What this means in practice for you and me is a diet reduced to between 34 to 54 per cent of normal calorie intake with a specific composition of proteins, carbohydrates, fats and micronutrients which could unlock the key to anti-ageing and maybe extend your life. It breaks down like this: on day one, the overall diet adds up to 1,090 calories - which must be consumed as 10 per cent protein, 56 per cent fat and 34 per cent carbohydrate. Days two to five contain 725 calories each, split into portions of 9 per cent protein, 44 per cent fat and 47 per cent carbohydrate.
It sounds all so complicated. So why is this new diet so much better than the 5:2, I ask Prof Longo (catching him during a lunch break, which he says rarely consists of actual, you know, lunch). "I don't like to say better or worse," he protests. "But the composition is important; we worked hard to find the exact amounts, so that you don't feel like you are fasting, but your body gets the same effects. The time (five-day stretches) is important because the body needs to do this for three or four days before reprogramming itself, and the process of regeneration and rejuvenation begins."
He adds of the 5:2, first outlined by BBC medic Dr Michael Mosley who co-authored the bestselling The Fast Diet book with journalist Mimi Spencer: "The 5:2 - it is not bad, but you need to do it every week. With the Fasting Mimicking Diet, you do it every three or four months, so that's less than half the time."
He also thinks there are two specific problems with 5:2. "We have seen that the high protein intake associated with 5:2 and the short duration of the fast (ie, 24 hours) seems to block the regenerative effect you get from fasting." Plus, the brain gets confused when you ask it to change behaviour often; fasting and then not fasting, especially if you swap the days around a lot may affect our Circadian rhythms, says Prof Longo. So you could end up struggling to sleep at night on the 5:2. He is most certainly not a fan of extreme fasting: "Water-only fasting should only be done in a specialised clinic and can increase the incidence of gallstones in women at risk if done improperly."
Petronella Ravenshear, a nutritional therapist in London, agrees that Prof Longo's way may be slightly easier on the body than other types of fasting. She says: "Fasting, or restricting calories, to improve health and increase lifespan, is not a new idea. But this fasting mimicking diet is less of a stressor on the body than complete fasting. It supplies most of the carbohydrates in the form of vegetables which are packed with phytonutrients and minerals and positively good for us, rather than grain-derived carbohydrates which don't supply much except sugar. One of the hormones this diet reduces is insulin-like growth factor (IGF-1), high levels of which are correlated with increased risk of cancer."
She adds: "What would be even better would be to encourage people on the non-fasting days to eat two to three times a day rather than going back to their old ways of grazing and snacking. Decreasing meal frequency is pretty painless and by reducing meal frequency we naturally reduce inflammation (the hallmark of modern chronic disease) in our bodies."
However, even Prof Longo says he still wants to see his findings confirmed from larger human trials (which are under way). His aim is not to see people lose weight necessarily, but to help end the global medical culture which revolves around pill-popping.
"The system is prehistoric, it's expensive, and it's making us all broke," he says of the curative medical tradition. Ideally, he hopes those following the FMD a couple of times a year will reduce the number of biomarkers for cancer sufficiently to prevent the disease.
So when can we start? Currently, Prof Longo warns against improvisation and says to consult a doctor or dietician before trying the FMD. He is currently writing a definitive guide to the FMD. L-Nutra will produce a five-day nutrition pack, containing soups, drinks and bars, called ProLon. All profits from the book and ProLon will be donated to charity, he says. While new diets have created multi-million empires for some, it seems this is not a money-making project.
Nor is it an attempt to rewrite global nutrition. Prof Longo says: "We don't want to change what people eat normally; we are not advertising restriction. In general, I would say, if you follow the FMD twice a year, as I do, and combine it with general nutritional advice like that, it will make a tremendous difference to your longevity."
The Fake Fast Diet
Day One
FMD permitted calories: 1,090 max
Breakfast: Black or Green Tea; one boiled egg (78 calories) + one slice whole wheat toast (68 calories)
Lunch: black coffee or tea; small green salad with avocado, dressed with olive oil (300 calories)
Snack: two almonds (28 calories)
Dinner: Large helping of mixed green vegetable soup with borlotti beans, and slice of whole wheat bread (616 calories)
Day Two
FMD permitted calories: 725 calories max
Breakfast: Black or green tea; one poached egg with a grilled tomato (100 calories)
Lunch: Miso soup (21 cals)
Snack: 7 walnut halves (90 cals)
Dinner: Vegetable chilli with kidney beans and two tsp sour cream (514 cals)
Day Three:
FMD permitted calories: 725 calories max
Breakfast: Black or Green tea: one slice whole wheat toast with two tsp of cashew butter (150 calories)
Lunch: Espresso; Smoked Salmon (100g) with watercress (200 cals)
Snack: Blueberries (100g) (57 cals)
Dinner: Large portion vegetable soup (318 cals)
Day Four:
FMD permitted calories: 725 calories max
Breakfast: Black or Green tea; half an avocado on one slice of whole wheat toast (220 cals)
Lunch: Espresso; 250ml glass of almond milk (60 cals)
Snack: two squares 70 per cent dark chocolate (110 cals)
Dinner: Large green salad with 100g prawns, dressed with olive oil and lemon juice (335 cals)
Day Five
FMD permitted calories: 725 calories max
Breakfast: black or green tea; two boiled eggs (156 cals)
Lunch: Half an avocado on toast; miso soup (210 cals)
Snack: An apple (60 cals)
Dinner: Large portion of vegetable soup with 10g toasted pine nuts (299 cals)