The Sirtfood diet: what it promises and what the evidence says

What is the Sirtfood diet?

It was devised by two British nutritionists, Aidan Goggins and Glen Matten, and is organised into two phases. Its authors promise a loss of around 3.2 kg in the first week .

Phase 1, one week. From day 1 to day 3, you have three green juices a day and just one solid meal containing “sirt” foods, for up to around 1,000 kcal a day. The juices contain kale, rocket, parsley, celery, lovage, green apple, ginger and lemon, with half a teaspoon of matcha tea. From day 4 to day 7, the number of juices drops to two, a second meal is added and the limit rises to around 1,500 kcal.

Phase 2, two weeks. Calories are no longer counted. You have three meals a day containing “sirt” foods and one green juice.

The book lists twenty “sirt” foods. Ten are vegetables, herbs and fruit: kale, rocket, red onion, celery, lovage, parsley, capers, red chicory, strawberries and Medjool dates. The other ten are walnuts, buckwheat, soy, extra virgin olive oil, dark chocolate with 85% cocoa, turmeric, chilli, coffee, matcha tea and red wine. Almost all are plant-based and almost all fit into a healthy diet. What is debatable is what is attributed to the list.

What are sirtuins, and where does the promise come from?

Sirtuins are a family of enzymes, proteins that speed up reactions inside cells. They are involved in how cells manage energy, inflammation and cellular ageing. They are studied mainly in animals and in the laboratory; in those studies, fasting, calorie restriction and exercise activate them.

The book takes a leap. If certain polyphenols in food, plant compounds, also activated them, eating those foods would make you lose weight as if you were restricting calories, but without going hungry. That leap has not been demonstrated in people. A review of twelve randomised trials found that, overall, weight-loss treatments did not clearly change blood sirtuin 1 compared with control groups [1]. It increased only in people with a metabolic disease [1]. If losing weight does not move the marker, it is a long way from proving that a plate of rocket moves it and that this leads to weight loss.

In other words, the authors have taken a current research concept and made it the star of their method.

Why does the Sirtfood diet lead to weight loss?

Because you eat much less. Around 1,000 kcal a day is far below what most adults use. Any plan like that will make the scales go down in a week, whether it includes kale or rice. Some of what you lose in those first few days is water and the muscles’ sugar stores, and it comes back as soon as you eat normally again.

The menu’s composition matters less than people think. An analysis of 32 studies in which food intake was fully controlled found that replacing carbohydrates with fat, at the same calorie intake, barely changed energy expenditure or fat loss [2]. It also noted that the body activates mechanisms that resist weight loss [2]. What causes weight loss on the Sirtfood diet is the restriction, not the sirtuins.

What are the risks of the 1,000-calorie phase?

The first is the calorie figure itself. The UK health institute’s obesity guideline (NICE) reserves plans of 800 to 1,200 kcal for programmes with a specialist team and long-term follow-up [3]. It also does not recommend them as a long-term strategy for managing obesity [3]. Doing it on your own with a book is the opposite. At Alimmenta, we do not recommend plans below 1,200 kcal a day. Going below that figure requires medical supervision and can cause nutrient deficiencies.

• Hunger, fatigue and irritability. With so little energy, it is common to feel weak, irritable and unable to concentrate.
• Juices instead of food. Juicing removes the pulp, and with it the fibre. Juices are less filling than whole vegetables and fruit, and their sugars are absorbed more quickly.
• Too little protein. The method barely addresses it. Protein helps preserve muscle while you lose weight and feel less hungry by your next meal.
• Wine on the list. Alcohol is not recommended as part of a healthy diet, whether or not it contains polyphenols.
• A list that can be misleading. Promoting twenty foods sidelines fish, eggs, dairy products and many vegetables that also contribute to a healthy diet.

The initial phase is not for you if you are pregnant or in the post-partum period, whether or not you are breastfeeding, or if you are a minor. It is also not for you if you have diabetes and use insulin or medicines that can lower your blood sugar too much, or if you have had an eating disorder . In those situations, discuss any change of this scale with your doctor first; if you have had an eating disorder, also speak to a psychologist.

What mistakes do we see in consultations?

The mistake we see most often in consultations is regaining the weight. An analysis comparing 14 popular named diets found modest weight loss at six months, much of which had disappeared by twelve months [4]. The problem is not so much the speed of the first week as the fact that the method offers no plan for the fourth week. This is the pattern we describe in our guide to fad diets: how to recognise them and understand their risks.

The second mistake is thinking you were missing “sirt” foods. If you have been doing “everything right” for some time and your weight is not changing, the problem is usually somewhere else. In why am I not losing weight? Common mistakes and how to address them, we go through the ones we see most often.

What can be salvaged from the Sirtfood diet?

Quite a lot, if you leave aside the 1,000-kcal limit. Leafy vegetables, onion, berries, walnuts, soy, extra virgin olive oil, pure cocoa, coffee and tea are very similar to the Mediterranean dietary pattern. The PREDIMED study is a large randomised trial conducted in Spain. It found that a Mediterranean diet with extra virgin olive oil or nuts reduced major cardiovascular events compared with a low-fat control diet [5]. It is a way of eating that is suitable for the long term.

On your plate, that could mean, for example, a rocket salad with chickpeas, walnuts and olive oil; buckwheat stir-fried with vegetables and egg; or strawberries with plain yoghurt for dessert. Add what the list leaves out, such as fish, eggs, dairy products and other pulses. That gives you a better foundation than the original.

How to lose weight without another week of juice

Take more time and make less drastic cuts. Spain’s GIRO guideline treats obesity as a chronic disease, requiring care from several professionals and long-term follow-up [6].

The long-term approach is set out in our weight-loss diet guide. It is based on a moderate calorie deficit, protein at every meal, strength exercise and habits you can sustain.