Everything you should know about the 3-day sardine fast

What is a sardine fast?

The idea of eating nothing but sardines for health reasons goes back to 2012, when Dr Frederick Hatfield tried it after being diagnosed with cancer. He put himself on an extreme plan - eating only one to two cans of sardines per day - with the aim of pushing his body into a strict ketogenic state. The thinking was that by eliminating carbohydrates, he could reduce the sugar available for the cancer to feed on. In more recent years, this approach has gained viral attention, largely due to Dr Annette Bosworth’s promotion of the ‘three-day sardine diet’ as a way to jumpstart metabolism.

The sardine fast is based on cutting out carbohydrates to drastically limit the amount of sugar in your body. With fewer carbs coming in, your insulin levels drop. That shift signals your liver to start converting stored fat into ketones for energy - a metabolic state known as ketosis. Supporters say this shifts your metabolism from ‘sugar-burning’ to ‘fat-burning,’ making it easier for your body to access fat stores for fuel while sidestepping some of the hunger - and maybe even the energy crashes - that often come with traditional calorie restriction. Essentially, it mimics a fasting state, but unlike a strict water fast, the sardine fast at least has protein and healthy fats to keep you energised throughout the day.

Dr Meena Malhotra, a double board-certified physician in Integrative Medicine at Heal n Cure Medical Wellness Center, USA, explains that while the sardine fast involves a very limited diet, it is gentler than a water fast because it still provides protein, fat, and micronutrients.

Compared to a standard ketogenic diet, the sardine fast is far more restrictive in food choices but shorter in duration.

“The original idea was to aim for simplicity because sardines are protein-dense, high in omega-3 fats and low in carbohydrates,” she says. “So the goal was to reduce decision-making and calm insulin swings for a brief reset. The idea wasn't really a long-term plan.”

The effects of a three-day sardine fast can vary, meaning one person’s metabolic response may not mirror another’s.

Dr Wesley Buckle, a licensed naturopathic doctor at The Oasis Addiction Treatment Center in California, USA, outlines some of the potential changes you might notice in your body if you decide to try this dietary approach.

“A person will likely see early water weight loss,” he explains. “With carbohydrates near zero, glycogen stores drop, and the body releases the water stored alongside them. Ketones may suppress appetite for some people, and levels of your hunger hormone ghrelin may decrease after the first day or two.”

He notes, however, that short-term energy restriction can also lower leptin levels - a hormone that helps regulate your appetite and signals fullness to your brain - which may increase hunger in some people.

Malhotra adds that most people will see lower insulin levels and a shift towards fat burning fairly quickly.

Sardines are relatively high in purines and sodium, which is why general health guidance suggests you limit your oily fish intake to around two portions - roughly 280 grams - per week. This works out to around two to three cans of sardines weekly. A sardine fast, by contrast, typically involves eating two to four cans a day, which significantly exceeds these standard recommendations.

Mercury is a neurotoxin that can affect your nervous system, especially with repeated exposure. It’s naturally found in oceans and waterways, where microorganisms convert it into methylmercury - a form that builds up in fish over time.

Although a sardine fast is short-term, and sardines contain far less mercury than larger fish such as tuna, intake guidelines exist for a reason. If you’re healthy, the chances of significant heavy metal build-up or major nutrient imbalances during a 72-hour sardine fast are low. That said, ‘low’ doesn’t mean zero. Eating only sardines might give you more nutrients than a water fast, but that doesn’t automatically make it a better choice - just potentially less extreme.

Eliminating carbohydrates and fibre for three days can also cause uncomfortable side effects.

Buckle notes that a significant drop in carbs may lead to constipation, fatigue, headaches, irritability, and reduced exercise tolerance.

Malhotra adds that a lack of fibre can disrupt your gut bacteria and affect your digestion.

“Increased sodium can be problematic if you're sensitive to salt or have high blood pressure,” she warns. “Purines may raise uric acid, so people with gout or kidney issues should avoid this fast.

“An easier approach is simply adding sardines a few times a week to meals with vegetables, grains or legumes.”

As with any restrictive diet, it’s important to consult a health professional or carefully consider whether a sardine fast is suitable for you.

Malhotra explains that if you enjoy sardines, are metabolically healthy, and are looking for a short, structured reset, you may tolerate the fast well.

“However, people who are pregnant, have a history of eating disorders, gout, kidney disease, or require regular carbohydrate intake should avoid it,” she advises.

Buckle adds that people with uncontrolled hypertension are also not good candidates for this fast.

Both experts emphasise stopping the fast immediately if you experience any of the following:

• Dizziness.

• Fainting.

• Nausea.

• Heart palpitations

• Weakness.

• Persistent headaches.

• Feeling generally unwell.

“If signs of an allergy appear, stop eating sardines immediately,” Buckle says. “It’s also best to reintroduce other foods gradually to reduce gastrointestinal upset.”