Francoise Wilhelmi de Toledo combines a passion for her subject with a precision when discussing it that one would expect of a doctor and scientist with a raft of publications to her name.
"Real medicine is lifestyle, it is how we live," she says. "Drugs, any drugs, must be complementary to that."
As medical director of the renowned Buchinger Wilhelmi Clinic in Germany, she is an authority on therapeutic fasting and responsible at least in part for the current interest in its role in the management of chronic diseases including obesity, diabetes, high blood pressure, high cholesterol and cancer. And, of course, as a means of weight control made popular by the diet du jour, the 5:2.
It was at an obesity conference in Millancay in France in 2008 that Dr Wilhelmi de Toledo met the French documentary maker Sylvia Gilman. In the course of their discussions she ignited Gilman's interest in the evidence from centres in Russia, Germany and the United States to support fasting as a medical intervention.
An acclaimed documentary, The Science of Fasting, first shown on the Franco-German TV channel Arte in 2011, was the result. It explored the history of fasting over more than 50 years; from the work of Dr Otto Buchinger, the German pioneer of medical fasting via the Goriachinsk sanatorium in Russia, where fasting is a central element of public health, to laboratories at the University of Southern California (USC) where Professor Valter Longo is decoding the genetic mechanisms of fasting which, it is now known, can trigger changes in gene expression.
In 2012, for BBC Two's Horizon, Michael Mosley, a doctor and journalist took this on further, with a personal investigation into fasting as a way to "live longer, look younger and lose weight". It generated huge media interest and the multimillion-selling Fast Diet or 5:2 - intermittent fasting on two days out of seven - was the result.
How does Dr Wilhelmi de Toledo view the 5:2? "Well, I regard it as a nudge in the right direction," she says smiling.
Fasting, as part of a lifestyle is, she believes, undoubtedly a good thing but her focus is on making it part of the armamentarium available to doctors coping with an epidemic of lifestyle diseases in the West that threaten to cripple healthcare systems. She says there is strong evidence gathered over many decades to show how it can lower blood pressure, reduce excess fat and glucose in the blood, modulate the immune system, increase the effect of the mood and sleep-regulating neuro-transmitter serotonin, boost protein repair, and reduce inflammation.
Fasting has been likened to a "reset" button that returns the human body to its - healthy - factory settings. A study published last year by Prof Longo's team at USC, drawing on animal and human trials, concluded that three days of fasting can rejuvenate the immune system, triggering the production of new white blood cells. Other studies show that fasting can enable healthy cells to endure better the toxic impact of chemotherapy while cancer cells die more rapidly. It is a fascinating area of research that draws on the body's evolutionary adaptation.
"Human beings are not programmed for abundance," Dr Wilhelmi de Toledo says.
"Humans are programmed for loss." The capacity to fast derives from periods when our ancestors ate more than they needed and built up fat reserves and surplus nutrients, such as vitamins and minerals, in summer and autumn. In winter and spring, when access to food was much reduced, they endured periods of fasting in which their metabolism switched automatically from "external nutrition to nutrition taken from fat reserves".
In the absence of carbohydrates as a source of energy (glucose) for the cells, fatty acids, from fat supplies, were broken down in the liver to produce molecules known as ketone bodies which were used for fuel instead.
Of course we retain this ability to fast and exist on a ketogenic diet but rarely utilise it in the affluent West because food shortages are largely unknown. Nor is there much incentive for investment to research fasting and its documented benefits, despite preliminary evidence that it may help in Parkinson's, multiple sclerosis and Alzheimer's. In Russia, there is a vast, largely unexplored archive built up by a psychiatrist Dr Yuri Nikolayev, who used fasting or "the hunger cure" to treat a range of mental disorders.
This lack of interest frustrates Dr Wilhelmi de Toledo.
"Take type 2 diabetes," she says. "This is a disease we know that we can cure [through fasting]. But there is an industry that sells all these drugs and devices. We have a type of medicine [in fasting] that is highly successful but there is no return on investment."
It was as a 17-year-old in Geneva that Dr Wilhelmi de Toledo embarked on her first fast with the aid of a book, because she "was at odds with my weight and wanted to match the ideal of the slim beauty". She says it was a revelation, that she felt "buoyant, sometimes euphoric" while fasting.
Her second fast was under medical supervision at the Buchinger Wilhelmi Clinic in Uberlingen and throughout her medical studies she returned there to fast. It was where she met her husband, Raimund Wilhelmi, the grandson of the famous Dr Otto Buchinger. A doctor with Germany's Imperial Navy, Buchinger contracted rheumatic fever aged 39 in 1917 and was discharged as permanently disabled. A 19-day water fast restored the use of his joints. From that point on, to his death at the age of 89, he dedicated himself to research into fasting, laying the foundation of the clinic that opened in 1953, which treats more than 3000 people a year from all over the world.
Some are seeking help for intractable health problems while for others weight loss is the primary goal. Many, however, are seeking respite from stress of work in the "spiritual dimension of fasting" that Dr Wilhelmi de Toledo claims is one of its most beneficial side effects. Programmes, supervised by the clinic's team of seven doctors, range from the minimum 10-day fast to a 28-day fast: this includes a digestive rest day to prepare the gut for deprivation and a third of this period is dedicated to the "refeeding" phase. Educating fasters for a return to their normal routine - and an improved diet and lifestyle - is a crucial element.
Dr Wilhelmi de Toledo still fasts twice a year, during a 12-day annual retreat, and to counteract a severe seasonal allergy to birch pollen. For the future, a renewed emphasis on data collection is the priority and the clinic has taken on a new research assistant to oversee this. Suspicion and cynicism about fasting is still rife among doctors and nutritionists and she is determined to challenge it. "We want to document and show that fasting is therapeutically efficient, safe and enjoyable," she says.
The science, it would seem, is increasingly on her side.