Salmon roe – sometimes referred to as "salmon caviar" – is the internal egg mass found in female salmon. It is served in a variety of dishes, including sushi and pasta, or as a garnish or condiment to seafood or eggs. Salmon roe is rich in protein, vitamins, and the omega-3 fatty acids eicosapentaenoic acid, or EPA, and docosahexaenoic acid, or DHA.
Omega-3 fatty acids are long-chain polyunsaturated fatty acids. They are considered essential fatty acids because the human body cannot make them. Alpha-linolenic acid, or ALA, an omega-3 fatty acid that serves as a precursor to other essential fatty acids, can be converted in the body to the longer-chain omega-3 fatty acids, EPA and DHA, but the conversion rate is low. For this reason, nutrition experts recommend eating foods naturally high in EPA and DHA, which are present in fatty fish such as salmon (and their roe), herring, mackerel, anchovies, sardines, and tuna. The Institute of Medicine has determined that an adequate intake of ALA for men and women is approximately 1.6 and 1.1 grams per day, respectively, which equates to ~500 milligrams of EPA and DHA.
Between 40 and 70 percent of the DHA in salmon roe is in phospholipid form, compared to fish, with just 1 to 3 percent. Most of this is present as phosphatidylcholine. The form of DHA present in fish or roe influences not only how the fatty acid is metabolized, but also how it is transported into the brain.
Transport of free-DHA and DHA-lysoPC into the brain. The non-esterified form of DHA (free-DHA) detaches from albumin in the plasma and is transported along the outer membrane leaflet of the blood-brain barrier (BBB) via passive diffusion. APOE3 produced in astrocytes plays a role in maintaining the tight junctions, which keep the outer membrane leaflet of the BBB intact.
The phospholipid form of DHA is metabolized to non-esterified DHA (called free-DHA) or to a different phospholipid form (called lysophosphatidylcholine DHA, or DHA-lysoPC). Free-DHA is easily transported across the outer membrane (called a leaflet) of the blood-brain barrier via passive diffusion. DHA-lysoPC, however, is transported across the inner membrane leaflet of the blood-brain barrier via a specialized transporter protein called Mfsd2a. Mice that lack the Mfsd2a transporter have 60 percent less DHA in their brains, have small brains, and typically exhibit a variety of motor and cognitive defects. Salmon roe, due to its higher concentration of DHA in phospholipid form which metabolizes to DHA-lysoPC, may take special advantage of this form of transport utilizing Mfsd2a.
DHA-lysoPC appears to be the brain's preferred source of DHA. Studies demonstrate that DHA-lysoPC levels are approximately 10-fold higher in the brains of young rats and piglets compared to free-DHA. Animal evidence also suggests DHA-lysoPC is critical for normal fetal and infant brain development as well as eye development. This form of DHA is only transported through the Mfsd2a transporter and is required for growth of new membranes in neurons and other cell types during the time when the brain is rapidly growing. Infants who have a mutation in the gene that encodes for the Mfsd2a transporter develop microcephaly and exhibit early neurodegeneration that progressively worsens with age, suggesting that DHA-lysoPC transport into the brain may be important to maintain brain function during the aging process.
Dementia affects the lives of nearly 50 million people worldwide, a number expected to triple by the year 2050. Approximately 70 percent of all dementia cases are related to Alzheimer's disease, a progressive neurodegenerative disorder characterized by memory loss, cognitive decline, and behavioral changes. A key risk factor for Alzheimer's disease is the presence of the gene for apolipoprotein E4, or APOE4, which is present in approximately 25 percent of the population. Having one copy of the gene increases the risk of developing the disease 2- to 3-fold, while two copies increase the risk as much as 15-fold.
"A key risk factor for Alzheimer's disease is the presence of the gene APOE4, which is present in 25% of people. Having one copy of the gene increases the risk of developing the disease 2- to 3-fold, while two copies increase the risk as much as 15-fold" Click To Tweet
Several critical changes occur in the brain of a person with Alzheimer's disease. The most significant of these include the accumulation of amyloid-beta plaques and the formation of tau tangles (both of which interfere with the brain's normal functioning), and reduced glucose transport into the brain. DHA promotes brain health and reduces the risk of developing Alzheimer's disease by promoting glucose uptake in the brain. DHA also slows the progression of Alzheimer's disease and improves symptoms in APOE4 carriers. These benefits are generally observed only when DHA is consumed in the diet as opposed to via dietary supplements, which do not have the phospholipid form of the fatty acid.
This difference in response may lie in the way in which the brain transports the different forms of DHA, as described above. Several studies have found that APOE4 disrupts the tight junctions of the blood-brain barrier, leading to a breakdown in the barrier's outer membrane leaflet and a subsequent loss of barrier integrity. An end result of this loss is impaired diffusion of free-DHA. The transport system that moves the free-DHA into the brain may be faulty in people who have the APOE4 variant, putting them at increased risk for developing Alzheimer's disease. However, the metabolism and transport across the blood-brain barrier of DHA-lysoPC can bypass the tight junctions, providing a better means of DHA transport for people with the APOE4 variant, potentially lowering their risk of developing Alzheimer's disease.
To date, research on the effects of DHA in the brains of people with Alzheimer's disease has been based on DHA in fish oil supplements. Future research should consider the effects of DHA in phospholipid form, especially from rich sources such as fish roe or krill, which can have as much as one-third to three-quarters of the DHA present in phospholipids.
Consumption of salmon roe is growing in popularity. For some people, however, an appreciation for any type of roe is an acquired one, due to the tiny eggs' unique texture and briny flavor. In particular, they burst with an audible “pop” between the tongue and the roof of the mouth, releasing the lightly flavored contents. Compared to the roe of other fishes, salmon roe has a considerably larger egg size, conspicuous pop, and mild, sweet finish, faintly reminiscent of salmon. Many roes, especially brined ones, readily absorb other flavors, especially those of certain metals.
Processing techniques vary according to the intended final use for the roe and include brining (the most common), curing, marinating, or fermenting. These processes greatly enhance the food safety profile of salmon roe and can guide consumers' decisions about which salmon roe products to purchase for optimal quality and safety.
Salmon roe processing typically occurs under very stringent safety procedures to reduce the risk of potential contamination. The egg mass is sometimes left intact (commonly referred to as a "skein"), or the mass can be separated and processed as individual eggs (commonly referred to as "ikura").
Shortly after the eggs are removed from the fish they are graded based on freshness and size. Freshness dictates the eggs' firmness. Higher quality eggs are inherently firmer, require shorter brining time, and are less salty. Less fresh eggs are softer and require longer brining (which firms the eggs), and the end product is saltier. The highest grade is “#1, Gold,” which is assigned to the freshest, firmest, largest eggs. Grades 1 through 3 are successively less fresh and smaller.
After grading and sorting, the eggs are brined (during which foreign matter is separated from the eggs), and then packaged. Part of the packaging process of frozen roe typically involves a nitrogen gas flush that displaces the oxygen and seals the package. This allows the eggs to be stored in the freezer for several months and prevents oxidation of the omega-3 fatty acids they contain.
Proper storage and handling of salmon roe are critical to avoid food safety risks. In particular, Listeria monocytogenes, a bacterium that thrives even in cool temperatures and high salt concentrations, poses a significant threat to ikura, which is consumed raw. Evidence suggests that storing ikura at temperatures of 3°C reduces the risk of Listeria contamination.
Although parasitic contamination of salmon roe is rare, some parasite larvae may be present in the eggs. These are typically removed during the brining process, however. In addition, food safety regulations in the EU stipulate that all fish and fish products that are to be consumed raw or nearly raw must be frozen at -20°C for at least 24 hours to kill parasites. Regulations in the US are even more stringent and stipulate that the freezing process to destroy parasites should be -20°C for 7 days or -35°C for 15 hours.
The stringency with which salmon roe is handled and processed typically eradicates most of the common food safety risks. However, the US Food and Drug Administration and the US Dietary Guidelines for Americans recommend that women who are pregnant should not consume raw fish or raw fish products. Pregnant women should speak with their healthcare provider when deciding whether to eat salmon roe.
The greatest risk of microbial contamination is often post-processing in restaurants or the consumer's home kitchen. Consumers should keep frozen salmon roe in the freezer until ready to use and then eat the product within a short period of time (less than 1 week).
Although mercury is present in nearly all fish and shellfish, salmon is typically lower in this toxic metal and is listed in the "best choices" for fish consumption by the FDA. Interestingly, a recent randomized controlled trial showed that pregnant women who supplemented with 600 milligrams of DHA during their third trimester had sons with larger total brain volumes, total gray matter, corpus callosum, and cortical volumes when compared to boys born to mothers who took a placebo. These findings suggest that the omega-3 fatty acids from fish actually protect the brain from the toxic effects of mercury even in the developing fetus (which is the most susceptible to mercury effects).
"A randomized controlled trial showed that supplementing with 600 milligrams of DHA during the third trimester of pregnancy resulted in sons with larger total brain volumes, total gray matter, corpus callosum, and cortical volumes compared to placebo." Click To Tweet
Polychlorinated biphenyls, or PCBs, are toxic compounds historically used in a variety of industrial and chemical applications. Although PCBs were banned in the 1970s, they still persist in the environment, and many freshwater, saltwater, and farmed fish are contaminated with PCBs. The amount of PCBs in salmon is considerably lower in wild-caught salmon compared to farmed salmon, and amounts are even lower in roe.  Farmed salmon from Europe tend to have higher PCB levels than those farmed in the Americas.
Since PCBs can bioaccumulate in human muscle and adipose tissue, brain, liver, and lungs and have long elimination half-lives, ranging from 10 to 15 years, limiting PCB exposure by consuming wild-caught salmon or their roe rather than farmed salmon may be advisable. Some, but not all, PCBs are excreted in sweat. Practices such as intense exercise and sauna use promote heavy sweat losses.
Fish, fish products, and shellfish are among the most common allergenic foods. Roughly 1 percent of people worldwide are allergic to fish and fish products. Symptoms range from mild itching to life-threatening changes in blood pressure or shortness of breath. People with fish allergies should not eat salmon roe.
Salmon roe is a flavorful, nutrient-rich food that is served in a variety of ways. Consumption of salmon roe may promote fetal neural development and reduce the risk of developing Alzheimer's disease in adults. Although consuming roe may carry some health risks, proper storage and handling of the product can reduce these risks.