The Health Detective’s Handbook
Drawing on over 40 years of experience, nutrition and exercise specialist Susan Birch explains in her new book how metabolism, hormones, food, movement, sleep and stress work together to influence your energy, weight, mood and overall health.
In this extract she looks at the influence of our ancestral genetics on our diets.
This extract is from The Health Detective’s Handbook: Finding your path to lifelong wellness, reproduced with kind permission of the author.
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Genetic adaptation – the slow pace of change
The transition from hunter-gatherer societies to agriculture marked a major shift in human diets. Over the past century, this shift has accelerated, leading to the year-round consumption of processed foods, containing refined carbohydrates and PUFAs (polyunsaturated fatty acids). This is a stark contrast to the seasonal diets of our ancestors. Alongside these dietary changes, modern food manufacturing has introduced entirely new ingredients that our bodies haven’t evolved to process.
Our genetic programming still aligns with ancestral conditions: whole foods, food scarcity, and seasonal variation. We haven’t adapted to the constant availability of ultra-processed, energy-dense foods, which make overeating easy and contribute to the growing burden of chronic metabolic disease. This mismatch between our physiology and modern diets has profound health consequences.
Genetic adaptation is a slow process, unfolding over thousands of years in response to environmental pressures. But the rapid dietary changes of recent decades have outpaced our evolutionary capacity to adapt. This imbalance is a key driver behind the prevalence of chronic diseases today.
Modern medicine complicates this dynamic further. Treatments for lifestyle-related conditions reduce the selective pressures that would normally drive genetic adaptation. When pharmaceuticals manage diseases like obesity and diabetes, the natural motivation to modify our behaviours and change our diet weakens. This delay in genetic evolution, combined with medical interventions, perpetuates many modern health issues.
Lactose tolerance is an example of slow genetic adaptation. Populations with a history of dairy farming, such as those in Europe, Africa and the Middle East, developed this trait around 7500 years ago. Other populations, lacking this history, remain largely lactose intolerant. Similarly, some populations have adapted to wheat and grains, like those in the Fertile Crescent, while others, such as Northern Europeans and non-Western groups, often exhibit gluten intolerance and inflammation.
In East Asia, people adapted to a rice-based diet, while in the Americas, maize became a staple. East Asians also adapted to metabolise soy and its phytoestrogens efficiently, potentially contributing to lower rates of breast cancer and heart disease. In contrast, populations newly introduced to soy may face digestive and hormonal issues due to genetic variations in enzyme activity and hormonal regulation.
These examples illustrate how genetic diversity shapes our dietary tolerances, and why evolutionary history matters when choosing what to eat. Understanding these variations helps explain why some diets work better for certain people, and highlights the value of food choices that are most consistent with our genetic heritage.
Author bio
Susan Birch is the Health Detective, a passionate health advocate with a Master’s in Health Sciences and over 40 years’ experience in nutrition and exercise science. Now based in the Eastern Bay of Plenty, she enjoys mountain biking, kayaking and weightlifting.
The Health Detective’s Handbook is available via the author’s website, or via Amazon.
