The Sirtfood diet has been in the news again this week after singer Adele showed off her slimmed-down figure on US comedy show Saturday Night Live.
But what exactly is the Sirtfood diet, and does it work?
What's the premise?
Two nutritionists in the United Kingdom launched the Sirtfood diet in 2016.
The premise is that a group of proteins called sirtuins, which are involved in regulation of metabolism, inflammation and ageing, can be accelerated by eating specific foods rich in a class of phytonutrients called polyphenols.
Phytonutrients are chemical compounds plants produce to help them grow well or defend themselves. Research is continuing to shed light on their potential benefits for human health.
Common Sirtfoods include, apples, soybean, kale, blueberries, strawberries, dark chocolate (85 percent cocoa), red wine, matcha green tea, onions and olive oil. The Sirtfood diet gets some of its fame because red wine and chocolate are on the list.
The diet involves two phases over three weeks. During the first three days, total energy intake is restricted to 4,200 kilojoules per day (or 1,000 Calories).
To achieve this, you drink three sirtfood green juice drinks that include kale, celery, rocket, parsley, matcha green tea and lemon juice. You also eat one "Sirtfood" meal, such as a chicken and kale curry.
On days four to seven, you have 2-3 green juices and one or two meals up to a total energy intake of 6,300 kJ/day (1,500kcal).
During the next two weeks — phase two — total energy intake should be in the range of 6,300-7,500 kJ/day (1,500-1,800 kcal) with three meals, one green juice, and one or two Sirtfood snacks.
There's a diet book available for purchase which gives you the recipes.
After three weeks, the recommendation is to eat a "balanced diet" rich in Sirtfoods, along with regular green juices.
The idea of losing a lot of weight in just three weeks will appeal to many people.
The eating plan encourages a range of polyphenol-rich foods that are also good sources of vitamins, minerals and dietary fibre, and would be recommended in a range of diets designed to assist with weight management, or as part of a healthy, balanced eating plan.
A weight loss diet will be effective if it achieves sustained total daily energy restriction. So the biggest benefit of the Sirtfood diet is the daily energy restriction — you are likely to lose weight if you stick to it.
Also, the exclusion of energy-dense, ultra-processed "junk" foods will help lower the risk for chronic disease.
But there are drawbacks to consider too.
It would be wise to watch the portion size for some of the foods listed, such as red wine and chocolate.
Like most restrictive diets, phase one may be challenging and is not recommended for people with underlying health conditions without the supervision of a health professional.
The rapid weight loss in the first phase will reflect a loss of water and glycogen, the stored form of energy in muscles and the liver, rather than being all body fat.
The food list includes specific products that may be hard to locate in Australia, such as lovage, a European leafy green plant whose leaves can used used as a herb, roots as a vegetable and seeds as a spice. Some other items on the list can be expensive.
Most research has looked at the sirtuin-mediated effects of energy restriction in worms, mice or specific body tissues. No studies have tested the effect of diets that vary polyphenol content on the action of sirtuins in mediating weight loss.
A search on PubMed, the scientific database of research studies, didn't locate any human trials of the Sirtfood diet. So the short answer about whether the Sirtfood diet works or not is we don't know.
The authors' claims about effectiveness are based on anecdotal information from their own research and from personal testimonials, such as the one from Adele.
Considering the hype surrounding the Sirtfood diet against a checklist on spotting a fad diet sounds alarm bells. For example:
does it promote or ban specific foods?
does it promote a one-size-fits-all approach?
does it promise quick, dramatic results?
does it focus only on short-term results?
does it make claims based on personal testimonials?
Looking at the Sirtfood diet, the answers to most of these questions seem to be "yes", or at least a partial yes.
The best diet for weight loss is one that meets your nutrient requirements, promotes health and well-being, and that you can stick with long-term.
Clare Collins, Professor in Nutrition and Dietetics, University of Newcastle; Lee Ashton, Postdoctoral research fellow, University of Newcastle, and Rebecca Williams, Postdoctoral Researcher, University of Newcastle.