Have you heard of the 72 Hour Sardine Challenge? If you struggle with high blood glucose or have trouble getting into ketosis, this simple, fish-forward fast might be worth trying. Read on for a clear, practical guide to the sardine fast and tips to make it easier.
I decided to try the challenge after watching Dr. Annette Boz present results from clients who tested it. The idea is straightforward: for 72 hours you eat only sardines (or similar small oily fish), along with water, black coffee, or plain tea. The clock begins with your first can and ends with your last.
If you want a quick reference, there is a free Sardine Challenge Cheat Sheet (PDF) that summarizes the rules, portions, and a popular sauce recipe many people ask about.
Note: Many people record their experience and results on video — you can find walkthroughs and three-day experiment videos by searching online if you want a visual guide or inspiration before you start.
Guidelines for the 72 Hour Sardine Challenge
- Eat only sardines for the full 72 hours. Small oily fish like herring or mackerel are often considered acceptable alternatives.
- There is no set limit on the number of cans—eat more if you’re hungry. The goal is to keep carbohydrate intake extremely low while maintaining protein and healthy fats.
- Only consume water, black coffee, or plain tea besides the sardines. No other foods or caloric beverages.
- When you feel hungry, eat sardines. Prioritize whole cans or portions so you stay satisfied without adding other foods.
- Sardines come packed in water or olive oil. You can drain them or keep the oil; you may also drain and add your own preferred oil (avoid seed oils).
- Do not use sardines packed in seed oils or soya oils—choose olive oil, water, or naturally preserved options.
- Sardines in tomato sauce or mustard are fine as long as the ingredient list has no added sugar. You may add zero-carb condiments to improve flavor.
- Smoked or kippered sardines are acceptable but can be higher in sodium—consider this if you are limiting salt.
- You can eat them hot, cold, or at room temperature depending on preference.
What is the Goal of the Sardine Challenge?
- The main goal is to lower blood glucose and raise ketone levels quickly. Many people who are insulin resistant report significant improvements in glucose and ketone readings after three days.
- If you can measure blood glucose and ketones (for example, with a GKI calculator or a meter), you’ll be able to track changes precisely. If you can’t test, you can still try the challenge, but testing helps quantify progress.
- Some participants describe the practice as a sardine fast and hope for autophagy. While autophagy is often associated with prolonged fasting, its occurrence during a sardine-only three-day fast is not guaranteed—if autophagy is your primary objective, a water fast may be more reliable.
- What you can expect is a marked improvement in GKI (Glucose Ketone Index), often reaching therapeutic ketosis ranges that can last for days after the fast ends.
- Another clear benefit is a large influx of omega-3 fatty acids, which are strongly anti-inflammatory and may support metabolic health.
- Many people also report suppressed carb cravings and a reset of taste preferences following the challenge.
Sardine Challenge Resources
- Free PDF Sardine Challenge Cheat Sheet — a handy printable that covers the rules, tips, and a popular sauce recipe.
- Search for three-day challenge videos and personal result videos if you prefer to see someone’s step-by-step experience before trying it yourself.
- Dr. Annette Boz’s presentations and case summaries are useful background reading or viewing when available.
- Look up a GKI (Glucose Ketone Index) calculator online to translate glucose and ketone numbers into a single therapeutic index.
- Blood glucose and ketone meters such as those sold for home testing will report the values you need to calculate GKI, if you want objective data.
GKI Levels of Ketosis
- Greater than 9: Not in ketosis
- 6–9: Low-level ketosis
- 3–6: Moderate ketosis
- 1–3: High nutritional ketosis
- Less than 1: High therapeutic ketosis
How to Ease Your Way Into Sardines
Not everyone loves the idea of canned sardines straight from the tin. If you’re hesitant, try these strategies to build tolerance and even enjoyment:
- Try several brands and varieties—taste and texture vary widely. Give at least a few brands a fair try before deciding you don’t like them.
- Chill a can and eat it cold with a light drizzle of mustard, a splash of apple cider vinegar, or a few drops of hot sauce for contrast.
- Pan-fry small sardines in butter or olive oil until crispy; serve with a low-carb dip like keto tartar sauce for a crunchy, approachable version.
- Make air-fried sardine bites or lightly bread them with low-carb coatings for a more familiar texture.
- Experiment with simple recipes like sardine salads, lightly breaded fried sardines, or other family-style preparations that mask strong flavors while keeping the fish front and center.
- Start by including sardines in one meal a day and gradually increase frequency until you’re comfortable with the idea of three days of sardines only.
Many people who were initially skeptical end up enjoying sardines regularly after giving them a fair try. The combination of convenience, nutrition, and metabolic benefits makes this a practical short-term strategy for many aiming to reset blood sugar and deepen ketosis.