The truth behind the viral 'sardine fast' that promises weight loss in just three days

Eating canned sardines for breakfast, lunch and dinner might not sound appealing. But that's exactly what followers of a viral diet trend are doing in hopes of losing weight, burning fat and improving their health in just three days.Known as the 72-hour Sardine Fast, the challenge is exactly what it sounds like – eating little else besides canned sardines for three days. Supporters claim it can help people enter ketosis – when the body burns fat for energy instead of glucose – improve blood sugar, reduce inflammation and break through stubborn weight-loss plateaus.The challenge was popularized several years ago by physician Dr Annette Bosworth, known online as Dr Boz, who claims it can encourage the body to burn fat while mimicking some effects of fasting. In updated instructions released in May 2026, she encouraged followers to eat sardines with their oil whenever hungry for 72 hours while avoiding carbohydrates. She also credits ketogenic-diet researcher Dr Dominic D'Agostino with inspiring the trend.But while sardines are packed with protein, omega-3 fatty acids and other important nutrients, eating them exclusively can have consequences, particularly for people taking certain medications.Experts speaking with the Daily Mail have revealed everything you need to know about the sardine fast. Practitioners of a three-day sardine fast claims it leads to quick weight loss. Experts told the Daily Mail the picture is more complicated than that What is the three-day sardine fast?Unlike a traditional fast, the sardine fast allows participants to eat throughout the day, as long as they stick to sardines and avoid other foods.Dr Dominic D'Agostino, a researcher specializing in ketogenic metabolism who's credited in part with inspiring the trend, describes the approach as a short, calorie-restricted ketogenic diet that shares some metabolic features with fasting. In the keto diet, participants eat very few carbohydrates while prioritizing healthy fats and protein. D'Agostino's version involves eating two to three cans of sardines daily for three days, along with water and electrolytes. That comes out to around 380 to 575 calories and 45 to 68g of protein daily, depending on the product.This is a severely restricted calorie intake for most adults. Health authorities in the US recommend about 2,000 calories a day spread across three meals, though this shifts depending on overall health goals. Claim: You'll lose weight quicklyThe sardine fast can produce a noticeable drop on the scale, but much of the initial weight change may come from water rather than body fat. Because sardines contain almost no carbohydrates, the body begins using stored carbohydrate, called glycogen. Glycogen is stored alongside water, which is released as those stores decline.D'Agostino said the resulting weight loss reflects a combination of body fat, glycogen, water and reduced food in the digestive tract. He reported losing around six pounds during his own three-day sardine fasts, with body-composition scans suggesting that some of the reduction was fat. However, he acknowledged that changes in hydration and glycogen can distort these measurements. Some weight also returns when normal carbohydrate intake resumes.Sardines have no special fat-burning ingredient, D'Agostino told the Daily Mail. Instead, their high protein content may help suppress hunger, while the restrictive diet naturally limits calorie intake.Registered dietitian Bonnie Taub-Dix agreed that weight loss is possible but questioned whether such a restrictive approach offers lasting benefits. 'If you eat only sardines, you'll probably lose weight, but not because they're magical,' she said. Claim: It puts your body into ketosis Sardines have been linked to ketosis, which may result in weight loss, but it may not lead to lasting changesEliminating nearly all carbohydrates can trigger ketosis, a metabolic state in which the body burns more fat for energy and produces ketones as an alternative fuel.D'Agostino said people starting from a typical higher-carbohydrate diet may enter nutritional ketosis within 24 to 48 hours, although timing varies according to prior eating habits, physical activity and individual metabolism.But ketosis doesn't necessarily mean more body fat loss. The body burns fat from both food and stored body fat, so higher ketone levels don't automatically mean more weight loss. 'Over time, body fat loss is governed by the energy deficit,' D'Agostino explained.This means losing weight over time depends on burning more calories than you consume rather than solely ketosis. Dr Dominic D'Agostino helped inspire the sardine fast and champions it for weight loss and ketosisClaim: It 'resets' your metabolism and lowers blood sugarSupporters claim the sardine fast can reset metabolism and improve insulin resistance.'Reset your metabolism' is a marketing phrase, not a scientific one, D'Agostino said. During severe carbohydrate restriction, insulin levels generally fall, liver glycogen declines and the body increasingly relies on fat for fuel. Blood sugar may also decrease.D'Agostino pointed to a study in which adults with obesity and insulin resistance following a calorie-restricted, low-carbohydrate diet experienced an average reduction in liver fat of nearly 30 percent after just 48 hours, along with improvements in how the liver responds to insulin. However, the study did not examine the sardine fast specifically. D'Agostino believes these rapid changes help explain why short periods of severe carbohydrate restriction may offer metabolic benefits. Still, a lower blood sugar reading after three days doesn't mean insulin resistance has been cured. He emphasized that lasting metabolic improvements depend on sustained changes in diet and body composition.Claim: It reduces inflammation Sardines are high in sodium, which may pose risks for people with high blood pressureSardines are rich in EPA and DHA, omega-3 fatty acids involved in regulating inflammation and supporting cardiovascular health.D'Agostino said the combination of omega-3 fatty acids, ketosis and calorie restriction could influence inflammatory pathways within days. However, whether these changes translate into measurable reductions in inflammation, including C-reactive protein, within just 72 hours is unclear.Taub-Dix offered a more cautious interpretation. 'Protein can help you feel satisfied, and omega-3 fats may help support a healthy inflammatory response,' she said. 'But let's not confuse the benefits of these nutrients with promises of dramatic results in just 72 hours.'Claim: Sardines provide all the nutrients your body needsA typical 3.75oz can of sardines has about 23g of protein, along with omega-3 fatty acids, vitamin B12, vitamin D and calcium when the soft, edible bones are included. Sardines are also low in mercury compared with many larger fish.However, they contain no fiber, virtually no carbohydrates and little vitamin C. 'More isn't always better, especially when eating more of one food means crowding out so many others,' Taub-Dix said.Although three days of restricted eating is unlikely to cause a significant vitamin deficiency in an otherwise healthy adult, the lack of fiber may lead to constipation or digestive discomfort. Severe carbohydrate and calorie restriction may also cause headaches, fatigue, dizziness or irritability.Truth: The sardine fast could have lasting health consequences Taub-Dix warned that eating four to six cans of sardines daily could provide substantial sodium, depending on the brand, posing risks for people with high blood pressure or those advised to limit intake.But the more serious concerns involve how extreme dietary restriction interacts with certain medical conditions and medications.D'Agostino identified medication-related complications as among the most important dangers. People taking insulin or sulfonylureas, medications used to lower blood sugar, may experience hypoglycemia, or dangerously low blood glucose, when carbohydrate intake suddenly drops.Those taking SGLT2 inhibitors, another class of diabetes medications, face a different risk: euglycemic diabetic ketoacidosis, a potentially life-threatening condition in which dangerous levels of acid accumulate in the blood even when glucose readings appear relatively normal.D'Agostino cautioned that this differs from nutritional ketosis and requires emergency medical care. Persistent vomiting, abdominal pain, confusion, fainting or unusually deep and rapid breathing warrant immediate medical attention.Sardines are also high in purines, which can raise uric acid levels and trigger gout attacks. People with kidney disease may need to limit certain minerals, including phosphorus and sodium, so large quantities of sardines can potentially be problematic.Pregnant or breastfeeding women, children, underweight individuals and people with serious medical conditions should also avoid this type of unsupervised diet.The takeaway: Should you do the sardine fast? Dietitians urge adding sardines to a regular diet rather than making them the sole focusD'Agostino sees the sardine fast as a potentially useful way to trigger short-term metabolic changes, including ketosis and lower insulin levels. However, he acknowledges that no clinical trials have directly tested the challenge or established whether its benefits last beyond three days.Taub-Dix argues that people can obtain sardines' nutritional benefits without eliminating other foods. 'I'd rather see people add sardines to their diets instead of making them the sole source of nutrients and making everything else disappear from their plates,' she said. She suggests incorporating sardines into salads, whole-grain pasta or toast, or serving them alongside vegetables.Johannah Katz, MA, RD, is a registered dietitian based in Florida.