she could still move all her fingers, Harriet decided simply to go home. ‘I used an old wrist support and strapped two fingers together, which was uncomfortable but worked for the time being,’ she says. ‘But after about two weeks, I realised my hand had gone purple and I thought, this isn’t normal.

‘I went to the doctor and a scan revealed I’d broken two very small bones in my left hand and one in my wrist.’

Despite this, Harriet considered the whole thing to be mainly an inconvenience. ‘I was stupid for not seeing a doctor sooner,’ she admits.

However, after telling friends she’d broken her hand from a simple fall, they encouraged her to have a bone density scan. The results showed she was suffering from osteopenia, a bone-thinning condition that precedes osteoporosis – a debilitating condition also known as brittle bone disease.

Affecting some seven million people in the UK, osteopenia causes bones to lose strength but usually has no symptoms, meaning many people have no idea they have it until they break one. Left untreated, it can progress to osteoporosis, which leaves bones so fragile they can fracture after a minor fall or even a cough or sneeze.

‘I’d never heard of osteopenia,’ says jeweller Harriet, now 55. ‘I knew a little bit about osteoporosis, but thought it was a condition affecting only old people.

Harriet Kelsall, then 46, was out for a run when she stumbled and hit the pavement hard. ‘I grazed my knee and there was blood everywhere,’ she says

‘If it wasn’t for my friends pushing me to go for the bone density scan, I definitely wouldn’t have known I had it.’

Since her diagnosis, and on her GP’s advice, Harriet has started hormone replacement therapy (HRT), takes a high-dose vitamin D supplement and has overhauled her lifestyle – all measures proven to help improve or maintain bone density and cut the risk of osteopenia progressing to osteoporosis.

She now exercises about five times a week – combining sessions on her Peloton stationary bike, a weekly run and weight training – while cutting back on refined sugar and increasing her protein intake to support muscle and bone health.

‘I understand now that it’s all about making sure your muscles can support your bones,’ she says.

‘If I do fall over, hopefully my muscles will help protect them.

‘I haven’t had another scan yet to see if my bone density has improved, but I’ve tripped a few times since and been thrown off my paddleboard without breaking anything – which I’m taking as a good sign.’

Harriet’s diagnosis may seem surprising for someone relatively young. But while bone density loss is most strongly linked to ageing – it’s estimated up to 40 per cent of over-50s have osteopenia – doctors say they are seeing a growing number of younger adults diagnosed with the condition.

Experts describe it as an early warning sign, meaning your bones are weaker than normal but not weak enough to be classified as full-blown osteoporosis. Among the condition’s most famous sufferers is Hollywood actress Gwyneth Paltrow, now 53, who was diagnosed at just 37 after blood tests showed her vitamin D levels were critically low.

While osteopenia is still thought to be uncommon in under 50s, experts now believe they have pinpointed a series of potential triggers that could be driving diagnoses in younger people.

Crucially, they also argue that tackling these triggers could prevent countless cases from occurring in the future.

Professor Alex Comninos, consultant endocrinologist and head of the Imperial College Healthcare NHS Trust Endocrine Bone Unit, now sees a steady stream of under‑50s with fragile bones.

‘Most didn’t realise they were at risk until they’d had a fracture,’ he says.

So what exactly is this condition, and what are its causes?

Osteopenia is when bone mineral density is lower than normal, weakening the skeleton and increasing the risk of fractures. It causes no pain or early warning signs, with many unaware they have it until it is picked up after a fracture or during a bone density scan carried out because of other risk factors.

Osteopenia is diagnosed using a DEXA scan, which measures bone mineral density and compares it with that of a healthy adult of the same sex.

Unlike osteoporosis, which often requires medication to reduce the risk of fractures, osteopenia can often be improved through lifestyle changes such as weight-bearing and resistance exercise, and adequate calcium and vitamin D intake.

Importantly, osteopenia does not inevitably progress to osteoporosis, with the risk depending on age, sex, genetics, hormone levels, lifestyle and any underlying medical conditions. Factors such as heavy alcohol use, lack of exercise and poor nutrition also play a role.

‘A lot of people view osteopenia and osteoporosis as inevitable conditions that happen as a normal part of ageing,’ says Dr Richie Abel, associate professor of musculoskeletal sciences at Imperial College London. ‘It should be viewed for what it is – a totally preventable and treatable disease.’

He adds that some scientists are now viewing osteoporosis as a childhood disease – not a disease of old age.

Among the condition’s most famous sufferers is Hollywood actress Gwyneth Paltrow, now 53, who was diagnosed at just 37 after blood tests showed her vitamin D levels were critically low. She is pictured working out in a video on TikTok this week

‘When you’re young, you build, grow and develop the bones in your body,’ he says. ‘If you eat well, exercise regularly and have no nasty diseases or treatments, you should be able to build solid, healthy bones.

‘Scientists believe we reach peak bone mass between 25 and 30. This means that even throughout early adulthood, people should be really conscious of building a fit, healthy body – which means avoiding known harms such as smoking.

‘If you don’t build this bone when young, there is less to lose when you get older, meaning you’re more likely to have a fracture.’

The problem is that only those who suffer a serious fracture are routinely scanned. But in recent years, experts say there have been a number of breakthroughs in understanding the potential causes. Perhaps the strongest evidence is for the link between osteopenia and low vitamin D and calcium levels.

Vitamin D helps the body absorb calcium and other minerals from food, building strong bones.

Experts worry today’s young people, who spend more time indoors and are often less physically active, may fail to build the strongest skeleton possible.

Because sunlight is the body’s main source of vitamin D, spending little time outdoors – as well as obesity, which can trap the vitamin in body fat – may increase the risk of deficiency.

Smoking and heavy drinking can also accelerate bone loss.

Smoking reduces protective oestrogen levels and interferes with the body’s use of calcium and vitamin D, while excessive alcohol disrupts bone formation and calcium absorption, increasing the risk of osteopenia and osteoporosis.

Researchers believe several factors may be driving the rise in younger people developing the condition – one of the biggest being low oestrogen.

While levels naturally fall after the menopause, they can also drop much earlier because of early menopause – when periods stop before the age of 45 – or premature ovarian insufficiency (POI), where the ovaries stop functioning before 40.

Oestrogen can also be depleted by cancer treatments such as chemotherapy, surgery to remove the ovaries and conditions that disrupt normal hormone production, including eating disorders, being significantly underweight and excessive exercise.

Professor Comninos says oestrogen is one of the body’s main bone-protecting hormones. When levels fall too early, bone breakdown begins to outpace rebuilding, leaving them weaker and more prone to fractures.

He regularly sees women in their 20s, 30s and early 40s with low bone density caused by anorexia, not eating enough to fuel intense exercise, premature menopause or POI. For women whose oestrogen levels fall prematurely, particularly because of POI, doctors will often recommend HRT.

Autoimmune conditions can also play a role. Coeliac disease, for example, damages the lining of the small intestine, making it harder to absorb calcium and vitamin D – two nutrients essential for healthy bones.

The condition appears to be on the rise in the UK, driven by better diagnostics, greater clinical awareness and a genuine increase in cases. Studies suggest 70 per cent of new coeliac patients have low bone density.

‘If you’re given the diagnosis of osteopenia, it’s not a reason to panic – it’s a wake‑up call to think about your bone health,’ says Professor Comninos.

The good news, he says, is that simple changes to diet, vitamin D, exercise and – where needed – hormones or medication can stop osteopenia progressing and, in some cases, actually improve bone strength.

‘It’s all about leading a healthy lifestyle,’ says Kirsty Carne, senior specialist osteoporosis nurse at the Royal Osteoporosis Society. She recommends its calculator at thegreatbritishbonecheck.org.uk to assess your risk.

‘We always recommend avoiding smoking as it can have quite a significant impact on bone health,’ she says. ‘And while it’s absolutely fine to have a drink now and again, excessive alcohol can interfere with bone growth and density.’

Alongside a healthy diet, experts say the most effective intervention is exercise. When you lift weights or do impact movements, the physical pull of your muscles and gravity trigger bone-forming cells to build a thicker, more resilient skeleton.

Weight-bearing exercises – walking, dancing or jogging – stimulate bone formation by placing strain on the skeleton. And exercises such as tai chi improve balance and muscle strength, reducing the risk of falls that could lead to fractures.

‘You can start small and find your own way of doing exercise – like joining a dance club or doing salsa classes,’ says Dr Abel.

‘If you find something you enjoy, you will keep going with it rather than getting bored and quitting. This will be more beneficial in the long run, improving both your mental and physical health.’

Experts stress that it is never too early to start protecting bone health. ‘Prevention is so important as – in the worst case – osteoporosis can lead to life-changing fractures that could put you in a care home when you’re older and carry a significant risk of mortality,’ adds Dr Abel.

‘It is very important that people understand just how damaging osteoporotic fractures are. When the choice is between preventing them or not, the answer is simple.’