TV doctor Michael Mosley's death has been met with an outpouring of people describing how he revolutionised their approach to health.
Former deputy Labour leader Lord Tom Watson called him a "hero", crediting his book The Fast Diet with aiding his seven-stone weight loss.
Ted Verity, editor of Mail Newspapers - which published Mosley's regular column - said he believes Mosley's work will have "extended, and even saved, the lives of countless readers".
The scope of Mosley's work was huge and saw him ingest tapeworms, take magic mushrooms, and let leeches gorge on his blood.
One of his enduring legacies is the 5:2 diet and the popularisation of intermittent fasting.
But what are the diets, and what do they claim to do?
What is intermittent fasting?
Intermittent fasting, or time-restricted eating (TDE), was embedded in Mosley's approach.
It can take different forms, including fasting on certain days of the week or restricting your "eating window" each day.
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Some people take a 16:8 approach - fasting for 16 hours, eating meals within the space of eight hours - while others stretch the eating period to 10 hours and others adopt OMAD - one meal a day.
What is the 5:2 diet?
The 5:2 diet involves cutting calories to 500-600 on two days of the week, and eating normally the rest of the week.
Mosley was first introduced to it through a 2011 study and it became the backbone of his 2013 book The Fast Diet.
On the diet's website, Mosley said those following it can expect to lose around 1lb (0.5kg) a week, and enjoy health benefits from improvements to blood pressure and cholesterol levels to better insulin sensitivity.
On non-fasting days, people are encouraged to eat relatively healthily but "with little thought to calorie control and a slice of pie for pudding if that's what you want".
The Blood Sugar Diet
Mosley found personal success with the 5:2 diet when he reversed his type 2 diabetes.
He wrote on the diet programme's website that he was not "exceptional" in this success.
"Studies carried out by Professor Roy Taylor and his team at Newcastle University have shown that losing 10-15% of body weight can reverse type 2 diabetes in 84% of recently diagnosed diabetics, and 50% of those who have been diabetic for more than 10 years."
The Blood Sugar Diet is based on eating a low carbohydrate Mediterranean-style diet, aimed at weight loss and - unsurprisingly - improving blood sugar levels.
Mosley's book of the same name came out in 2016.
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The Fast 800
Both the Blood Sugar Diet and the original 5:2 were precursors to Mosley's updated diet plan: the Fast 800.
As the name suggests, it combines fasting with eating 800 calories.
There are two ways the 800 calories are consumed: in one phase of the plan, 800 is the total calories for the day, while in another, users adopt a 5:2 approach, but the calorie allowance on fasting days is bumped up to 800.
The Very Fast 800 and the Fast 800 Keto
There are two "rapid weight loss" plans: the Very Fast 800 and the Fast 800 Keto.
Both of them are recommended for people who are "significantly overweight or obese" and only for a maximum of 12 weeks or until a healthy BMI is reached, whichever is first.
The Very Fast 800 involves eating 800 calories a day, with a focus on low-carb Mediterranean-style food.
People following the diet can choose to split their calories between either two or three meals - if two, they may combine it with time-restricted eating and adopt a limited eating window.
Be the first to get Breaking News Install the Sky News app for freeThe Fast 800 Keto focuses on very low-carb, high-protein foods, with a slightly higher calorie intake - up to 1,000.
The purpose of minimising carbohydrates is to push the body into the metabolic state of ketosis, where the body burns fat after running through carbohydrates and the glucose stored in your muscles and liver.
During ketosis, fat is turned into ketone bodies in the liver to generate energy until more carbohydrates are consumed.
The New 5:2
The New 5:2 follows the same principles as the original 5:2 diet, but with an 800 calorie allowance on fasting days.
Mosley said 800 calories appeared to be the "magic number" when it came to weight loss.
The Fast 800 programme website says it is "high enough to be manageable... but low enough to trigger a range of desirable metabolic changes".
The Way of Life
The Way of Life is the final strand of the Fast 800, but doesn't involve calorie counting or fasting.
It is aimed at people wanting to maintain the benefits of the 5:2 diet, without continuing fasting.
It focuses on "sensible portions" of a low-carb, Mediterranean diet, full of fibre, protein, and healthy fats.
What's the evidence for intermittent fasting?
Research has identified several benefits of intermittent fasting besides weight loss, including reduced cholesterol, improved insulin sensitivity, reduced inflammation markers, and improved leptin sensitivity, according to nutritional neuroscientist Dr Amy Reichelt.
As Mosley found in his own experience, the approach has been shown to improve insulin sensitivity, which helps in better blood glucose control and can reduce the risk of developing type 2 diabetes.
However, Dr Reichelt notes that most studies focus on the 4:3 diet (alternate day fasting) rather than 5:2.
Research shows alternate-day fasting can reduce the levels of the appetite-regulating hormone leptin, which tells the brain the stomach is "full".
Some rodent studies also suggest intermittent fasting could lower the risk of neurodegenerative diseases because fasting triggers autophagy - a process where the body removes damaged cells and regenerates new ones - which may contribute to increased longevity.
How can intermittent fasting reverse diabetes?
One of the primary issues in the development of type 2 diabetes is insulin resistance "where the insulin that is produced is not as effective as it should be in bringing down glucose levels", Professor Frank Joseph, a consultant specialising in diabetes, tells Sky News.
During fasting, the body uses stored glucose - glycogen - in the liver, then uses fat as an energy source.
"This switch can help reduce insulin resistance, as the body becomes more efficient at using insulin to move glucose into cells," he explains.
Fasting lowers insulin levels, which helps the body to burn fat more effectively, and lower insulin levels can reduce insulin resistance.
Obesity is a significant risk factor for type 2 diabetes, so reducing body weight can also improve insulin sensitivity and decrease blood glucose levels, he adds.
What are the downsides to intermittent fasting?
People can experience "episodes of overwhelming hunger" in the first days of a large calorie deficit, Dr Reichelt says, speaking to Sky News.
Feeling fatigued, cognitively slower, and having a negative mood are also to be expected, she says.
Then there's the issue of gaining weight back if you stop the diet.
"During long periods of calorie restriction, your metabolism decreases, so shifting back to a 'normal' way of eating can trigger weight gain," she explains.
"Any kind of restrictive diet can cause a rebound weight gain effect, where people can rapidly gain more weight when they shift back to normal eating habits."
There is also a risk of nutritional depletion when you’re only eating 800 calories a day, Prof Joseph says.
People following the diet must be careful to consume enough vitamins and minerals. In trials, people used pre-made soups and shakes to ensure this, and this is the basis for the NHS type 2 diabetes path to remission programme.
Is intermittent fasting sustainable long-term?
How sustainable it is is very individual, Prof Joseph says: "Some people swear by it and will stick with it, some will cycle between other diets and yo-yo."
The biggest barrier to sustaining it is simply that "people feel hungry when they try this degree of restriction", he says. It's important the diet is "only a part of the whole 'behaviour' change", he says.
"Success depends on a holistic approach that looks at a combination of planning, balance, flexibility, support, and a positive mindset."
Read more:
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Why have some of the diets caused controversy?
Lose A Stone In 21 Days With Michael Mosley, which aired on Channel 4 in 2020, followed the principles of the Fast 800.
It was criticised by the eating disorder charity Beat for "promoting extreme weight loss and crash dieting".
"Research has shown that dietary restraint, including the restriction of calories, has been found to be a risk factor in the development of an eating disorder," the charity said.
"The programme caused enough stress and anxiety to our beneficiaries that we extended our Helpline hours to support anyone affected and received 51% more contact during that time," it added.
The Fast 800 website says anyone who is underweight or has a history of disordered eating should avoid the programme.
Dr Reichelt also stressed this, saying: "Anyone with a history of eating disorders - including anorexia nervosa, bulimia, and binge eating disorders, or compulsive eating behaviours - should exercise caution around any kind of diet shift as it can trigger a relapse of symptoms."
