This viral diet sounds like a health disaster (but it actually isn’t)
Key Takeaways
The sardine fast involves drinking water and eating only sardines for 3 days.
Advocates for the sardine fast say it can “jumpstart” the metabolism, reducing inflammation and hunger.
While it may sound strange (and, to some, downright unpleasant), it is not considered unsafe health-wise to eat only sardines for a few days.
The sardine fast has gone viral online as a nutrition “hack” with basically just one rule: Eat only sardines for 3 days straight. It began gaining traction in 2023 as part of the ketogenic diet and intermittent fasting boom.[]
While it promises to eliminate unhealthy food cravings, reduce inflammation with omega-3s, stabilize blood sugar, and control hunger hormones, do these health claims hold any water?
Try the diet
Followers of the sardine fast are instructed to choose canned or jarred sardines (flavored with carb-free seasoning as needed) for a convenient form of carb-free sustenance. Fasters are allowed to drink black coffee, tea, and plain water.
Annette Bosworth, known as Dr. Boz, promotes this short-term dietary strategy on YouTube.[]
Dr. Boz explains that prepacked sardines are “really hard to overeat” because they’re filling and promote a ketone surge that suppresses appetite. She swears by the sardine fast to give her patients a “metabolic workout” and overcome mindless eating.
Dr. Boz has tried the fast herself, even going so far as doing what she calls a “sardine cleanse,” which extends the protocol for 1 full week.
A recipe for health?
There’s little argument about the nutritional benefits of sardines. Aside from being rich in omega-3s, they’re full of other anti-inflammatory, heart-healthy micronutrients, including the following:[]
Arginine
Calcium
Iron
Potassium
Magnesium
Taurine
Zinc
Sardines are relatively inexpensive and easy to find in the store. They come in different varieties, skin on or off, and may be prepared with various flavorings.
If you’re concerned about the mercury in fish, sardines should be the least of your worries. These smaller fish are lower on the food chain and contain much less mercury than larger fish like tuna. They’re also rich in selenium, which is believed to help mitigate mercury’s toxic effects.[]
As a result, studies suggest that higher sardine intakes (as much as 100 grams, 5 days per week) may be safe and beneficial vs the typically suggested intakes of one to two servings of seafood each week.[] However, it’s important to note that, per authors of one study, such high consumption may be impractical and unsustainable over the long term.[]
Move over, keto
Sardines have clearly earned their place as part of a healthy diet, but what about a strict sardine fast? By omitting carbohydrates through the consumption of protein- and fat-rich sardines, sardine fasters are essentially embarking on a short-term ketogenic diet (but slightly higher in protein).[]
A small study (n=14) on healthy adults found that a short-term ketogenic diet led to lower blood glucose and insulin concentrations during exercise and at rest compared to a mixed diet.[] Other changes in the short-term ketogenic diet group included weight loss and lower efficiency during high-intensity exercise. Researchers noted that the weight loss appeared to be muscle (not fat) in body composition studies. However, water loss caused by muscle glycogen depletion in a short-term ketogenic diet could be responsible for this perceived loss of muscle, and previous studies have indicated that ketogenic diets significantly reduce fat mass over muscle in the longer term.[]
While it’s easy for medical providers to OK a 3-day sardine fast to healthy, fit patients, individuals with health conditions, especially diabetes, may warrant a little more caution. Nonetheless, an older study determined that a very low-calorie ketogenic diet (with somewhat higher protein, similar to the conditions of a sardine fast) can be safely implemented in patients with type 2 diabetes.[]
Benefits noted in the study included more exceptional weight loss, better glycemic control, and good tolerability compared to a standard, low-calorie-diet recommendation for diabetes. While additional monitoring and medication adjustments may be appropriate for diabetes patients, a sardine fast isn’t necessarily a dangerous or harmful proposition.
What this means for you
It’s easy to dismiss short-term diets as fads, but there’s some potential value to consider in the sardine fast. Sardines are a safe and nutritious food, and eating them for 3 days may be unpleasant but most likely not unhealthy. Despite obvious exceptions for patients with eating disorders, keeping an open mind with otherwise healthy individuals who want to explore this “dietary challenge” could introduce a beneficial food to their roster and tune them into their body’s hunger and satiety cues.