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One day last summer, Alison slipped off her jewellery, stepped into a hospital gown and lay down inside a full-body MRI scanner.

As the machine issued calming instructions - breathe in, hold, breathe out - it captured thousands of images, from her head to her toes.

A tech worker and mother-of-two in her 50s, Alison (whose full name can’t be shared under participant privacy rules) had joined a nationwide health study after spotting a flyer in her local library.

Her mother died young of cancer, and women like her - of Caribbean background - were under-represented in research and often overlooked. Signing up, she says, was a way to be counted, “so that there’s data from people like me”.

What Alison didn’t realise was that she was part of one of the most ambitious studies of human health ever undertaken.

Since it launched in 2006, UK Biobank, a government-backed effort to transform medical research, has been building a vast database on the health and lifestyles of 500,000 people aged 40 to 69 when they enrolled.

Blood and other biological samples are taken and physical measurements recorded. Participants provide key information such as their education level, location, ethnic background, and living circumstances.

Crucially, they also consent to long-term tracking of their healthcare records.

Since 2014 the project has also carried out a series of full-body scans on participants, which generate more than 12,000 images per person.

The five-hour process, which scientists aim to repeat two or more years later, includes MRIs of the brain, heart, liver and abdomen; DEXA scans to assess bone density and body fat; and ultrasounds of the carotid arteries.

With 100,000 participants scanned so far - and more still being invited - the study is offering scientists an unprecedented window into how diseases take hold, slowly and silently, years before symptoms appear.

Its cloud-based platform is now used by more than 21,000 researchers across 60 countries, including early career scientists and those in low-resource settings, who receive free computer time. To date, the data have fuelled more than 16,000 scientific publications.

“This massive imaging project is making the invisible visible,” says Rory Collins, principal investigator and chief executive officer of UK Biobank.

“This is a study of the interaction of genes, environment, and lifestyle”, all of which are “determinants of disease”.

The project has produced more than one billion images - more than 10 times the size of any previous undertaking - fuelling breakthroughs in everything from AI-driven diagnostics to early disease prediction.

One of the most striking demonstrations of UK Biobank’s potential came during the Covid-19 pandemic.

Thousands of participants had undergone brain imaging before and after the outbreak - allowing researchers to study the impact of infection.

They found measurable brain changes even among people with mild Covid, including shrinkage in areas linked to smell, memory and emotion.

The findings reshaped scientists’ understanding of the virus’ neurological toll and showed the unique value of repeat imaging, which allows scientists to observe how a disease unfolds.

Funded by the government’s Medical Research Council and charities including the Wellcome Trust, UK Biobank grew out of a realisation at the turn of the century that understanding heart attacks or diseases such as dementia requires studying not just sick patients but huge numbers of healthy people over time.

Collins and others had seen how smaller studies could give misleading results, especially for risk factors such as blood pressure. They saw huge value in pairing genetic data with long-term health tracking.

The approach has already paid off with a better understanding of diagnosing and treating diabetes.

Type one diabetes was long thought to affect only children, and doctors assumed that people who got the disease in middle or old age had Type two, Collins says.

But UK Biobank research has showed that Type one occurs at the same rate throughout life. With clearer data, scientists realised that many older adults had been misclassified and given the wrong treatment.

When combined with genetic, lifestyle, and clinical data, the scans are also helping scientists detect diseases earlier, understand how they develop and, in some cases, rethink what health risk looks like.

Take body fat. A person’s body mass index, or BMI, has long been used as a rough proxy for health.

UK Biobank imaging shows that two people with the same BMI can carry fat in radically different ways - some in places that raise the risk of diabetes and heart disease, others in ways that may be protective.

“Body mass index is a very crude measure,” Collins says. “The risk associated with different distributions is likely to be massively different.”

Studies have used UK Biobank scans to spot early signs of heart damage, liver disease and even brain shrinkage linked to mild alcohol use.

Another study found that one in 10 middle-aged people, with no symptoms, had a build-up of calcium deposits in their abdominal aorta - the abdomen’s largest artery - a dangerous condition linked to heart disease that often goes undiagnosed.

Researchers are using AI to mine the vast trove of data, training models to predict diseases like Alzheimer’s or to build a “digital twin” of a patient - so researchers can establish a benchmark and compare how sick or healthy a person is.

As the number of disease cases among the participants grows and more repeat scans come online, researchers say the most transformative discoveries are still to come. As Collins put it: “We ain’t seen nothing yet”.

Alison says taking part in the research is one of the most meaningful things she’s ever done.

“They are connecting things that people haven’t previously even considered,” she says. “It’s laying the foundation for us to start seeing the deeper connections in the body and in our lives.”

What UK Biobank is revealing, scan by scan and layer by layer, is that disease doesn’t arrive out of nowhere.

It accumulates quietly, shaped by genes, environment, and habits. By making those changes visible long before symptoms appear, researchers hope to catch illness in the act - and eventually, to stop it.

It’s a shift not just in medicine, but in mindset: from treating disease after it strikes, to understanding, and potentially interrupting, how it takes shape in the first place.