Intermittent fasting (IF) has been the hot ‘new ‘diet over the past decade. That’s a long time for a fad diet to remain popular.
Ten years on, is there evidence to support the IF health claims?
What is Intermittent fasting?
IF was popularised by Dr Michael Mosley initially as The Fast Diet, then The 5:2 diet and more recently as The Fast 800 Diet and The Fast 800 Keto.
The 5:2 diet, as it became known, prescribes limited food intake for 5 consecutive days, followed by 2 days to eat whatever you like. It also promotes extended periods of fasting (non-eating time), on the restriction days.
“It’s human nature for people to want to reward themselves after doing very hard work, such as exercise or fasting for a long period of time, so there is a danger of indulging in unhealthy dietary habits on non-fasting days.”
Dr. Frank Hu, chair of the department of nutrition at the Harvard T.H. Chan School of Public Health
The Fast franchise quickly morphed from a longevity program into just another calorie/kilojoule restriction regime, aka a crash diet.
Read more about it in my original review of The Fast Diet, in 2014, examining the scientific claims and naturopathic caveats.
Taking IF to the next level: The Fast 800 diet
In 2018 Mosley published The Fast 800 Diet, rapid weight loss program as a prevention or cure for Type 2 diabetes.
The regime is based on consuming only 800 calories (3347 kilojoules) a day. This is significantly less than the recommended daily intake of 2000-2500 cal /8700 kj. Mosley advises people to follow this rapid weight reduction phase for 2 to 12 weeks, promising a 14 kilo weight loss in the first 8 weeks. Once that is attained, the program moves to the 5:2 diet for maintenance.
It’s important to note that less than 1,000 cal/day for women and 1,200 for men, is considered a starvation diet. Starvation causes our metabolism to slow down, in order to stay alive because we’re not eating enough.
You don’t need a nutrition degree to be alarmed about the starvation angle, let alone concerns for legitimising disordered eating. As a naturopath this raises additional questions, including the impact of potential nutrient loss as well as mental health.
Is Intermittent fasting evidence-based?
A systematic review of IF, analysing six studies, concluded “Intermittent energy restriction was shown to be more effective than no treatment, however, this should be interpreted cautiously due to the small number of studies.”
The biggest issue is compliance. While those who can stick to the diet in the long term can achieve the outcomes spruiked by the author, studies frequently report a 50% or less compliance rate. The 5:2 diet was found to be too restrictive for the majority, not to mention the even more extreme 800 version.
Does weight loss = good health?
People who stuck to the diet lost weight, however this was not necessarily associated with good health. A study of 50 overweight men and women measured the effects of IF-induced weight loss on inflammatory, metabolic, and molecular pathways of healthy aging.
After six months, participants in the intermittent fasting group had lost about 8 per cent of their body weight – up to 20 kilograms – and 16 per cent of their body fat. Despite these significant results, they did not reduce inflammation, glucose tolerance or insulin sensitivity.
Intermittent fasting may not be all it’s cracked up to be study finds
Inflammatory markers did not improve with IF. Inflammation has been linked with almost every major health condition, including chronic pain, cancer and Alzheimer’s.
In fact, most of the health claims previously made by Mosely and other supporters of IF weren’t replicated in human studies.
The problem with diets
I loathe the term ‘clinically obese’. Although used liberally in medical literature, it’s effectively just another form of fat shaming. Realistically, almost everyone who has been deemed overweight or obese and directed by a health professional to lose weight, has made numerous previous attempts to change this.
Weight loss is a complex issue. Occasionally weight gain is connected to a metabolic disorder, such as an underactive thyroid, but more often than not it’s related to a variety of psychosocial and lifestyle factors.
We eat and drink for a myriad of reasons other than hunger. Extreme calorie restriction is too difficult for most people to follow, can be emotionally triggering and may have serious negative side effects. The multibillion dollar diet industry thrives on this failure rate.
Body dissatisfaction and disordered eating are on the rise and pose a significant risk to health. A cultural preference for thinness, doesn’t necessarily equate to physical or emotional wellness
Healthy intermittent fasting?
Before embarking on IF, or any other diet, consider the following.
Know your ‘why.’
For those who want to try IF, my first question is why? Is it solely to lose weight, or for good health? Do you actually need to lose weight? Have you dieted before, and if so, was the weight loss maintained for at least a year? How did dieting make you feel emotionally?
If weight gain is causing physical pain in your joints or your blood sugar levels are rising, rethinking the types of food you eat and how you exercise, can definitely help these conditions. But focusing on caloric intake alone, isn’t the healthiest solution.
Remember, being underweight is more likely to endanger our health than being overweight. Both ends of this spectrum are associated with increased death rates.
A cornerstone of IF includes extending the time between the last meal of the day and breakfast. Anecdotally, emotional binging on food and drinking alcohol (another source of calories/kilojoules) are more likely to happen at night. This leads many IF adherents to delay eating until after midday. Some will only eat one meal a day.
There’s strong evidence to suggest that breakfast is the most important meal and the timing of it is also important, especially for weight loss. A recent study found that significantly more fat was burned by people who ate the same evening meal at 6pm, rather than 10pm. It should be noted this study was run in Spain, where dining late is the norm.
Research on metabolism and weight loss has also validated the old adage ‘eat breakfast like a king lunch like a prince; dinner like a pauper’. This study supports eating early in the day (7-00 – 7.15am to be precise!), with smaller amounts at lunch and the least at dinner, as the healthiest option for losing weight. Other researchers suggest that 4 – 6pm being the latest time to eat.
‘Consuming’ also includes what you drink. Never swap food calories/kilojoules/’points’ for alcohol.
A beer belly really is a thing. Due to the way alcohol is metabolised, these calories/kilojoules tend to be deposited as abdominal fat. This alone is associated with increased risk of Type 2 diabetes, high blood pressure and some cancers. It’s not just about beer or men. For women even moderate drinking has been associated with increased belly fat and also higher levels of androgens (male hormones).
Holistically a healthy diet needs to be one that can be easily maintained, is enjoyable and positively impacts physical and emotional wellbeing.
In my clinic, especially when working with men diagnosed with metabolic syndrome or type 2 diabetes, the best results have come from reducing alcohol, increasing whole plant-based foods and finding a form of exercise they enjoy. This works without a crash diet and it’s sustainable in the long term.
The traditional Mediterranean way of eating remains the most evidence-based, successful diet. Perhaps this is because it’s less a diet and more a roadmap of healthful way of life.
Update: June 2023
Don’t just take my word for it. Another analysis based on the latest research has come to a similar conclusion re IF.
Gill Stannard is a naturopath with over 30 years of clinical experience. She supports her clients to create a lifestyle that makes them feel well physically, emotionally and spiritually, with a Health at Every Size (HAES) philosophy.