There is plenty of evidence to suggest that a nutrient-rich, primarily plant-based diet promotes healing of the body. The most compelling evidence, however, is in the overall vitality of vegans and vegetarians.
Their skin tends to be more supple, their eyes brighter and hair glossier. They don't look their age. You might even say they look ageless.
Country singer Emmylou Harris (67) is an advertisement for the vegetarian lifestyle, while former supermodel Christie Brinkley (61), who describes herself as a "flexible vegan" (she eats cheese), has the glow of a woman half her age.
Then there's Susan Sarandon (68) who is now a 'flexitarian'. She recently added a little red meat to her otherwise predominately plant-based diet but doesn't look any less extraordinary.
Closer to home, raw food advocate and author Bernadette Bohan looks at least a decade younger than her 60 years. She recently released a new cookbook, Raw: Recipes for Radiant Living, which features immune-boosting, nurturing recipes made from minimally cooked and unprocessed ingredients.
Raw has broader appeal than her previous titles, partly because this time there's more mouth-watering photographs than theory; partly because raw food is the prevailing health trend of the moment. This is a cookbook that raw food purists will enjoy as much as those taking their first steps towards adding more plant-based foods to their diet.
No doubt the cover of the book, featuring a radiant-looking Bernadette in a slim-fitting white tube dress, will appeal to another market altogether: those that want to live longer and look better.
Bernadette is modest when I mention how well she looks for her age. "Well, it's a good job you don't see me first thing in the morning," she laughs.
She tries to change the subject but I'm determined to get the specifics. Does she have good genes, or a very good beautician? No and no, she rails.
She attributes it all to her diet, which she radically changed 15 years ago when she was diagnosed with cancer for the second time, more of which anon.
"Look, there is no magic cure," she continues. "You are going to age. But you can slow down the process by eating good food that nourishes the cells and strengthens the immune system."
As Bernadette overhauled her diet, she began to observe significant improvements to her overall wellbeing, including better eyesight and an easing of the symptoms of arthritis. She says she has more energy now than she had in her 30s.
"I've noticed that the figure doesn't change with this type of diet," she adds. "I had middle-age spread at 46. Of course, that's the least of your worries when you're diagnosed with cancer, but the spread disappeared when I changed my diet, and it hasn't come back.
"I spent my whole life trying to zip up my jeans. Now I share jeans with my 20-year-old daughter." She also gets more wear out of her own clothes: she has fitted the same dress for the past 15 years. "I wore it to my 50th and 60th birthday parties and hope to wear it to my 70th."
Bernadette is as fastidious about what she puts on her body as what she puts in her body. The water she bathes in is filtered and she uses an Australian skincare range called Sukin Organics. "It's inexpensive, it smells gorgeous and it doesn't include irritants such as sulphates and parabens."
I wonder if the dietary changes helped her manage the menopause. "I absolutely sailed through it," she answers. "A very good friend of mine was going through a particularly bad menopause when I was undergoing chemotherapy and radiation [for breast cancer]. I told her that the treatment would likely put me through an early menopause.
"Well, as if that wasn't bad enough, she told me that the menopause would be worse than the treatment!"
Bernadette certainly experienced some early symptoms: "I had massive hot flushes - I used to think there was steam coming out of the top of my head!" But as she changed her diet she could feel the feel the symptoms easing off.
"Brassicas such as cabbage are really helpful and stodgy foods or foods high in sugar should be avoided.
"I have another friend who follows a diet similar to mine. During a consultation with a doctor she was told she was post-menopausal. She didn't even know she was going through it."
Bernadette has completely overhauled her lifestyle, yet she concedes that her lifestyle wasn't particularly unhealthy to begin with: "I really thought I was living a very healthy life," she explains. "I didn't eat many sugary things. I ate fruit and veg every day. Yes, I had a glass of red wine in the evening, but that was about the height of it."
These days she has a vegetable juice every morning. She eats sprouted beans and seeds. She uses a dehydrator (which removes the moisture from fresh food while leaving the nutrients intact) to make vegetable crisps and dried fruit snacks.
She also takes supplements: "Chlorella is great if you have sugar cravings." She has gone from getting her 'five a day' to making fruit and vegetables the mainstay of her three meals a day.
If you think this all sounds very virtuous but not particularly enticing, rest assured, the proof is in the pudding, or rather the lemon meringue pie featured in her latest cookbook. It satiates the sweet tooth with raisins, desiccated coconut and the natural sweetener, stevia.
Elsewhere there's a "divine and decadent" tiramisu, which is made sweet using dates and shredded coconut, as well as an ice cream recipe made from cream nut milk. "This is super if you are lactose-intolerant or trying to avoid dairy milk," adds Bernadette.
Mains include carb-free spaghetti with sweet pepper sauce using courgette instead of pasta and a roasted garlic cottage pie, while snacks include onion crisps and smoked chipotle hummus.
"I really think this book is going to let people see that raw food doesn't mean living on a leaf of lettuce or a stick of carrot," continues Bernadette. "I am a mammy, so if I can do it, anyone can do it."
Of course, a stick of carrot here and there helps too. The first step towards a raw lifestyle is simply adding more raw vegetables to your diet.
"The cooking process," explains Bernadette, "destroys 50pc of the minerals, 75pc of the vitamins and 100pc of the enzymes, hormones, oxygen and phytonutrients in food… All cellular life depends on enzymes, so much so that without them, there would be no life."
Bernadette thinks of it in terms of 'live foods' and 'dead foods'. "Live foods are foods that have not been denatured by heat, and as such their nutrients and enzymes remain intact… if you have digestive issues, enzymes are really important to include in your diet."
There is also evidence to suggest that enzyme-rich food can help in the fight against cancer, and Bernadette has survived cancer twice. In 1988 she was diagnosed with lymphoma for which she was treated with steroids. Fifteen years ago she was diagnosed with breast cancer. She underwent chemotherapy, radiotherapy and surgery but also began to research other ways that she could help her body recover and heal.
The turning point came when she began to follow the simple advice of Hippocrates, the Father of Medicine, who said "Let food be thy medicine and medicine be thy food" along with the modern research conducted by the likes of Dr Colin Campbell, who discovered that predominately plant-based diets can lower the risk of chronic diseases such as heart disease, diabetes and cancer. She soon felt stronger and looked better. Crucially, the cancer hasn't returned.
Bernadette went on to share her story in books such as The Choice, The Survivor's Mindset and Eat Yourself Well, but she is quick to add that this is not about changing your lifestyle to promote healing.
"It's not a miracle or magic cure," she says. "This is about giving the immune system the best chance to fight disease." She makes a strong case, but is it really possible to follow this diet without feeling deprived of protein?
Yes it is, says Bernadette. She contends that vegetarians can have the "protein levels of the world's top athletes and sportspeople by eating plant-based foods".
The trick is making sure portions are both adequate and consistent. "Protein is a bit like water in that you can only absorb and use a certain amount at any one time," explains Orla Walsh, a dietician/nutritionist with the Marie Keating Foundation.
"Therefore it is important to eat enough protein at each meal. You can meet your protein requirements with a vegetarian diet. However education is key... you need to know where to find it and how much you need to eat for an adequate intake."
Orla also stresses that while a plant-based diet is the healthiest approach, this doesn't have to mean a plant-only diet.
Consultant nutritionist Elsa Jones agrees. She is also an advocate of adding more plant-based protein to the diet. "I always encourage my clients to include more plant-based sources of protein in their diets as over-consuming animal protein is not good for health.
"Beans, lentils, nuts and seeds are not only good sources of plant protein, they're also high in fibre and loaded with vitamins and minerals that help us look and feel our best.
"This could explain why a diet high in plant-based foods is strongly associated with increased longevity."
It is not without its challenges, though: "In the past, there was a tendency for vegetarians to overly rely on carbohydrates or synthetic forms of vegetable protein at mealtimes," continues Elsa, "but I think nowadays most vegetarians are well educated in terms of which foods they need to be eating to feel satisfied and well."
And sometimes that means eating a little meat. 'Flexitarianism' - vegetarians that occasionally add meat to their diet when they are feeling low in energy - is on the rise, and many will agree that allowing a little leeway makes it much easier to adapt to a plant-based diet and not fall off the wagon completely.
Bill Clinton, once a champion of the previously mentioned Dr Colin Campbell's China Study (the most comprehensive nutrition study to date), has since moved to the Paleo plan.
Former vegan Anne Hathaway realised she needed to eat meat when she was filming a water scene for the film Interstellar. "[I was] running through water and then you wear a 40-pound suit on top of it, so for me it was intense. I was facing… I don't know how many days in a row of, like, garbanzo beans on a plate".
Orla adds: "The recommended nutrient intake for certain vitamins such as vitamin B12 and Vitamin D are difficult to meet on a vegan diet. Therefore many vegans need to supplement their diet to achieve nutritional balance and avoid deficiencies."
Bernadette's husband and son still like to eat meat occasionally. Her youngest, Julie (20), was the most responsive to the new eating plan as she was five when her mother became interested in raw food. Meanwhile, her daughter Sarah (30) joined her mum and sister by embarking on a plant-based diet three years ago.
Perhaps the strongest message in Bernadette's new book is that this doesn't have to be an all-or-nothing lifestyle, although the author reckons that we'll all be closer to all rather than nothing in the years to come.
"This is just like the tobacco industry," she says, before paraphrasing the famous Schopenhauer quote, "all truth is ridiculed in the beginning".
"I was telling people how bad sugar was for the body 15 years ago. Yes, I would have loved for people to listen but I'm just glad that people are now getting educated and wising up. Eventually we will reach a critical mass."
As for those doubters and naysayers, surely the vitality of Bernadette, who turned 60 in August, will give them some food for thought.
'Raw: Recipes for Radiant Living' is published by Gill & MacMillan RRP €22.99.