A 3 oz (85 g) serving of raw Pacific oysters has 69 calories and 8 g of protein. Yet it supplies nearly 6 times the Daily Value of vitamin B12 (13.6 µg, 567%). It also gives more than a full day of zinc (14.1 mg, 128%), copper (149%) and selenium (119%).
Eastern oysters and cooked or canned oysters are different USDA records with different numbers, so the figures here are for raw Pacific oysters.
Oysters are filter feeders. Under average conditions, a mature oyster filters about 11 to 45 liters (3 to 12 US gallons) of water a day. That is also why it concentrates whatever the water holds, including bacteria and viruses.
Oysters usually spawn as males first and as females later. The Pacific oyster genome carries expanded families of heat shock protein 70 genes. The sequencing team thinks they are central to surviving the stressful tidal zone.
Raw oysters are where the care is needed. They can carry norovirus and Vibrio vulnificus, a bacterium that can kill people with liver disease or a weak immune system. The FDA and CDC, the US food and disease agencies, say only heat makes oysters safe. Hot sauce, lemon juice and alcohol do not.
Human trials on oysters themselves are few. Most trials below tested oyster extract supplements, which the cards say plainly.
Health benefits of oysters
The top findings from 28 studies, strongest evidence first
What the stars mean
- Strong. Several randomized trials, pooled in a meta-analysis, point the same way, or regulators (FDA or EFSA) have approved the claim.
- Moderate. Good randomized trials support it, but there are fewer of them, or their results vary.
- Limited. Small, short or single trials, or mixed results. Promising, but it needs confirmation.
Population and lab studies get no stars: they show a link or a mechanism, not proof.
How we rate evidence →- ★Less cholesterol absorbed than with chickenIn 8 healthy men, a 3-week diet of oysters mixed with clams cut cholesterol absorption to 42%, against 54% on chicken. Blood cholesterol and blood fats did not differ, and it was a very small study.
- ★Mixed results for muscle strength with oyster extractIn 52 postmenopausal women with low muscle mass, 12 weeks of oyster extract raised thigh and grip strength. In 54 older walkers with low muscle mass, it did not raise strength or muscle mass; these were extracts, not whole oysters.
- ★Lower liver enzymes with oyster extractIn 40 older women with obesity, 12 weeks of the extract lowered the liver enzymes ALT, AST and GGT. In a pilot trial of 84 regular drinkers, GGT crept up on placebo but tended to fall on the extract.
- ★Short children grew more with oyster extractOver 24 weeks, 80 children aged 6 to 9 below the 25th height percentile grew 3.22 cm on oyster extract, against 2.38 cm on placebo. A second trial in 66 children with mild short stature also found more height growth.
- ★Higher glutathione after 8 days of oyster extractIn 7 healthy men with no comparison group, 8 days of oyster extract raised glutathione, an antioxidant the body makes, about 1.5-fold. It was tiny and short, and it measured blood markers, not illness.
All 28 studies, strongest evidence first
Oysters and clams cut cholesterol absorption to 42%, against 54% on chicken
In this feeding trial, 8 healthy men with normal cholesterol ate three diets, 3 weeks each. The main animal protein was oysters mixed with clams, crab or chicken. Total cholesterol, LDL, HDL and triglycerides did not differ between the diets. It was very small, and the oysters came mixed with clams.
In older walkers, oyster extract did not raise muscle strength or mass
This placebo-controlled trial included 54 adults aged 50 to 78 with low muscle mass, who all walked 30 to 60 minutes a day. For 12 weeks they took placebo (a dummy pill) or 1,200 mg a day of Crassostrea gigas extract fermented with a lactic acid bacterium. Knee strength, grip strength, muscle mass and blood markers did not differ, and no side effects were reported. This tested a supplement, not whole oysters.
Oyster extract raised thigh and grip strength in postmenopausal women
This double-blind, placebo-controlled trial gave 52 postmenopausal women with low muscle mass 1000 mg of the fermented extract a day for 12 weeks. Blood IGF-1, a growth factor, was higher than on placebo, while muscle mass, body composition and the inflammation marker hsCRP did not differ. The first two authors list Marine Bioprocess Co. as their affiliation. The paper says the funder had a role in interpreting and writing up the results.
In older women with obesity, oyster extract lowered liver enzymes
This double-blind, placebo-controlled trial included 40 women with a body mass index of 25 or more. For 12 weeks, 20 took the fermented extract and 20 took placebo. The liver enzymes ALT, AST and GGT and free fatty acids all fell. The authors say the short intake period and small sample limit the results, which apply only to older women with obesity.
Oyster extract kept liver enzyme GGT lower than placebo in regular drinkers
This pilot trial included 84 adults with a drinking habit and slightly high GGT, 42 in each group. GGT crept up on placebo and tended to fall on the extract. The gap between groups was just significant at week 12, while neither group changed significantly on its own. The efficacy analysis covered 38 people on placebo and 36 on the extract.
Short children on oyster extract grew 3.22 cm, against 2.38 cm
This double-blind trial included 80 children aged 6 to 9 with height below the 25th percentile. They took 500 mg a day of fermented oyster extract or placebo for 24 weeks. Bone age and IGF-1 also rose more than on placebo, while growth hormone did not differ. Marine Bioprocess Co. makes the product and holds patents, and some authors are affiliated with it. The paper says the sponsor had no role in data analysis or interpretation.
Oyster extract improved height growth in children with mild short stature
This double-blind trial had 40 children per group, aged 6 to 9, with heights between the third and 25th percentiles. They took 500 mg a day or placebo for 24 weeks, and 66 finished, 33 in each group. Bone age and IGF-1 also improved more than on placebo, and sex hormones did not rise. The authors declare no conflicts of interest.
Oyster extract for 8 days raised reduced glutathione about 1.5-fold
In this preliminary before-and-after trial, 7 healthy men aged 23 to 37 took 3 doses of 2 capsules a day of Crassostrea gigas extract. Glutathione is an antioxidant made by the body, and serum protection against free-radical damage to red blood cells also improved. It was tiny and short, and it measured blood markers, not illness.
Low-zinc diet cut testosterone from 39.9 to 10.6 nmol/L
This study is about zinc, not oysters, which are rich in zinc: 4 young men restricted dietary zinc for 20 weeks. In 9 older men who were marginally low in zinc, 3 to 6 months of zinc supplements raised testosterone from 8.3 to 16.0 nmol/L. Testosterone also tracked cell zinc levels in 40 men, but the groups were tiny. It did not test whether extra zinc raises testosterone in men who already have enough.
In China, shellfish was linked to lower type 2 diabetes risk
This cohort study tracked 51,963 men and 64,193 women in China, with shellfish as one group (oysters not separated), alongside fish and omega-3 fats. Men in the fourth fifth of shellfish intake had 0.66 times the risk of the lowest fifth, and the trend was also inverse in women. Food questionnaires show a link, not cause.
In Norwegian women, shellfish was linked to higher type 2 diabetes risk
This cohort followed 60,777 Norwegian women for a mean of 7.2 years, and 681 developed treated type 2 diabetes. Shellfish was one group, with oysters not separated, and no link appeared for total seafood, fatty fish, total meat or red meat. Mice fed a shellfish-based Western diet also had poorer glucose tolerance. The result runs against the Chinese cohort above, so it is not settled.
Crab and oysters were not linked to age-related macular degeneration
This cross-sectional study (a single snapshot) covered 2,520 people aged 65 to 84 in Salisbury, Maryland. Both foods are high in zinc, yet no relationship appeared. Advanced macular degeneration was less likely in people who ate fish and shellfish high in omega-3 fats (odds ratio 0.4). But crab and oysters were counted together and showed no link.
Shellfish was the most common food allergy among US adults, affecting 2.9%
This survey asked 40,443 adults and counted shellfish as one group, not oysters alone. Among adults with any food allergy, 51.1% had a severe reaction and 48.0% developed the allergy as adults.
Oyster growers eating more oysters had higher blood cadmium
This study covered 61 non-smoking oyster growers and family members in British Columbia. Pacific oysters there often exceed 2 µg/g (micrograms per gram) of cadmium by wet weight. Oyster intake predicted blood cadmium but not urine cadmium, and 36% had urine cadmium above 1 µg/g creatinine. The authors conclude the oyster cadmium is absorbed, and these were people with high oyster intake.
Florida Vibrio: 83% of bloodstream infections went with raw oysters
This report covered 690 Vibrio infections in Florida from 1981 to 1993. Of the patients with bloodstream infections linked to raw oysters, 44% died. Of the bloodstream infections, 91% occurred from April through October. Liver disease was present in 48% of patients and linked to death (1.9 times the risk).
Of US Vibrio vulnificus bloodstream cases, 96% had eaten raw oysters
This CDC review covered 422 infections reported from 23 states between 1988 and 1996. Of the bloodstream cases, 61% died, and underlying liver disease went with death. Every oyster trace-back with complete information pointed to the Gulf of Mexico. Most of those oysters (89%) were harvested in water above 22 °C.
Liver disease: 80 times illness risk, over 200 times death risk
This report covered 72 Florida cases of Vibrio vulnificus from raw oysters, and 36 (50%) died. The headline risks compare people with and without liver disease. The death rate was 63% in people with liver disease and 23% in those without. The authors urge education about the risk and thorough cooking of shellfish.
After California’s 2003 warm-season limit, raw-oyster Vibrio cases fell to zero
From 2003 to 2010, California had no Vibrio vulnificus cases from raw Gulf oysters. Before the rule, 0 to 6 cases and 0 to 5 deaths a year had raw oysters as their only exposure. Similar cases in other states rose slightly. The rule covered Gulf oysters sold from April 1 to October 31, unless processed to reduce the bacteria to nondetectable levels.
Raw Alaskan oysters: a 29% attack rate among people who ate them
This Vibrio parahaemolyticus outbreak study looked back at cruise passengers (132 interviewed, 22 cases) and used statewide surveillance that found 62 patients. Raw oysters were the only significant predictor of illness. All the oysters were harvested when mean daily water temperature exceeded 15.0 °C. Water at the farm had warmed 0.21 °C a year since 1997.
In a Florida outbreak, eating only well-cooked oysters did not clearly protect
Of 223 oyster eaters, 129 (58%) became ill, against 2 of 76 non-oyster eaters. Most oyster eaters (67%, 149 of 223) said they ate only cooked oysters, grilled, stewed or fried. For them the relative risk was 0.68, a difference that could be chance. The authors link the outbreak to overboard dumping of feces and say cooking left consumers with no assurance of safety.
Steamed oysters at only 21 to 74 °C caused a norovirus outbreak
This case-control study (comparing ill and well diners) covered restaurant patrons in North Carolina, and over 200 people reported illness. Steamed oysters carried an odds ratio of 12, and 20% of 41 households had secondary cases. The authors say undercooked contaminated oysters pose a similar risk to raw ones.
A 2005 US hepatitis A outbreak from oysters included 39 cases
This short outbreak report found it was the first large hepatitis A outbreak linked to shellfish in the United States in more than 15 years. It was also the first in which an identical virus sequence came from both the food and the patients.
Tabasco on shucked oysters cut surface Vibrio sharply but barely inside
In this lab test, freshly shucked oysters got Tabasco sauce or a horseradish cocktail sauce and sat for 10 minutes. The cocktail sauce did not lower surface bacteria, and neither sauce cut the bacteria within the oyster much. The authors advise people at risk to avoid raw oysters.
Boiling shucked oyster homogenate 3 minutes brought two norovirus stand-ins below detection
This lab study used murine norovirus and Tulane virus, which stand in for human norovirus. One minute at 63 °C cleared Tulane virus, and one minute at 67 °C cleared the murine virus. The authors note that FDA recommends boiling shucked oysters for at least 3 minutes. They did not test human norovirus.
Over 6 minutes at 80 °C inactivated tested viruses except hepatitis A
This lab study used live eastern oysters and heated their tissue. Human norovirus, stand-in viruses, hepatitis A virus and rotavirus all built up to high levels in the oysters. Oyster tissue protected the viruses during heating, and hepatitis A virus was the most heat resistant.
Oysters left 5 hours unchilled carried more Vibrio than iced oysters
This test ran at two Alabama sites and one in Delaware Bay. At one Alabama site, the count was 5.1 against 3.6 log MPN/g for oysters iced at once, a log scale of bacteria per gram. Layered ice gave lower Vibrio levels than plain refrigeration, and in New Jersey 7 hours in the shade gave the highest level (5.3 log). Several authors are FDA scientists, and they say faster cooling can lower the risk of illness.
Oyster meat increased sexual behavior in male mice at 250 mg/kg
Mouse study comparing oyster meat with a version digested by the enzyme alcalase, which did better on antioxidant tests. Mouse behavior does not show an aphrodisiac effect in people.
Pacific oyster peptides raised testosterone in male mice and mouse cells
Peptides made from Crassostrea gigas by enzyme digestion also raised cAMP and the activity of nitric oxide synthase in mouse penile tissue. It was lab work only, and no people were tested.
Who should be careful with oysters
Most healthy people can eat cooked oysters freely. Take care if one of these applies:
- Liver disease or a weak immune system. Do not eat raw oysters if you have liver disease, diabetes, cancer, HIV or another condition that weakens immunity. CDC says about 1 in 5 people with a Vibrio vulnificus infection die. In a Florida report, people with liver disease were over 200 times more likely to die from it. Thoroughly cooked oysters are the safe choice.
- Pregnancy, young children and older adults. FDA lists these groups, along with people with weakened immune systems, among those who should avoid raw oysters. In healthy people, raw oysters mostly cause diarrhea and vomiting. Norovirus outbreaks have followed oysters that were steamed but not hot enough.
- Shellfish allergy. Shellfish allergy affected 2.9% of US adults in a survey. Allergy to crustaceans such as shrimp and to mollusks such as oysters can occur separately, so ask an allergist before trying oysters.
How to choose, store and eat oysters
- Cooked: FDA advises boiling or simmering shucked oysters for at least 3 minutes. Or fry them at 375 °F for 3 minutes, or bake them at 450 °F for 10 minutes. For oysters in the shell, boil 3 to 5 minutes after the shells open.
- Raw: only for healthy adults who accept the risk. Hot sauce cut bacteria on the surface of shucked oysters but little inside. Lemon juice and alcohol do not kill Vibrio, so none of them is a substitute for cooking.
- Which oyster: the numbers on this page are for raw Pacific oysters. Eastern oysters and cooked or canned oysters have different nutrient values.
Buy oysters with closed shells and throw away any with open shells. Also throw away any that do not open during cooking. Keep them cold: in an FDA-authored study, oysters held 5 hours in air before refrigeration carried more Vibrio vulnificus than oysters iced at once.
No official residue data for oysters were found for this page, so residues are left out.
Sources
- Vibrio and oysters. CDC.
- About Vibrio infection. CDC.
- How to generate awareness of Vibrio vulnificus in raw oysters: a media relations and community outreach health education kit. FDA. 2004.
- Selecting and serving fresh and frozen seafood safely. FDA.
- Food allergies. FDA.
- Regulation (EU) No 1169/2011, Annex II: substances or products causing allergies or intolerances.
- Warren CM, Aktas ON, Gupta RS, Davis CM. Prevalence and characteristics of adult shellfish allergy in the United States. J Allergy Clin Immunol. 2019.
- Haq SM, Dayal HH. Chronic liver disease and consumption of raw oysters: a potentially lethal combination. Am J Gastroenterol. 2005.
- Watanabe F, Yabuta Y, Tanioka Y, Bito T. Biologically active vitamin B12 compounds in foods for preventing deficiency among vegetarians and elderly subjects. J Agric Food Chem. 2013.
- Jones JL, Lydon KA, Kinsey TP, et al. Effects of ambient exposure, refrigeration, and icing on Vibrio vulnificus and Vibrio parahaemolyticus abundances in oysters. Int J Food Microbiol. 2017.
History of oysters
People ate oysters long before anyone farmed them. Shell middens (ancient heaps of shells) in New South Wales, Australia, have been dated at 10,000 years. Japan is said to have cultivated oysters since at least 2000 BC.
In Rome, Sergius Orata of the Roman Republic is considered the first major oyster merchant and cultivator. He is said to have built channels and locks to control the tides, and the Romans said he could breed oysters on his roof. Romans also farmed the Thames Estuary from about the 1st to the 4th century and sent oysters to Rome.
In the 19th century oysters were cheap food for working people. New York Harbor held the largest oyster beds in the world. In the late 19th century, six million oysters could sit on barges at once. Overharvesting, disease brought by foreign species, effluent and silt destroyed most of the beds by the early 20th century.
The Pacific oyster is now the most widely grown bivalve in the world. It reached California waters in 1929 and became the basis of the West Coast oyster industry. Taxonomists have since moved the Asian Pacific species of Crassostrea into a new genus, so Crassostrea gigas is often written Magallana gigas today.
In 1888, according to the story, Lee Kum Sheung discovered oyster sauce by accident. The old rule to eat oysters only in months with an R has a kernel of truth, because Vibrio is more common in warm months. But FDA calls it a myth that avoiding those months keeps you safe.
Sources
- Oyster. Wikipedia.
- Oyster farming. Wikipedia.
- Salvi D, Macali A, Mariottini P. Molecular phylogenetics and systematics of the bivalve family Ostreidae based on rRNA sequence-structure models and multilocus species tree. PLoS One. 2014.
- Zhang G, Fang X, Guo X, et al. The oyster genome reveals stress adaptation and complexity of shell formation. Nature. 2012.
- How to generate awareness of Vibrio vulnificus in raw oysters: a media relations and community outreach health education kit. FDA. 2004.
Frequently asked questions about oysters
Are oysters good for you?
They are nutritious, with a food-safety catch when raw. Three ounces (85 g) of raw Pacific oysters have 69 calories and 8 g of protein. They also have 13.6 µg of vitamin B12 (567% of the Daily Value) and 14.1 mg of zinc (128%).
Human trials on oysters themselves are scarce. In 8 men, an oyster and clam diet lowered cholesterol absorption to 42% against 54% on chicken, but blood cholesterol did not change. The main downside is that raw oysters can carry Vibrio vulnificus and norovirus.
Are raw oysters safe to eat?
Not for everyone, and nobody can tell by sight or smell. In a CDC review of 422 US Vibrio vulnificus infections, 96% of the bloodstream cases had eaten raw oysters and 61% died. Another report found people with liver disease were 80 times more likely to fall ill and over 200 times more likely to die.
FDA and CDC advise people with liver disease or weak immunity to eat oysters cooked only. CDC says most oyster-related Vibrio infections in other people cause diarrhea and vomiting.
Do hot sauce, lemon juice or alcohol make raw oysters safe?
No. FDA says the active ingredients in hot sauce work no better than plain water on harmful bacteria, and alcohol does not destroy them. CDC says the same about hot sauce, lemon juice and alcohol. In a lab test, Tabasco sauce cut Vibrio on the surface of shucked oysters but did little inside.
Only heat reliably kills the bacteria.
Should you only eat oysters in months with an R?
It is a rough guide, not a safety rule. Vibrio is more common in warm months. In Florida, 91% of bloodstream infections happened from April through October, and 89% of traced Gulf oysters came from water above 22 °C.
But FDA, citing CDC, says 40% of cases occur from September through April. A norovirus outbreak from steamed oysters also hit a North Carolina restaurant in December.
Are oysters an aphrodisiac?
There is no scientific evidence that they are, according to FDA. Zinc matters for testosterone: restricting dietary zinc for 20 weeks cut testosterone from 39.9 to 10.6 nmol/L in 4 young men. But that shows a shortage hurts, not that extra zinc helps men who have enough.
Oyster peptides raised testosterone in mice. No human trial of oysters for desire or sexual function turned up in a PubMed search.
How much zinc and vitamin B12 are in oysters?
A 3 oz (85 g) serving of raw Pacific oysters has 14.1 mg of zinc, 128% of the Daily Value. It also has 13.6 µg of vitamin B12 (567%) and 1.34 mg of copper (149%). Selenium is 65.5 µg (119%) and iron 4.34 mg (24%). Per 100 g, that is 16.6 mg of zinc and 16 µg of B12.
Eastern oysters and cooked or canned oysters have different values, and cooking can break down part of the B12 in foods.
Do oysters contain cadmium?
Some do. Pacific oysters from Washington to Alaska often exceed 2 µg/g of cadmium by wet weight, which is high compared with Atlantic oysters. Among 61 non-smoking oyster growers and family members in British Columbia, oyster intake predicted blood cadmium. And 36% had urine cadmium above 1 µg/g creatinine.
They ate far more oysters than most people, and no study here measured harm from occasional servings.
Can you be allergic to oysters?
Yes. Shellfish was the most common food allergy among US adults, at 2.9%, though that counts all shellfish together. Allergies to crustaceans such as shrimp and to mollusks such as oysters can occur separately.
US labeling rules name crustacean shellfish such as crab, lobster and shrimp, while EU rules list molluscs as their own allergen. So read labels and ask an allergist if you have reacted to shellfish.