Fish is fully digested and absorbed within 24 to 48 hours, but certain compounds from fish, especially mercury, can linger in your body for months. The answer depends on what you mean by “in your system”: the physical food itself clears your digestive tract relatively quickly, while nutrients like omega-3 fatty acids stay elevated in your blood for days, and trace contaminants like mercury take roughly 2 to 3 months to drop by half.
Digestion: 24 to 48 Hours
Fish is one of the faster proteins to digest. After you eat a serving, your stomach acid and digestive enzymes begin breaking down the protein within minutes. The bulk of nutrient absorption happens in the small intestine over the next several hours, and any remaining waste moves through the large intestine. For most people, the entire process from eating to elimination takes somewhere between 24 and 48 hours, though individual factors like meal size, fiber intake, and gut health can shift that window in either direction.
People taking antacid medications may digest fish protein more slowly because reduced stomach acid gives digestive enzymes less help in breaking down proteins. This is also relevant for fish allergies: when digestion is impaired, intact allergen proteins are more likely to reach the intestinal lining and trigger a reaction. Under normal conditions, though, fish proteins are broken down efficiently and don’t persist in any meaningful way.
Omega-3 Fatty Acids: About 3 Days
The omega-3 fats in fish, primarily EPA and DHA, are a big reason people eat fish in the first place. After a single serving, EPA levels in your blood peak around 6 hours, dip slightly, then rise again at the 24-hour mark. DHA follows a similar pattern but drops off more slowly. Both remain above your baseline levels for at least 72 hours after one meal.
This is why dietary guidelines recommend eating fish two to three times per week rather than loading up once a month. Spacing out servings keeps omega-3 levels consistently elevated. The EPA and FDA recommend 2 to 3 servings per week from low-mercury fish, or 1 serving per week from moderate-mercury varieties.
TMAO: Gone Within a Day
You may have seen headlines about TMAO, a compound your body produces after eating fish and other seafood. TMAO has been linked in some research to cardiovascular risk, which understandably worries people. The good news: circulating TMAO levels return to baseline within 24 hours of eating fish. This has been demonstrated across multiple types of fish and seafood, and it held true for every participant with normal kidney function studied. TMAO from fish is a short-lived spike, not a lasting buildup.
Mercury: Weeks to Months
This is where fish truly “stays in your system.” Nearly all fish contains some methylmercury, a form of mercury that accumulates in fish tissue and is absorbed very efficiently by your gut. Once in your bloodstream, methylmercury binds to proteins and distributes throughout your body, including your brain.
The half-life of methylmercury in humans, meaning the time it takes for your body to clear half of what you absorbed, averages about 80 days. Estimates across different studies range from roughly 40 to 120 days depending on the population and methods used. A large study of 304 adults found a population mean half-life of about 80 days, with a range of 64 to 97 days covering most people. In pregnant women, some estimates put the average even higher, around 103 days.
To put that in practical terms: if you eat a high-mercury fish like swordfish or king mackerel, about half the mercury from that meal is still circulating 2 to 3 months later. It takes several half-lives for levels to drop to negligible amounts, so a single high-mercury exposure could take 6 months or longer to fully clear. This is why mercury builds up in people who eat high-mercury fish regularly, as new exposure arrives before the old exposure is fully eliminated.
Low-mercury fish like salmon, sardines, tilapia, and shrimp contain far less methylmercury per serving, so even with the slow clearance rate, regular consumption stays within safe levels for most adults. The concern is primarily with large predatory fish that concentrate mercury up the food chain.
PCBs and Other Persistent Pollutants
Some fatty fish, particularly farmed salmon and certain freshwater species, also carry trace amounts of industrial pollutants called PCBs. These are stored in body fat and cleared extremely slowly. Lower-chlorinated forms have half-lives of 1 to 6 years, while more persistent forms can take 8 to 24 years to reduce by half. At typical dietary exposure levels from occasional fish consumption, the amounts are very small, but they do accumulate over a lifetime. This is one reason fish advisories exist for certain lakes and rivers where contamination is higher.
What This Means for How Often You Eat Fish
The nutrients from fish clear your system in days, which is why regular consumption matters for sustained benefit. The contaminants clear in months to years, which is why variety and fish choice matter for long-term safety. Sticking to 2 to 3 servings per week of low-mercury species gives you consistent omega-3 levels without meaningful mercury accumulation. If you eat a high-mercury fish occasionally, your body will clear most of that mercury over the following 4 to 6 months, assuming you aren’t repeating the exposure frequently.
For people who’ve been eating high-mercury fish regularly and want to reduce their levels, simply switching to lower-mercury options allows the body’s natural elimination process to gradually bring mercury concentrations down over several months.