Absence of Neu5Gc and Presence of Anti-Neu5Gc Antibodies in Humans—An Evolutionary Perspective
- 1Department of Pathology, Biomedical Research and Training Facility 2, Glycobiology Research and Training Center, University of California, San Diego, La Jolla, CA, United States
- 2Department of Anthropology, University of California, San Diego, La Jolla, CA, United States
The glycocalyx of human cells differs from that of many other mammals by the lack of the sialic acid N-glycolylneuraminic acid (Neu5Gc) and increased abundance of its precursor N-acetylneuraminic acid (Neu5Ac). Most humans also have circulating antibodies specifically targeting the non-human sialic acid Neu5Gc. Recently, several additional mammalian species have been found to also lack Neu5Gc. In all cases, loss-of-function mutations in the gene encoding the sialic acid-modifying enzyme CMAH are responsible for the drastic change in these species. Unlike other glycan antigens, Neu5Gc apparently cannot be produced by microbes, raising the question about the origin of these antibodies in humans. Dietary exposure and presentation on bacteria coating themselves with Neu5Gc from the diet are distinct possibilities. However, the majority of the non-human species that lack Neu5Gc do not consume diets rich in Neu5Gc, making it unlikely that they will have been immunized against this sialic acid. A notable exception are mustelids (ferrets, martens and their relatives) known for preying on various small mammal species rich in Neu5Gc. No studies exist on levels of anti-Neu5Gc antibodies in non-human species. Evolutionary scenarios for the repeated, independent fixation of CMAH loss-of-function mutations at various time points in the past include strong selection by parasites, especially enveloped viruses, stochastic effects of genetic drift, and directional selection via female immunity to paternal Neu5Gc. Convergent evolution of losses of the vertebrate-specific self-glycan Neu5Gc are puzzling and may represent a prominent way in which glycans become agents of evolutionary change in their own right. Such change may include the reconfiguration of innate immune lectins that use self-sialic acids as recognition patterns.
The glycocalyx of all vertebrate cells is decorated with abundant terminal sialic acids. These acidic nine-carbon backbone sugars cap the ends of tens to hundreds of millions of glycan chains per cell. In mammalian species and other vertebrates, the sialic acids N-acetylneuraminic acid (Neu5Ac) and its derivative N-glycolylneuraminic acid (Neu5Gc) are the two most common forms, each a family of molecules with various modifications of the canonical, 9-carbon monosaccharide (1). Until recently, humans were the only mammalian species known to lack the sialic acid Neu5Gc, as our lineage fixed the loss-of-function mutation affecting the single copy CMAH gene that encodes the sialic acid-modifying enzyme CMAH over 2 million years ago in the lineage leading to H. sapiens (2, 3). More recently, several other species of mammals have been documented to also lack Neu5Gc due to ancient mutations fixed over 30 million years ago in these lineages (4–6). The loss of function of the CMAH enzyme prevents the modification of the precursor monosaccharide to the derived sialic acid Neu5Gc (in their respective sugar-nucleotide form, CMP-Neu5Ac and CMP-Neu5Gc). As illustrated in Figure 1, the lack of this enzymatic function can lead to drastic changes in the molecular composition of the glycocalyx of cells throughout the body. Recent evidence has shown that humans are not alone in this loss, instead several other species of mammals have independently fixed different loss-of-function mutations of their Cmah gene at various time depths during evolution, leading to loss of Neu5Gc in entire lineages or just individual species (6). These losses have occurred through exon deletion, premature stop codons, or frameshift mutations in the gene encoding the CMAH enzyme (4–6). The picture emerging is that of a phylogeny of mammals punctuated with taxa that have lost the capacity to synthesize Neu5Gc (Figure 2). These taxa include New World primates (>100 species of South and Central American primates known as Platyrrhynes), Mustelidae (57 species of small carnivores including ferrets, martens and weasels), pinnipeds (33 species comprising seals, sea-lions and walruses), Procyonidae (~15 species including racoons, ring-tails and coatis,) hedgehogs (17 species), bats from at least two different lineages, sperm whale (a single species), and white-tailed deer (a single species) (4–6). For most of these groups, only a few representative taxa and a few individuals have been studied at the genomic level, and so there is the possibility that the Cmah gene remained intact or polymorphic in some of the taxa.
An obvious prediction is that additional taxa with inactivated Cmah genes will be discovered as additional complete genome sequences are obtained. These cases of convergent molecular evolution result in an overall reconfiguration of the outermost layer of the glycocalyx, now lacking Neu5Gc and carrying an excess of Neu5Ac, given that human and non-human cells retain comparable levels of sialic acid (10) (see Figure 1B for red blood cells and Figure 1C for sperm cells). Among the many functions of the glycocalyx, molecular identity is paramount (11–13). The molecular patterns, as defined by composition and structure of the glycocalyx have evolved into self-associated molecular patterns (SAMPs) (14), that contribute to efficient surveillance by innate immune receptors including complement factor H and Siglecs, which can inhibit immune response upon engagement with SAMPs (14, 15). Losing Neu5Gc would dramatically alter self-recognition. This would have required evolving altered receptor specificities, affinities, and knock-on effects in signaling pathways due to altered engagement of innate receptors. The biochemical impact of the altered sialome on the human glycocalyx could have had many other effects, including changes in inflammation and metabolism (16, 17).
Another potential consequence are autoreactive antibodies produced against the lost sialic acid. Indeed, despite the absence of endogenous Neu5Gc, experimental studies in humans and in Cmah(−/−) mice have revealed that dietary Neu5Gc, in both free and glycoconjugate-bound forms, can become incorporated into tissues in trace amounts. This incorporation occurs especially in tissues with rapid growth and/or turnover rates, including epithelia, endothelia, fetal tissues, and carcinomas (18–20). It has also been established that all humans have various levels of circulating antibodies specific for glycans carrying this foreign molecule, essentially making Neu5Gc a “xeno-autoantigen,” which can cause “xenosialitis,” an inflammation due to reaction against a xeno-sialic acid that is now part of “self” molecules (21–24). Surprisingly, even humans on diets extremely rich in Neu5Gc do not appear to accumulate beyond trace levels of this dietary xenoglycan.
How ingested Neu5Gc becomes incorporated into the human body remains incompletely understood. There is evidence that Neu5Gc is converted to GalNGc and can then be incorporated into the glycosaminoglycan chondroitin sulfate, an important component of extra-cellular matrices and skeletal bone (25). This incorporation has recently allowed the identification of GalNGc in bones and in fossilized bones as old as 3 million years, opening the possibility to study ancient glycomes of extinct hominins (26). There is much ongoing research to understand the potential effects of incorporation of dietary xeno-sialic acid and targeting antibodies against xeno-sialic acid, xenosialitis in the context of cancer and autoimmunity and even unexplained infertility, where chronic immune reactions to incorporated xenoglycans could contribute to xenosialitis (22–24, 27–29). Aside from humans, natural levels of anti-Neu5Gc antibodies in other species lacking Neu5Gc have not been studied to date. However, anti-Neu5Gc antibodies have been seen in chickens, where antibodies can be efficiently generated upon immunization (18) and are the basis of immune assays for the detection of Neu5Gc in human samples (30).
Natural Immunization Against NEU5GC-glycans
There are four main differences between immunization against Neu5Gc and other xenoglycans, such as the disaccharide alpha-Gal, or alloglycans such as ABO oligosaccharide antigens. First, in the case of other xenoglycans, immunization against the missing, terminal “self”-glycan is thought to be caused by encounters with microbial glycans with the same structure (31, 32). Considering that the synthesis of endogenous Neu5Gc has never been documented for any microbe, it would appear unlikely that this microbial priming method occurs for Neu5Gc (33, 34). Despite the apparent lack of Neu5Gc synthetic capacity in microbes, however, at least one microbe, Non-typeable Haemophilus influenzae (NTHi), can scavenge dietary Neu5Gc and incorporate it into its own glycolipids. There is evidence that young human infants are “xeno-autoimmunized” against Neu5Gc by early H. influenzae infection and this method has also been utilized for experimental immunization of Cmah(−/−) mice in the laboratory (35). Immunization thus seems to depend on diets rich in Neu5Gc from red meats or certain marine sources (fish eggs or echinoderms (27, 36). Secondly, unlike other xenoglycans, it is important to stress that the monosaccharide Neu5Gc itself is immunogenic, none of the constituent monosaccharides of alpha-Gal (galactose) or ABO antigens (fucose, galactose, N-Acetylgalactosamine, and N-Acetylglucosamine) are foreign to individuals lacking these structures and once ingested, they are incorporated as non-antigenic glycans or metabolized (37). The antigenicity of other xenoglycans, is largely determined by glycosidic linkages, rather than by the nature of the monosaccharide: galactose-alpha-1,3-galactose for alpha-Gal; fucose-alpha-1,2-galactose for the H antigen; H antigen with N-Acetylgalactosamine for A antigen; or H antigen with alpha-1,3-galactose for B antigen. Thirdly, unlike the other immunogenic glycans, Neu5Gc can be part of numerous different antigens depending on the identity of the sialoglycoconjugate they occur on. Finally, sialic acids are one to several orders of magnitude more abundant than either alpha-Gal or ABO glycans (38, 39). These three differences: dietary origin, antigenicity of the monomer itself, and ubiquity/abundance on the cell surface make Neu5Gc a unique antigen, whose loss may lead to wide-ranging physiological effects (37, 38).
While humans have many dietary sources for Neu5Gc, among the New World primates, there are very few species that eat vertebrate meat. Capuchin monkeys (genera Cebus and Sapajou) are known to prey on young coati (40), relatives of racoons belonging to the family of Procyonidae, and on lizards or birds, but these prey species all lack endogenous Neu5Gc (5, 41). It is thus very unlikely that these New World primates are immunized against Neu5Gc in the wild, but captive capuchin monkeys may be exposed to Neu5Gc through monkey chow containing red meat (Primate Info Net, University of Wisconsin). Hedgehogs and other insectivores, consume mostly insect prey that lack sialic acids and thus can be safely expected not to be naturally immunized against Neu5Gc (6). The same can be said for the different bat species that lack Neu5Gc, as these all feed on insects, fruit, or nectar (42). Pinnipeds (seals, sea-lions and walruses) are all strict carnivores and some of their prey include fish and marine invertebrates that could contain Neu5Gc (43). Studies of pinniped immune responses to sialic acids are urgently needed. The one species of whale also lacking Neu5Gc is the sperm whale (Physeter catodon) (6), whose diet consists mostly of giant squid and other cephalopods (squid and octopus) with occasional fish (44). Again, such a diet is unlikely to expose sperm whales to large amounts of Neu5Gc (45), leading to the prediction that they will not have circulating antibodies against the xenoglycan. Mustelids are the one group of species for which it can be assumed that dietary exposure and immunization occurs, as they are all known to feed on a variety of small mammals and vertebrates (46).
Evolutionary Mechanisms for the Fixation of Loss-of-function Mutation
The loss-of-function mutations of the Cmah gene are by definition recessive, as one copy of the functional gene suffices to generate a Neu5Gc positive phenotype in a diploid organism.
Balancing Selection Maintaining Polymorphisms
Some animals, including several dog and cat breeds, are polymorphic for Neu5Gc expression. While overall tissue expression is thought to be low, expression on blood cells in these animals can be high (47, 48). Polymorphisms involving Neu5Gc on the ganglioside GD3 exist in felids and are called AB blood groups in domestic cats (not related to primate ABO blood groups), where cats lacking Neu5Gc-GD3 have circulating antibodies specific for Neu5Gc (47). Dog breeds also differ in their expression of Neu5Gc on red blood cell glycolipids (48).
Due to the recessive nature of loss of function mutations, their increase in frequency within a population must be mediated by selection on homozygous carriers, who have fitness advantages conferring higher survival and/or reproductive success. For example, selection for polymorphisms involving a loss-of-function mutation could be based on the accompanying ablation of the glycan used as a receptor by pathogens (49–52).
In contrast a number of human-specific pathogens evolved specificity for Neu5Ac, including the causative agent of human malignant malaria P. falciparum,the toxins of cholera agent V. cholerae(56) and typhoid fever agent S. typhi (57), and most influenza A viruses (58). Loss-of-function mutations, especially in polymorphic populations, could also provide partial protection from enveloped viruses that bear the antigenic glycan acquired from the cell membrane of the previous, Neu5Gc positive host. The latter mechanism would be analogous to such protection in alpha-Gal negative Old World primates (59–61) and across ABO mismatched humans (62–65). Such protective mechanisms are thus observed both, between species and within species with existing (balanced) polymorphisms. Balanced polymorphisms are maintained by frequency-dependent selection, i.e., selection favoring the rare variants, thus preventing their extinction but also preventing their fixation (see Figure 3) (66). Such dynamic co-evolutionary processes between pathogens and their hosts are the inspiration behind the terms evolutionary arms race and Red-Queen effects (15, 67).
Fixation of the loss-of-function allele, on the other hand, could happen either via directional selection or genetic drift, where small founder populations consist mostly of homozygous carriers of the loss-of-function mutation. First defined using plants in 1962 (68) and recently applied to primates by Galili (65), the idea of “catastrophic selection” combines these ideas with very strong selection. It is not clear how such “catastrophic selection” differs from short episodes of strong selection, possibly accompanied by demographic bottlenecks, which could also result in the fixation of loss-of-function mutations. Alternatively, selective pressure for Cmah loss-of-function could occur through reproductive conflict as discussed below.
Female Immune-Mediated Selection Against Paternal Neu5Gc
Mammalian sperm are highly sialylated as a mechanism to enhance sperm survival and function along the perilous journey through the female reproductive tract to the site of fertilization in the oviduct (69–71). Mammals make anti-sperm antibodies when directly exposed to sperm (72). Major human sperm antigens include, highly sialylated GPI-anchored glycoproteins such as CD52 (73, 74), which in males that have a functional CMAH allele, carry Neu5Gc (75). Theoretically, immunization of females homozygous for the loss-of-function allele of Cmah could occur via insemination by males that have Neu5Gc-bearing sperm. Indeed, we have shown experimentally, using Cmah(−/−) mice immunized against Neu5Gc, that their immune response against Neu5Gc bearing sperm severely reduces female fertility (8). In a further study, we demonstrated that Neu5Gc bearing sperm, both, sperm from either wild type mice or from Cmah(−/−) mice exposed to seminal fluid from wildtype mice containing Neu5Gc-rich CD52, are both targeted by antibodies and are increasingly phagocytosed by female uterine immune cells (75). These insights have potential relevance for human fertility where Neu5Gc or anti-Neu5Gc antibodies in the reproductive tract are common among infertility patients, but not healthy controls (29).
In addition to blocking fertilization, it is possible that anti-Neu5Gc immunity from a primed CMAH(−/−)mother (29) could also negatively affect a CMAH(+/−)embryo or fetus in a manner similar to hemolytic diseases of the newborn caused by ABO glycan mismatches.
Reproductive xenosialitis could thus be a plausible mechanism mediating directional selection, leading to the fixation of the loss-of-function allele in the population, irrespective of the mechanism(s) involved for the initial selection favoring the mutation (see Figure 4).
Conclusions and Perspectives
It is interesting that watershed events, such as the loss of Neu5Gc from the glycocalyx of human cells have occurred numerous times in many mammalian and other vertebrate species. These cases of convergent evolution represent precious opportunities for increased understanding of evolutionary processes. In some respects, Neu5Gc is an ideal self-molecule as it is “private” to vertebrates and, based on current data, has yet to be successfully mimicked by microbes. Against the background of this benefit, the loss of Neu5Gc appears paradoxical and may implicate strong selective regimes, either catastrophically caused by pathogens, or under directional sexual selection via female immunity to paternal xenoglycans. Massive genetic drift, or combinations of milder selection and founder events, can also not be excluded.
More information on species expected to encounter Neu5Gc in their diets, i.e., mustelids, pinnipeds, and humans, is needed to begin answering several outstanding questions in the field: For instance, what are the potential protective functions of anti-Neu5Gc antibodies in species that lack this sialic acid, especially as regards ongoing protection from cross-species infections by enveloped viruses bearing Neu5Gc on their viral envelopes? Or on the flip-side, what are the potential liabilities of anti-Neu5Gc antibodies due to autoimmunity against incorporated dietary Neu5Gc? Evolutionary events such as the ones discussed here exemplify how glycans, rather than representing the end result of different evolutionary histories and contingencies, can become an evolutionary force of their own and constrain future evolution of entire lineages including subsequent compensatory evolution of glycan binding immune receptors (15, 76).
MA and PG did the research and wrote the paper. PG produced the figures.
Mathers Charitable Foundation, Mizutani Foundation for Glycosciences, Grant 170174.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Keywords: sialic acid, Neu5Gc, Neu5Ac, xenoglycan, anti-glycan antibodies, xeno-antigen
Neu5Gc in red meat and organs may pose a significant health hazard
Neu5Gc, a non-human sialic acid sugar molecule common in red meat that increases the risk of tumor formation in humans, is also prevalent in pig organs, with concentrations increasing as the organs are cooked, a study by researchers from the UC Davis School of Medicine and Xiamen University School of Medicine has found.
The research, published in Glycoconjugate Journal Sept. 9, suggests that Neu5Gc may pose a significant health hazard among those who regularly consume organ meats from pigs.
Neu5Gc is naturally found on cell surfaces in most mammals but not in humans. It gets incorporated into human cells by eating meats, organs and some dairy products.
Previous studies have shown when Neu5Gc is incorporated into human tissues, the immune system recognizes it as a foreign threat, producing antibodies to counter it. Repeated consumption of these meats then causes chronic inflammation, which has been known to increase risks of tumor formation. Neu5Gc has been linked to cancer as well as cardiovascular and other inflammatory diseases, including some bacterial infections.
The UC Davis and Xiamen University study is the first to find Neu5Gc in substantially higher levels in pig organs, including the spleen, lungs, heart, kidney and liver, than in skeletal muscle, which cooking of the meat exacerbates.
“We were rather surprised that organ meats from pigs have alarmingly high levels of Neu5Gc,” said co-author Frederic A. Troy II, professor and chair emeritus in the Department of Biochemistry and Molecular Medicine and adjunct professor at Xiamen University School of Medicine.
“Although we do not know quantitatively what levels trigger an immune response, if you’re going to eat organ meat, you’re going to have a potentially greater risk of certain inflammatory diseases,” he said.
The risk of Neu5Gc toxicity is particularly high in China and other countries where people tend to consume large amounts of organ meat. In the U.S. and other western nations where there has been a rise in the culinary “nose-to-tail” movement in recent years, more chefs are cooking all parts of animals. Given the results from this study, the authors urge people to be cautious about the types of meats they ingest.
Troy and colleagues recently reported high levels of the free form of another sialic acid, Kdn, in breast, cervical, liver, lung, throat, ovarian and uterine cancers. They also showed that a polymeric form of Neu5Ac, a polysialic acid, is a metastatic factor when expressed on the cell surfaces of a number of human cancers.
For the current study, the researchers assessed the levels of three sialic acids ― Neu5Ac, Neu5Gc and Kdn ― located at the end of sugar chains frequently attached to glycoproteins and gangliosides in cellular membranes.
Glycoproteins, such as polysialylated neural cell adhesion molecules (NCAMs), have many functions during brain development that modulate cell-cell adhesive interactions involved in synaptogenesis, neural plasticity, myelination and neural stem cell proliferation and differentiation. Gangliosides serve as markers for cellular recognition and modulate axon-myelin interactions, axon stability, axon regeneration and modulate nerve cell excitability.
The researchers measured sialic concentrations in pig spleens, kidneys, lungs, hearts, livers and muscle at three, 38 and 180 days (adult) of age. Compared to skeletal muscle, the concentrations of Neu5Gc were high in all organs, particularly heart, spleen, kidney and lungs. Cooking increased sialic levels in most organ tissues.
Though the study was conducted in pigs, these results have ramifications for organ meat from other animals.
“The basic, fundamental biochemical pathways for synthesis and metabolism of the sialic acids are essentially identical processes common in all evolutionary species from ‘bacteria to brains,’ Troy said. “Therefore, the translational aspect of our findings to other mammalian species is essentially a given from a biochemical perspective.”
In contrast to mice and rats, neonatal pigs are genetically closer to humans, and share similar physiology and anatomical structures with human infants. Importantly, the piglet brain more closely resembles the human brain in anatomic structure and developmental growth patterns, Troy added.
The study also sheds light on the developmental biology of sialylation, as the molecular mechanisms regulating the age-related developmental expression and function of the sialic acids are poorly understood.
“Our new findings show that there are clear changes in levels of these sialic acids in young and adults pigs as a function of aging, a finding that is neither well understood nor has been previously reported,” he said.
While it’s long been known that sialic acids have higher concentrations in animal meats, no one had ever precisely measured their concentrations in specific organs. To some degree, this was a result of technology.
“This study would not have been possible if not for the high sensitivity afforded by LC mass spectrometry,” Troy noted. “This advance in structural analysis thus allows studies that could not have been done five or 10 years ago.”
The research study is entitled “Developmental changes in the level of free and conjugated sialic acids, Neu5Ac, Neu5Gc and KDN in different organs of pig: a LC-MS/MS quantitative analyses.” Other authors included Suna Ji, Fang Wang, Yue Chen, Changwei Yang, Panwang Zhang, Xuebing Zhang and Bing Wang, all from Xiamen University.
This research was funded by grants from the Xiamen University School of Medicine,
N-Glycolylneuraminic acid (Neu5Gc) is a sialic acid molecule found in most non-human mammals. Humans cannot synthesize Neu5Gc because the human gene CMAH is irreversibly mutated, though it is found in apes. It is absent in human tissues because of inactivation of gene encoding CMP-N-acetylneuraminic acid hydroxylase. The gene CMAHencodes for CMP-N-acetylneuraminic acid hydroxylase, which is the enzyme responsible for CMP-Neu5Gc from CMP-N-acetylneuraminic (CMP-Neu5Ac) acid. This loss of CMAH is estimated to have occurred 2-3 million years ago, just before the emergence of the genus Homo.
Neu5Gc is closely related to the commonly known N-acetylneuraminic acid (Neu5Ac). Neu5Ac differs by a single oxygen atom that is added by the CMAH enzyme in the cytosol of a cell. In many mammals, both of these molecules are transferred into the Golgi so that they may be added to many glycoconjugates. However, in humans, Neu5Gc is not present.
With the loss of Neu5Gc gene and gain of excess Neu5Ac, it should have affected the interactions of pathogens and humans. Humans should have been less susceptible to Neu5Gc-binding pathogens and more susceptible to Neu5Ac-binding pathogens. It is suggested that human ancestors lacking Neu5Gc production survived a then-prevailing malariaepidemic. However, with the rise of Plasmodium falciparum, the parasite that causes malaria today, humans were once again endangered as this new strain of the malaria had a binding preference to the Neu5Ac-rich erythrocytes in humans. The latest research shows that humans who lack Neu5Ac on their red blood cells are less likely to get malaria from the parasites that cause it.
Neu5Gc is found in most mammals, with exceptions like humans, ferrets, the platypus, western dog breeds and New World monkeys. Trace amounts can be found in humans, even though the gene to encode for production of Neu5Gc was eliminated long ago. These trace amounts come from consumption of animals in human diet. Mainly, the sources are red meats such as lamb, pork, and beef. It can also be found in dairy products, but to a lesser extent. Some 1.1% of the identified Neu5Ac is actually Neu5Gc in commercial whey protein. Neu5Gc cannot be found in poultry and is found in only trace amounts in fish. This confirms that Neu5Gc is mainly found in foods of mammalian origin. Lanolin in shampoo also contains Neu5Gc.
In 2017, scientists succeeded in indirectly identifying the presence of Neu5GC from multiple ancient animal fossils dated to over a millions years ago, the oldest of which was dated to around 4 Mya.
Even though Neu5Gc is not known to be produced by any mechanism in the human body (due to lack of genes), our bodies do interact with trillions of microorganisms that are capable of complex biological reactions. Neu5Gc is reported to be found in concentration in human cancers, as well as in fecal samples, suggesting that humans ingest Neu5Gc as part of their diets. Uptake is thought to be by macropinocytosis, and the sialic acid can be transferred to the cytosol by a sialin transporter. It is possible that the immune system then recognizes the molecule as foreign, and that the binding of anti-Neu5Gc antibodies may then cause chronic inflammation. This assumption has yet to be concretely proven, however. Further studies have shown that humans have Neu5Gc-specific antibodies, often at high levels. Feeding Neu5Gc knockout mice Neu5Gc-rich diets along with anti-Neu5Gc antibodies (attempting to mimic a human system) causes systemic inflammation in the mice, and they are five times as likely to develop hepatocarcinomas.. However, a study released in September 2018 found no evidence that exposure to higher levels of anti-Neu5gc antibodies increased colon cancer risk. 
A baseline excretion of Neu5Gc exists, and it is incorporated into all body parts, some of which—mucins, hair, saliva, serum and blood, are commonly excreted. Neu5Gc is rapidly absorbed in the intestinal tract, some of which is converted to acylmannosamines by intestinal cells and bacteria, and reconverted back to Neu5Gc in the body. According to an absorption study, about 3–6% of the ingested dose of Neu5Gc was excreted within 4–6 hours, with the peak excretion rate at 2–3 h and a return to baseline levels within 24 h. In mucins, an increase was seen from day 1 to 4, with increased also found in hair after ingestion. This table and this table(S3) shows levels of Neu5Gc in common foods.
Sialic acids are negatively charged and hydrophilic, so they don’t readily cross the hydrophobic regions of cellular membranes. It is because of this that the uptake of Neu5Gc must occur through an endocytic pathway. More specifically, exogenous Neu5Gc molecules enter cells through clathrin-independent endocytic pathways with help from pinocytosis. After the Neu5Gc has entered the cell via pinocytosis, the molecule is released by lysosomal sialidase. The molecule is then transferred into the cytosol by the lysosomal sialic acid transporter. From here, Neu5Gc are available for activation and addition to glycoconjugates. Because Neu5Gc appears to be enhanced in naturally occurring tumors and fetal tumors, it is suggested that this uptake mechanism is enhanced by growth factors.
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