Does Nattokinase Break a Fast? Your Keto & Intermittent Fasting Guide
Short answer: No, nattokinase capsules will not break your fast. A standard nattokinase supplement contains zero calories, zero carbs, and zero sugar — so it won’t trigger an insulin response or kick you out of ketosis. In fact, taking nattokinase on an empty stomach may actually be the best way to take it.
But there’s more to the story, especially if you’re combining keto with intermittent fasting. Let’s break it all down.
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Will Nattokinase Break Your Intermittent Fast?
If you’re doing intermittent fasting — whether it’s 16:8, 18:6, or OMAD — nattokinase capsules are completely safe to take during your fasting window. Here’s why:
Nattokinase is a purified enzyme, not a food. A typical capsule contains the enzyme itself, a cellulose capsule shell, and maybe a tiny amount of rice flour as filler. We’re talking essentially zero calories and zero carbohydrates. That means no insulin spike, no blood sugar response, and no disruption to your fasted state.
In fact, many health practitioners actually recommend taking nattokinase on an empty stomach. The reason? When you take it with food, the enzyme can get busy breaking down proteins from your meal instead of doing its real job — supporting circulation and cardiovascular health. Taking it while fasting means more of the enzyme gets absorbed into your bloodstream where you actually want it.
What about extended or water fasts? If you’re doing a strict water fast (24+ hours) for autophagy or other therapeutic reasons, the answer gets a little more nuanced. While nattokinase itself won’t add calories, some purists consider any supplement a fast-breaker. If you’re doing an extended fast, talk to your healthcare provider about whether to continue any supplements during that window.
The bottom line for most people: If your fasting goal is weight loss, blood sugar management, or metabolic health (which is most of us in the keto community), nattokinase is perfectly fine during your fast.
Is Nattokinase Keto Friendly?
Yes — nattokinase is completely keto compatible. The supplement itself contains zero net carbs, so it won’t affect your daily carb count or knock you out of ketosis.
However, here’s what you need to watch out for: not all nattokinase supplements are created equal. Some brands add fillers, binders, or sweeteners that can sneak in a few carbs. Before you buy, flip that bottle over and check the “Other Ingredients” section on the label. You want to see simple ingredients like:
- Nattokinase (often listed with FU or fibrinolytic units)
- Vegetable cellulose (capsule shell)
- Rice flour or microcrystalline cellulose (common, keto-safe fillers)
Avoid any supplement that lists sugar, maltodextrin, or corn starch — those are carb bombs in disguise.
Is natto itself keto friendly? This is a different question. Natto (the fermented soybean dish that nattokinase comes from) contains about 12 grams of net carbs per cup. That’s a significant chunk of your daily keto carb budget. If you love natto, a small serving can fit into a well-planned keto day, but most people find it easier to take the supplement instead.
What is Nattokinase (and Why Should You Care)?
Nattokinase is an enzyme that comes from natto, a traditional Japanese fermented soybean dish. The enzyme is produced naturally during fermentation by a bacterium called Bacillus subtilis — and it’s been consumed in Japan for centuries.
What makes nattokinase interesting for the 40+ crowd (hi, that’s us) is its potential cardiovascular benefits. Research suggests nattokinase has fibrinolytic properties, meaning it may help break down fibrin — a protein involved in blood clot formation. Some of the studied benefits include improved circulation, support for healthy blood pressure, and a potential reduction in blood clot risk.
For those of us combining keto with an active lifestyle after 40, supporting cardiovascular health isn’t optional — it’s essential. That’s why nattokinase has become a popular addition to many keto supplement routines.
Important: Nattokinase can interact with blood-thinning medications like warfarin or aspirin. Always consult your healthcare provider before adding it to your routine, especially if you take any medications that affect blood clotting.
Best Time to Take Nattokinase on Keto
Timing matters with this supplement, especially if you’re fasting. Here’s our recommendation for combining nattokinase with a keto and intermittent fasting lifestyle:
Morning, during your fasting window is the sweet spot for most people. You get the benefit of an empty stomach (better absorption), your body is in a fasted state (no food competing for the enzyme’s attention), and research suggests nattokinase activity peaks about 2-4 hours after taking it and lasts up to 8 hours.
If you do two-a-day dosing, consider taking your second dose in the evening. Some research suggests that taking nattokinase at night — when blood flow naturally slows during sleep — may enhance its cardiovascular benefits.
How to Buy Nattokinase
Nattokinase supplements are widely available in health food stores, online retailers like Amazon, and pharmacies. When shopping, keep these tips in mind:
Check the FU count. FU stands for “fibrinolytic units” — it measures enzyme activity. Most studies use 2,000 FU daily. Look for supplements that clearly state their FU per capsule.
Look for third-party testing. Reputable brands will have their products tested by independent labs for purity and potency.
Read the ingredients list. As we mentioned above, choose supplements with minimal additives and no hidden carbs — especially important if you’re tracking macros on keto.
Check reviews. Look for feedback from other people using nattokinase alongside keto or intermittent fasting.
We put together a free cheat sheet with the carb counts for 20+ common medicines and supplements so you don’t have to Google every bottle in your cabinet. Grab it here.
Frequently Asked Questions about Nattokinase
Does nattokinase break a fast?
No. Nattokinase capsules contain essentially zero calories and zero carbohydrates, so they will not break an intermittent fast or trigger an insulin response. Many practitioners actually recommend taking nattokinase on an empty stomach for better absorption.
Should you take nattokinase on an empty stomach?
Yes, taking nattokinase on an empty stomach is generally recommended. When taken with food, the enzyme may act as a digestive enzyme and break down dietary proteins instead of being absorbed into your bloodstream for cardiovascular benefit.
Is natto keto friendly?
Natto (the fermented soybean food) contains about 12 grams of net carbs per cup, so it should be eaten in small portions on keto. However, nattokinase supplements contain zero net carbs and are fully keto compatible.
What is the best time to take nattokinase?
For people combining keto and intermittent fasting, taking nattokinase in the morning during your fasting window is ideal. The enzyme peaks in activity about 2-4 hours after ingestion and its effects can last up to 8 hours. If taking twice daily, add a second dose in the evening.
Can you take nattokinase with other keto supplements?
Nattokinase can generally be taken alongside common keto supplements like magnesium, electrolytes, and MCT oil. However, avoid combining it with blood-thinning medications or aspirin without consulting your doctor, as nattokinase has its own blood-thinning properties.
This is not medical advice, and you should always consult your medical professional before taking any supplement.
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