The viral sardine fast promises quick weight loss, but experts say it’s an extreme and unsustainable approach.
Social media is filled with new fad diets that promise quick and easy weight loss, and many people online are willing to try just about anything to lose a few pounds — including eating solely sardines for three whole days.
The viral sardine challenge involves consuming only sardines, black coffee, tea, and water for 72 hours. The ultimate goal is to lose weight, improve blood sugar levels, and boost metabolism.
The trend was popularized by Annette Bosworth, known online as Dr. Boz. The physician is a major proponent of the keto diet, which involves eating a very small amount of carbohydrates and a lot of fat, putting the body into a ketosis state in which it burns fat for energy instead of carbs.
@clinic.dr.boz #CapCut ♬ original sound - Dr.Boz
Dr. Boz claims that sardines can produce life-changing results for patients, and she has convinced many people online to try the trend. However, experts tell Healthnews it’s probably not the best approach to losing weight and improving your health.
“Sardines are a rich source of protein and healthy fats with virtually no carbs, so yes, consuming them exclusively for three days could help shift the body into ketosis,” Phyllis Pobee, M.D., a physician specializing in weight loss and personalized medicine, tells Healthnews. “However, the real question is whether this extreme, short-term ketosis leads to sustainable weight loss.”
The sardine fast is trendy on Reddit, TikTok
Just like any fad diet or lose-weight-quick scheme, the sardine fast has picked up steam on social media platforms. On Reddit and TikTok, users discuss the trend, ask questions about its potential risks, and share their experiences with it.
Some have shared videos of their disgust while forcing themselves to consume the tinned fish, claiming that it has enabled them to better understand their hunger cues since they don’t actually enjoy the taste.
@serena.carnivore Yes, part of why a sardine fast works is because you eat less. I mean, it’s hard to overeat sardines. But, more than that, they are so satisfying and keep you full for so long! They are packed with nutrients, protein, and healthy fats! #carnivore #motivation #carnivorediet #heartdisease #instagram #meatheals #yestomeat #nutrition #diet #autoimmune #weightloss #fitness #weightlifting #cholesterol #bodybuilding #cancer #depression #zerocarb #keto #diabetes #liondiet #ketodiet #LCHF #lowcarb #bingeeating #recovery #eatingdisorders #carnivorerevolution #sardines ♬ original sound - Serena.Carnivore
Others have taken the trend to the next level, extending the fast from the recommended 72 hours to 10 whole days.
@itscourtneyluna Sardine fast day 1/10 #sardinefast #sardines #wieiad #whatieatinaday ♬ original sound - Courtney Luna - Carnivore
Does the sardine fast work?
Before examining whether the sardine fast itself has any benefits, it is important to acknowledge that sardines are extremely nutritious. Sardines are packed with omega-3 fatty acids, which are excellent for brain health, heart health, and reducing inflammation, Pobee explains. They also contain a high amount of vitamin D and B12, which support bone health and energy production, respectively.
Emily Van Eck, MS, a registered dietitian, tells Healthnews that sardines are also packed with protein and calcium, and they’re a more sustainable fish than many large fish such as tuna or salmon.
“I do think it's great to explore the tinned fish section of the grocery store,” Van Eck says. “Having sardines, tuna, or another tinned fish every week or so for a lunch or two would greatly increase the omega-3's you're consuming. They're an easy, affordable, highly nutritious, shelf-stable protein source.”
That being said, Van Eck says she sees absolutely no benefit in eliminating all other foods from your diet and consuming only sardines for three days.
Pobee adds that while someone who tries this trend may see the number on the scale go down, quick drops in weight often reflect water loss — not fat loss. Once the fast is over, reverting back to a regular diet will likely cause the number to go right back up.
The risks of the sardine fast
Beyond being an ineffective way to lose and keep weight off, Pobee says this trend isn’t without health risks.
“While sardines are nutrient-dense, eating only one type of food for three days can lead to nutrient imbalances, as you're missing out on fiber, phytonutrients, and other essential vitamins and minerals found in fruits, vegetables, and whole grains,” she says.
Plus, if an individual’s genetic profile includes variations in genes related to detoxification or fat metabolism, like the APOE gene, a diet high in fats — especially from fish — might not be ideal. She says those who metabolize fats differently could also experience sluggishness or digestive distress during such a fast.
Beyond the lack of nutrients, Pobee says consuming high amounts of sardines in one go could also pose a risk of excessive sodium intake, especially for those with hypertension or salt sensitivity. While sardines are lower in mercury compared to larger fish, she says they still contain trace amounts of heavy metals that could accumulate if eaten in large quantities.
“Individuals with genetic predispositions for slower detoxification, such as a variation in the GSTM1 gene, may be at greater risk for toxin buildup, making this type of restrictive fast less ideal,” Pobee adds.
Short-term, extreme diets can also have a detrimental effect on your relationship with food, Van Ecks says.
“The more often you engage with quick-fixes for weight loss, following strict rules and restricting your body of essential nutrients it needs and wants, you risk worsening your relationship with food and getting even more fixated on the foods that the diet tells you not to eat,” she says. “In this case, carbohydrates.”
In regards to the nutrients missing from this diet, Van Eck is particularly concerned about the lack of fiber. She says fiber is an incredibly important part of our diet and individuals should ideally be getting at least 10 grams at each meal. Eating only fish includes zero fiber.
Fiber helps our digestion and gut microbiome and is a necessity for lifelong health and disease prevention, she says. And while three days without fiber is not going to result in any sort of long-term damage, Van Eck says it is not really a "healthy" thing to do by choice.
“While sardines offer a variety of health benefits, the idea of eating only sardines for three days is more of a gimmick than a sustainable or balanced dietary approach,” Pobee says. “A better strategy for weight loss and metabolic health is a varied, nutrient-dense diet personalized to your body’s unique genetic and metabolic needs.”
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