A breakfast, lunch and dinner of canned sardines may sound far from appetizing.
Yet followers of a viral diet challenge are doing exactly that, hoping to lose weight, burn fat and improve their health within just three days.
Called the 72-hour Sardine Fast, the regimen involves eating little beyond canned sardines for three consecutive days.
Advocates say the plan 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 overcoming weight-loss plateaus.
The trend was popularized several years ago by physician Dr Annette Bosworth, better known online as Dr Boz. She says the approach may prompt the body to burn fat while reproducing some of the effects of fasting.
In updated guidance published in May 2026, Bosworth advised followers to eat sardines packed in oil whenever they felt hungry during the 72-hour period and to avoid carbohydrates. She also credited ketogenic-metabolism researcher Dr Dominic D’Agostino as an influence.
Sardines do provide protein, omega-3 fatty acids and several important nutrients. However, relying on them exclusively can carry risks, particularly for people taking certain medications or managing existing health conditions.
Experts who spoke to the Daily Mail explain what the sardine fast may—and may not—do.
Followers of the three-day sardine fast say it can deliver rapid weight loss, but experts told the Daily Mail that the reality is more nuanced
What is the three-day sardine fast?
Unlike a conventional fast, this plan does not require participants to stop eating altogether. They can eat throughout the day, provided they consume sardines and avoid other foods.
Dr Dominic D’Agostino, whose research focuses on ketogenic metabolism and who helped inspire the trend, describes it as a brief, calorie-restricted ketogenic diet with some metabolic similarities to fasting.
The keto diet generally 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 serving size, that provides roughly 380 to 575 calories and 45 to 68 grams of protein per day.
For most adults, that is an extremely low calorie intake. Health authorities in the US use about 2,000 calories a day as a general reference across three meals, although individual needs vary based on health, age, activity and goals.
Claim: You’ll lose weight quickly
The number on the scale may fall noticeably during a sardine fast, but the early decline is likely to include a substantial amount of water rather than body fat.
Because sardines contain almost no carbohydrates, the body draws down its stores of glycogen, the form in which it stores carbohydrate. Glycogen is held with water, so that water is released as the stores shrink.
D’Agostino said the change in weight can reflect a mix of body fat, glycogen, water and less material 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.
Even so, he acknowledged that fluctuations in hydration and glycogen can make such measurements difficult to interpret. Some of the lost weight typically returns once a person resumes eating carbohydrates.
There is no unique fat-burning compound in sardines, D’Agostino told the Daily Mail. Their protein may help control hunger, while the highly limited menu makes it difficult to consume many calories.
Registered dietitian Bonnie Taub-Dix agreed that the plan could result in weight loss, but questioned whether such a restrictive strategy provides durable benefits.
‘If you eat only sardines, you will probably lose weight, but not because sardines are magical,’ she said.
Claim: It puts your body into ketosis
Sardines may help prompt ketosis, a state associated with fat use, but that does not guarantee lasting weight loss
Removing almost all carbohydrates can cause ketosis, a metabolic state in which the body turns increasingly to 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 eating habits, physical activity and individual metabolism.
Being in ketosis, however, does not automatically mean a person is losing more body fat. The body can burn fat from food as well as fat stored in the body, so elevated ketone levels are not a direct measure of weight loss.
‘Over time, body fat loss is governed by the energy deficit,’ D’Agostino explained.
In other words, sustained weight loss depends on consistently using more calories than are consumed—not on ketosis alone.
Dr Dominic D’Agostino helped inspire the sardine fast and supports its potential to promote ketosis and short-term weight loss
Claim: It ‘resets’ your metabolism and lowers blood sugar
Supporters also suggest that the sardine fast can “reset” metabolism and improve insulin resistance.
‘Reset your metabolism’ is a marketing phrase, not a scientific term, D’Agostino said.
When carbohydrate intake drops sharply, insulin levels generally decline, liver glycogen is depleted and the body relies more heavily on fat for energy. Blood glucose may fall as well.
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D’Agostino cited research involving adults with obesity and insulin resistance who followed a calorie-restricted, low-carbohydrate diet. After 48 hours, participants recorded an average reduction of nearly 30 percent in liver fat, alongside improvements in the liver’s response to insulin.
That research did not specifically test the sardine fast.
D’Agostino said these rapid metabolic shifts may help explain why brief periods of severe carbohydrate restriction could have some short-term effects.
But a lower blood sugar reading after three days does not mean insulin resistance has been reversed. Lasting metabolic improvements, he stressed, depend on ongoing dietary changes and shifts in body composition.
Claim: It reduces inflammation
Sardines can contain significant amounts of sodium, which may be a concern for people with high blood pressure
Sardines are a good source of EPA and DHA, omega-3 fatty acids that help regulate inflammation and support cardiovascular health.
D’Agostino said the combination of omega-3 fats, ketosis and calorie restriction could affect inflammatory pathways within a matter of days.
Still, it remains uncertain whether those changes produce a measurable reduction in inflammation markers, such as C-reactive protein, in only 72 hours.
Taub-Dix urged a more measured view.
‘Protein can help you feel full, and omega-3 fats may support a healthy inflammatory response,’ she said.
‘But we should not confuse the value of those nutrients with promises of dramatic results in just 72 hours.’
Claim: Sardines provide all the nutrients your body needs
A typical 3.75-ounce can of sardines contains about 23 grams of protein, plus omega-3 fatty acids, vitamin B12, vitamin D and calcium when the soft edible bones are eaten.
Compared with many larger fish, sardines also tend to contain relatively low levels of mercury.
But they provide no fiber, almost no carbohydrates and only small amounts of vitamin C.
‘More is not always better, particularly when eating more of one food means pushing so many other foods off the plate,’ Taub-Dix said.
Three days of restricted eating is unlikely to create a serious vitamin deficiency in an otherwise healthy adult. However, the absence of fiber can cause constipation or digestive discomfort.
A sharp reduction in both carbohydrates and calories may also trigger headaches, tiredness, dizziness or irritability.
Truth: The sardine fast could have lasting health consequences
Taub-Dix warned that eating four to six cans of sardines a day could deliver a considerable amount of sodium, depending on the brand. That may be particularly problematic for people with high blood pressure or anyone advised to restrict salt.
More serious concerns arise when an extreme diet is combined with certain illnesses or medications.
D’Agostino said drug interactions are among the most important risks. People taking insulin or sulfonylureas, which lower blood sugar, could develop hypoglycemia—dangerously low blood glucose—when their carbohydrate intake suddenly falls.
Those using SGLT2 inhibitors, another type of diabetes medication, face a separate danger: euglycemic diabetic ketoacidosis. This potentially life-threatening condition involves a dangerous buildup of acid in the blood even when glucose levels appear relatively normal.
D’Agostino emphasized that the condition is not the same as nutritional ketosis and requires emergency treatment. Ongoing vomiting, abdominal pain, confusion, fainting or unusually deep and rapid breathing should prompt immediate medical attention.
Sardines are also rich 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, making large quantities of sardines potentially unsuitable.
Pregnant and breastfeeding women, children, people who are underweight and anyone with a serious medical condition should avoid attempting this kind of diet without medical supervision.
The takeaway: Should you do the sardine fast?
Dietitians recommend adding sardines to a varied eating pattern instead of relying on them as a sole food
D’Agostino views the sardine fast as a possible way to produce short-term metabolic changes, including ketosis and lower insulin levels.
However, he acknowledges that no clinical trials have directly evaluated the challenge or shown that any benefits continue beyond the three-day period.
Taub-Dix said people can gain sardines’ nutritional benefits without cutting every other food from their diet.
‘I’d rather see people add sardines to their diets than make them the only source of nutrients and remove everything else from their plates,’ she said.
She recommends adding sardines to salads, whole-grain pasta or toast, or pairing them with vegetables.
Johannah Katz, MA, RD, is a registered dietitian based in Florida.