Can you really lose weight but eating ONLY tinned fish for three days?

A tin of sardines might not seem like an obvious weight loss tool - yet thousands of people online are turning to the oily fish as part of a short-term 'sardine fast', claiming it can reset their appetite and help them shed pounds with minimum effort.

But as the trend gains momentum, experts have warned that while sardines themselves are nutritious, the diet behind the hype may be less healthy than it sounds - and even dangerous.

Sardine fasting's origins are relatively modern, and can be traced back to 2023 when it was heralded as a way to beat weight-loss plateaus by controversial US doctor and keto diet influencer Annette Bosworth, also known as Dr Boz.

The keto diet is built upon a simple foundation; by not eating foods or drinks containing carbohydrates, your body has to use its existing fat reserves for energy, a bodily function known as ketosis.

Over the next two years, sardine fasting was adopted by low-carb and biohacking communities who backed up Dr Boz's theory.

They claimed that eating a sardines-only diet for three days kickstarted their metabolism, reduced their appetite and helped them lose weight quickly.

Some variations of the fast also allow you to add in MCT (a derivative of coconut oil) or olive oil, lemon juice and spices to make the otherwise bland diet more palatable - but carbs are strictly off the menu.

Last year, sardine fasting reached social media, where it has metamorphosed into a mainstream diet trend backed by influencers and adopted by people across the world - all of whom parrot Dr Boz's original claim that it helped them lose weight, fast.

Sardines, which are naturally high in protein and a rich source of omega-3 fatty acids - which are vital for heart health - are undeniably good for you, one expert told the Daily Mail.

'Omega 3 is a type of healthy unsaturated fat that's known for its anti-inflammatory properties,' said Lucy Jones, Registered Nutritionist for Yakult.

'It's thought that this property may help those experiencing inflammatory bowel disease, although more research is needed. Emerging research also suggests omega-3 may play a beneficial role in supporting the gut microbiota.'

And, if you opt for a tin of boned sardines, you'll be getting a boost of calcium from chowing down on their tiny, crunchy skeletons, too.

So far it all sounds great on paper - so what are the dangers?

Firstly, the risk of overdosing on oily fish, which can contain pollutants such as heavy metals. 

The NHS recommends that adults eat at least two portions of fish a week, one of which should be oily fish like sardines.

A typical portion is around 140g cooked, about a small tin. While most adults can safely eat up to four portions of oily fish a week, pregnant and breastfeeding women are advised to limit their intake to two.

'This is because oily fish are known to be higher in pollutants which could be harmful to health,' said Jones.

The other concern about sardine fasting is that eating just one type of food for a period of time - known as a monodiet - means that the body is missing out on the huge range of vitamins and minerals it would otherwise get from healthy, balanced meals.

Hazel Shore, a SCOPE-Certified Obesity Specialist Dietitian at who works at healthcare platform Phlo Clinic, warns that any claimed benefits are unlikely - but the risks associated with eating a restrictive diet are very real.

'Sardines are a fantastic food and they absolutely deserve a place in a healthy diet but humans weren't designed to eat one single food exclusively, even for a few days,' she said.

'From a nutritional perspective, it's not a balanced or beneficial approach.

'You're essentially removing whole food groups, particularly carbohydrates and plant foods which means you miss out on fibre, a range of vitamins, antioxidants and the diversity that supports gut health.'

Furthermore, people ditching their daily morning toast for a tin of fish might not suddenly feel like a new and improved version of themself - in fact, the opposite could happen.

Read More

Tofu unpressed: Why the fermented soybean block is the most misunderstood protein in the supermarket¿ and how to use it to make moreish dishes even meat eaters will enjoy

Shore explained that suddenly removing carbohydrates from your diet can cause psychological dips as your body adjusts.

'The body runs very comfortably on carbohydrates,' she said.

'When people drastically restrict them, they often feel low in energy, irritable, mentally foggy and generally unwell as the body scrambles to find alternative fuel sources.

'That "grumpy, tired and wired" feeling many people report on restrictive plans is a physiological response, not a lack of willpower.'

Instead of being single minded about sardines, Jones suggests harnessing their fishy goodness by making them a part of a healthy, filling, varied meal.

She said: 'Whilst sardines are high in protein and healthy fats, they have very little fibre.

'Pairing them with fibre-rich foods helps create a more balanced meal that supports gut health, energy levels and fullness.

'Think wholegrain carbohydrates, such as wholewheat pasta, brown rice or quinoa, alongside plenty of vegetables of pulses.

'For example, sardines work beautifully in a colourful salad with mixed leaves, tomatoes, cucumber, peppers, chickpeas and quinoa, finished with extra virgin olive oil and lemon juice.'

WHAT IS THE KETO DIET?

The Ketogenic diet defines a low-carb, high-fat way of eating. 

Following this eating plan forces the body into a metabolic state, known as ketosis, which starves the body of carbohydrates but not calories.

Carbs are shunned in the keto diet as they cause the body to produce glucose, which is used as energy over fat.

Keto diets therefore lead to weight loss as they make the body burn fat as its primary energy source.

On the diet, followers can eat:

• Meat
• Leafy greens and most vegetables 
• Full-fat dairy
• Nuts and seeds
• Avocadoes and berries
• Fats, such as coconut oil

People cannot eat:

• Grains, including rice and wheat
• Sugar, like honey and maple syrup
• Most fruit
• White or sweet potatoes