Throughout her career, Leah De Souza-Thomas has explored the same fundamental question from two very different perspectives: why do people get sick, and what can we do about it? As a public health epidemiologist, she has answered that question at the population level. Now, as an ION-trained nutritional therapist, she answers it one client at a time through her practice, The Thrive Practice.
A childhood fascination with the human body
Leah’s route into nutrition did not start in a kitchen. It started with a childhood spent reading physiology books for fun and a biomedical sciences degree that convinced her she never wanted to work in a lab. A brief detour into B2B events and media confirmed that wasn’t right for her either. What finally set her direction was watching the film Outbreak, a medical disaster film about military and medical researchers racing against time to stop a deadly, fast-spreading airborne virus. It pointed her towards epidemiology, and from there to a job that felt, at the time, like a dream: working in the HIV and STI department at the Health Protection Agency.
Over the following years, Leah worked across all three domains of public health, health protection, health improvement and healthcare public health, and at local, regional and national level. She spent eight years running national surveillance programmes, including work on HIV and gonorrhoea transmission, before moving into the Cardiovascular Disease Prevention Project as Public Health England was taking shape.
Finding the gap that nutrition could fill
It was during her time at Public Health England that Leah studied an applied clinical practice programme. The training gave her, in her words, excellent structure and systems for thinking about the body, but it exposed a gap in her own knowledge of food and nutritional science. Many of the practitioners she met on the course had trained at ION, so the choice of where to fill that gap was straightforward. She went on to complete ION’s Graduate Diploma.
For Leah, combining epidemiology with nutritional therapy was about working at both ends of the same problem. Public health, she explains, is powerful for prevention and population-level change, but far less equipped to support the individual behaviour change that actually shifts someone’s health outcomes day to day. Nutritional therapy gave her a way to work with individuals, alongside the population-level work she continues to do.
She also thinks about what actually shapes people’s health long before they ever reach a clinic: income, housing, commute times, caring responsibilities and what food is realistically available and affordable near where they live. What she says she took from ION, particularly through her clinical training, was the ability to hold both sides of that picture: the biochemistry and the lived reality of the person in front of her.
The Inspire programme and the Black Health and Wellbeing Day
Leah’s public health work has had a tangible impact beyond the clinic. During the COVID-19 pandemic, she was part of a Public Health England team that ran national listening sessions to understand why ethnic minority communities were being hit disproportionately hard by the virus. Hearing from over 4,000 stakeholders, the team identified a persistent theme: a mistrust of institutions that only made contact when they needed something, rather than offering ongoing support.
That insight led Leah to co-design the Inspire programme with a group of Black Londoners, and to pilot a Black Health and Wellbeing Day in Lambeth in October 2021. The event drew around 500 people and offered free food, health checks and access to medical professionals in a setting designed to feel welcoming rather than clinical. The results were striking: roughly 200 referrals into local services, 69% of diabetes risk checks flagging high or very high risk, and one case where a prostate cancer referral led to early diagnosis. The model was later adopted across eight other London boroughs, and Inspire has since grown into a London-wide community health engagement programme with its own steering group and health summits, all built on trusted relationships rather than leaflets.
Detective work, one client at a time
Today Leah runs a portfolio career, continuing her public health work alongside her nutritional therapy practice, where she supports clients dealing with gut, hormone, immune, energy and, more recently, weight concerns. She describes her clinical approach as detective work: taking a proper history, using functional testing only where it genuinely adds insight, and sequencing a plan over several months rather than changing everything at once. Many of her clients arrive having tried numerous approaches designed to sell rather than to fit their actual lives, often after being told their test results look normal despite feeling unwell.
At first, Leah was cautious about supporting clients with weight concerns, wary of the stigma attached to it.
Some people view weight loss as more of an aesthetic thing rather than a recognition that the weight is a symptom.
That is why her position now is that addressing the underlying picture, blood sugar regulation, hormones, stress physiology, tends to be more useful than chasing weight loss in isolation.
What Leah says she finds most rewarding is not an improvement in test results, but hearing clients say that “they feel well within their body again, that their energy and sleep have come back”. She is clear that she does not want clients to become dependent on her, but to feel empowered to understand their own biology well enough to sustain the changes themselves. She also hopes to grow her practice into a more multidisciplinary team, working alongside associates with complementary skills, so that more people can access the kind of individual-level support that population health alone can’t provide.
