Eating canned sardines at breakfast, lunch and dinner may not sound like an appealing meal plan.
Yet followers of a viral diet challenge are doing exactly that, hoping to lose weight, burn fat and improve their health in just three days.
Called the 72-hour sardine fast, the regimen involves eating little besides canned sardines for three consecutive days.
Supporters say the approach may help the body enter ketosis—a state in which it uses fat rather than glucose for energy—while lowering blood sugar, reducing inflammation and helping overcome weight-loss plateaus.
The challenge was popularized several years ago by physician Dr Annette Bosworth, who is known online as Dr Boz. She says it may encourage the body to burn fat while reproducing some of the metabolic effects associated with fasting.
In updated guidance issued in May 2026, Bosworth advised followers to eat sardines in their oil whenever they felt hungry during the 72-hour period and to avoid carbohydrates. She also credits ketogenic-diet researcher Dr Dominic D’Agostino with helping inspire the trend.
Sardines are rich in protein, omega-3 fatty acids and several essential nutrients. But eating them as the sole food source can also carry risks, particularly for people who take certain medications.
Experts speaking with Our News Outlet explain what people should know before attempting the sardine fast.
Followers of the three-day sardine fast say it can produce rapid weight loss. Experts told Our News Outlet that the reality is more complicated
What is the three-day sardine fast?
Unlike a conventional fast, the sardine diet does not require people to stop eating. Participants can eat throughout the day, provided they stick to sardines and avoid other foods.
Dr Dominic D’Agostino, a researcher focused on ketogenic metabolism who is credited with helping inspire the trend, describes it as a short-term, calorie-restricted ketogenic diet with some metabolic similarities to fasting.
A ketogenic diet severely limits carbohydrates while emphasizing fat and protein.
D’Agostino’s version calls for two to three cans of sardines each day for three days, along with water and electrolytes. Depending on the brand and product, that provides roughly 380 to 575 calories and 45 to 68g of protein per day.
For most adults, that is an extremely low calorie intake. US health authorities generally use about 2,000 calories per day as a reference point across three meals, although individual needs vary according to health and goals.
Claim: You’ll lose weight quickly
The sardine fast may cause the number on the scale to fall quickly, but much of the early change is likely to reflect water loss rather than body fat.
Since sardines contain almost no carbohydrates, the body starts drawing on stored carbohydrates known as glycogen. Glycogen is stored with water, so that water is released as glycogen levels decline.
D’Agostino said the weight change can represent a mix of body fat, glycogen, water and less food remaining in the digestive tract. He has reported losing about six pounds during his own three-day sardine fasts, with body-composition scans indicating that some of the loss was fat.
However, he acknowledged that shifts in hydration and glycogen can make those measurements difficult to interpret. Some of the weight typically returns once people resume eating carbohydrates.
Sardines do not contain a unique fat-burning compound, D’Agostino told Our News Outlet. Their high protein content may reduce hunger, while the highly restrictive plan limits calories almost automatically.
Registered dietitian Bonnie Taub-Dix agreed that weight loss can occur, but questioned whether such a restrictive strategy provides sustainable benefits.
‘If you eat only sardines, you’re likely to lose weight, but not because sardines are magical,’ she said.
Claim: It puts your body into ketosis
Sardines may help a person reach ketosis, which can contribute to weight loss, but that does not necessarily produce lasting results
Removing almost all carbohydrates can prompt ketosis, a metabolic state in which the body burns more fat for fuel and produces ketones as an alternative energy source.
People moving from a typical higher-carbohydrate diet may reach nutritional ketosis within 24 to 48 hours, D’Agostino said, although the timing depends on previous eating habits, activity levels and individual metabolism.
However, being in ketosis does not automatically mean a person will lose more body fat. The body can burn fat from both food and stored fat, so elevated ketone levels alone do not guarantee greater weight loss.
‘Over time, body fat loss is governed by the energy deficit,’ D’Agostino explained.
In other words, long-term weight loss depends on consistently using more calories than are consumed, not simply on reaching ketosis.
Dr Dominic D’Agostino helped inspire the sardine fast and promotes it as a way to pursue weight loss and ketosis
Claim: It ‘resets’ your metabolism and lowers blood sugar
Advocates say the sardine fast can reset the metabolism and improve insulin resistance.
‘Resetting your metabolism’ is a marketing expression rather than a scientific term, D’Agostino said.
When carbohydrate intake drops sharply, insulin levels generally fall, liver glycogen stores decline and the body increasingly turns to fat for fuel. Blood sugar may decrease as well.
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D’Agostino cited research in which adults with obesity and insulin resistance followed a calorie-restricted, low-carbohydrate diet. After 48 hours, participants saw an average reduction of nearly 30 percent in liver fat, together with improved liver responses to insulin.
That research did not specifically investigate the sardine fast.
D’Agostino believes findings like these may help explain why brief periods of severe carbohydrate restriction could produce metabolic changes.
Still, a lower blood sugar reading after three days does not mean insulin resistance has been reversed. Lasting metabolic improvements, he stressed, depend on sustained changes in diet and body composition.
Claim: It reduces inflammation
Sardines contain significant amounts of sodium, which may be a concern for people with high blood pressure
Sardines provide EPA and DHA, two omega-3 fatty acids involved in inflammation regulation and cardiovascular health.
D’Agostino said the combination of omega-3 fats, ketosis and calorie restriction could affect inflammatory pathways within a matter of days.
However, it remains unclear whether those changes would result in measurable reductions in inflammation markers, such as C-reactive protein, after only 72 hours.
Taub-Dix urged a more measured view.
‘Protein can help promote fullness, and omega-3 fats may support a healthy inflammatory response,’ she said.
‘But we should not mistake the benefits of those nutrients for a promise of dramatic results in just 72 hours.’
Claim: Sardines provide all the nutrients your body needs
A typical 3.75oz can of sardines contains about 23g of protein, as well as omega-3 fatty acids, vitamin B12, vitamin D and calcium when the soft, edible bones are eaten.
Compared with many larger fish, sardines are also relatively low in mercury.
But they contain no fiber, almost no carbohydrates and very little vitamin C.
‘More is not always better, particularly when eating more of one food pushes so many other foods off the plate,’ Taub-Dix said.
Three days of restricted eating is unlikely to cause a major vitamin deficiency in an otherwise healthy adult. Even so, the absence of fiber may contribute to constipation and digestive discomfort.
Severely cutting calories and carbohydrates can also lead to headaches, tiredness, dizziness and irritability.
Truth: The sardine fast could have lasting health consequences
Eating four to six cans of sardines a day may deliver a considerable amount of sodium, depending on the brand, Taub-Dix warned. That could be problematic for people with high blood pressure or anyone advised to restrict sodium.
More serious concerns arise when extreme dietary restriction is combined with certain health conditions or medications.
D’Agostino said medication-related complications are among the most important risks. People who use insulin or sulfonylureas to lower blood sugar may develop hypoglycemia, or dangerously low blood glucose, when their carbohydrate intake suddenly falls.
Those taking SGLT2 inhibitors, another type of diabetes medication, face a different danger: euglycemic diabetic ketoacidosis. This potentially life-threatening condition occurs when dangerous levels of acid build up in the blood even though glucose readings may appear relatively normal.
D’Agostino stressed that euglycemic diabetic ketoacidosis is not the same as nutritional ketosis and requires emergency treatment. Persistent vomiting, abdominal pain, confusion, fainting or unusually deep and rapid breathing are warning signs that require immediate medical attention.
Sardines are also high in purines, compounds that can increase uric acid and trigger gout attacks. People with kidney disease may need to limit minerals such as phosphorus and sodium, meaning large amounts of sardines could create additional problems.
Pregnant and breastfeeding women, children, people who are underweight and those with serious medical conditions should also avoid attempting this kind of diet without medical supervision.
The takeaway: Should you do the sardine fast?
Dietitians recommend including sardines as part of a balanced diet rather than making them the only food
D’Agostino views the sardine fast as a possible way to prompt short-term metabolic changes, including ketosis and lower insulin levels.
But he acknowledges that no clinical trials have directly tested the challenge or shown whether any benefits continue after the three-day period.
Taub-Dix says people can gain the nutritional advantages of sardines without cutting every other food from their diet.
‘I’d rather see people add sardines to their diets than make them the sole source of nutrients and remove everything else from the plate,’ she said.
She recommends adding sardines to salads, whole-grain pasta or toast, or serving them with vegetables.
Johannah Katz, MA, RD, is a registered dietitian based in Florida.
