An international team of researchers has identified a direct molecular link between meat and dairy diets and the development of antibodies in the blood that increase the chances of developing cancer.
This connection may explain the high incidence of cancer among those who consume large amounts of dairy products and red meat, similar to the link between high cholesterol and an increased risk of heart disease.
The study was led by Dr. Vered Padler-Karavani of the Department of Cell Research and Immunology at the Shmunis School of Biomedicine and Cancer Research at Tel Aviv University’s George S. Wise Faculty of Life Sciences.
The results of the research were published on September 23, 2020, in BMC Medicine.
Neu5Gc is a sugar molecule found in the tissues of mammals but not in poultry or fish.
Humans develop antibodies to Neu5Gc in infancy, when they are first exposed to dairy and meat products. While it is known that these antibodies increase the risk of cancer, especially colorectal cancer, no direct link had been found between the antibodies and meat and dairy consumption.
For the study, the researchers used samples from NutriNet-Santé, an extensive national nutritional survey conducted in France. Salam Bashir, a Ph.D. student in Dr. Padler-Karavani’s lab, together with other team members measured the amount of Neu5Gc sugar in a variety of dairy and meat foods common in the French diet and calculated the daily Neu5Gc intake of 19,621 adults aged 18 and over, who reported all of their food intake online over a period of several days.
The research team then took a representative sample of 120 participants and tested the levels of the anti-Neu5Gc antibodies in their blood.
Based on these findings and the quantification of Neu5Gc sugar in various food products from France, Dr. Padler-Karavani and her team created an index called the Gcemic index.
This index ranks foods whose excessive consumption can lead to an increase in the antibodies—and possibly to an increase in the risk of cancer.
“We found a significant correlation between high consumption of Neu5Gc from red meat and cheeses and increased development of those antibodies that heighten the risk of cancer,” Dr. Padler-Karavani says.
“For years there have been efforts to find such a connection, but no one did. Here, for the first time, we were able to find a molecular link thanks to the accuracy of the methods used to measure the antibodies in the blood and the detailed data from the French diet questionnaires.”
Dr. Padler-Karavani adds that this combination of methods allowed the researchers to predict that those who eat a lot of red meat and cheese will develop high levels and a different variety of the antibodies, and therefore may be at higher risk for cancer – especially colorectal cancer, but other cancers as well.
Nutrition can dramatically affect health, and different dietary habits have been associated with various human diseases such as cancer, cardiovascular diseases, type II diabetes, obesity, and hypertension [1–4]. In particular, high consumption of red meat has been frequently suggested as a risk factor for human cancers and cardiovascular diseases [1–4].
Although various mechanistic explanations have been proposed (e.g., high energy/fat diet, N-nitroso, nitrates, nitrites, heme iron, compounds produced by gut microbiome or during cooking), none seems to be specific for red meat or dairy . Recently, based on limited evidence in humans, the non-human carbohydrate N-glycolylneuraminic acid (Neu5Gc) that is present in mammalian-derived food (i.e., red meat and dairy) has been implicated as a new risk factor for colorectal cancer .
Neu5Gc is a common sialic acid type of sugar in mammals. It is a nine-carbon negatively charged monosaccharide that can be synthesized by most mammals and found at the tips of carbohydrate chains (glycans), glycoproteins, and glycolipids . Humans cannot synthesize Neu5Gc due to a deletion in the CMAH gene that encodes the cytidine 5′-monophosphate-Neu5Ac hydroxylase .
Yet, dietary Neu5Gc can be consumed then incorporated at low levels onto human cell surfaces, particularly in cancer, consequently displaying a broad assortment of immunogenic Neu5Gc-glycans [6, 8]. In fact, all humans examined thus far have a diverse collection of polyclonal anti-Neu5Gc antibodies [7, 9, 10].
Thus, circulating anti-Neu5Gc antibodies continuously encounter Neu5Gc-containing epitopes on human tissues and have been proposed to lead to xenosialitis , which in mice have been shown to exacerbate cancer [11, 12] and cardiovascular disease .
Diverse feeding methods in human-like Neu5Gc-deficient Cmah−/− mice failed to recapitulate diet induction of anti-Neu5Gc antibodies that supposedly occur in humans [11, 14], and those had to be generated by immunization to allow their investigation in mice [11, 12].
Yet, in human studies, glycan microarray analysis revealed that certain anti-Neu5Gc antibodies can serve as a carcinoma biomarker  and that high levels of total anti-Neu5Gc IgG are associated with increased colorectal cancer risk, but not with red meat intake .
Altogether, the co-existence and interactions between Neu5Gc on cells with circulating anti-Neu5Gc antibodies have been suggested to modulate inflammatory response characteristics to meditate diseases [5, 17]; however, a direct correlation between anti-Neu5Gc antibodies and the diet in humans has been elusive.
Neu5Gc on human tissues and cells most likely originate from various dietary sources, given the absence of an alternative biosynthetic pathway to the CMP-Neu5Ac hydroxylase. Food items derived from mammals contain glycoproteins and glycolipids, many of which are covered with sialic acids.
The two most common sialic acids in mammals are N-acetylneuraminic acid (Neu5Ac) and its hydroxylated form Neu5Gc, and their levels vary in different organisms and tissues . While Neu5Ac is a native “self” carbohydrate in humans, Neu5Gc is a non-human immunogenic carbohydrate .
Neu5Gc is abundant in red meat and dairy, while scant in some fish, and non-existent in chicken [11, 18]. In this study, we investigated the dietary effects on the global burden of world colorectal cancer, and the effects of dietary Neu5Gc on the levels and repertoires of circulating anti-Neu5Gc antibodies in humans using the French NutriNet-Santé cohort based on detailed 24-h dietary records, in order to provide a mechanistic explanation for the cancer risk associated with red meat consumption.
We further validated our findings by affinity purification of such antibodies and detailed glycan microarray analysis. Based on these findings, we developed tools to assess diet-related induction of anti-Neu5Gc IgG and tools for personalized dietary recommendations related to Neu5Gc intake.
International colorectal cancer (CRC) and red meat intake correlate in different nations
While many epidemiological studies support increased cancer risk with high meat intake, we wanted to explore this relationship at the national level, based on the available global national meat consumption and cancer risk data. Dietary habits can vary dramatically in different parts of the world .
To evaluate the effect of red meat intake on CRC in different world nations, national per capita meat intake from the Food and Agriculture Organization (FAO) of the United Nations, FAOSTAT database , and CRC age-standardized incidence and mortality rates from GLOBOCAN database  were extracted (Fig. 1; Additional file 2: Data file S1). Both CRC incidence and mortality positively correlate with meat intake (Fig. 1a).
CRC rates were lowest in Sri Lanka, India, and many African nations, while highest in Australia, USA, Europe, and South American nations (Additional file 2: Data file S1), showing dramatic differences between the highest and lowest meat intake quartiles (Fig. 1b).
Gender had no effect on CRC rates in nations of the lowest meat intake quartile (20.25 ± 1.17 g/capita/day, mean ± sem), but in nations of the highest meat intake quartile (156.4 ± 3.40 g/capita/day), there were higher incidence and mortality rates in men compared to women (Fig. 1c). These findings are consistent with international comparisons of cancer risk conducted on a limited number of nations > 40 years ago .
These striking age-standardized correlations do not account for other confounding factors (e.g., weight, physical activity, smoking, alcohol); however, there is vast literature supporting meat-related cancer risk, particularly in CRC, the third most common cancer worldwide [36–39].
As a result, meat was recently classified as carcinogenic by The International Agency for Research on Cancer [IARC; Consumption of processed meat was classified as carcinogenic (group 1), while consumption of red meat as probably carcinogenic to humans (group 2A)] .
Furthermore, in the third expert report on diet, nutrition, physical activity, and cancer of the World Cancer Research Fund (WCRF) and the American Institute for Cancer Research (AICR), strong evidence was found for the roles of processed meat and red meat in CRC risk, both judged to be “convincing” and “probable,” respectively .
The Continuous Update Project (CUP) from the leading authority WCRF is the world’s largest and most updated resource on cancer prevention, adjusted for body mass index (BMI or body fatness for some studies) and alcoholic drinks, thus excluding such confounding factors and strongly supporting the role of meat consumption in CRC risk .
The meat cancer risk had been partially explained by high-energy/fat Western diet, or various compounds in meat, such as N-nitroso compounds, salts, nitrates, nitrites, heme iron, saturated fat, estradiol, and trimethylamine–N-oxide (TMAO) produced by gut microbiome .
More recently, the non-human immunogenic carbohydrate Neu5Gc and the circulating antibodies against it in humans had also been suggested to contribute to meat-related cancer risk [3, 5], mostly relying on studies in mice.
France is among the top 15 nations of high meat intake (Additional file 2: Data file S1) with a confirmed meat-related risk of CRC  and breast cancer  even after adjustment of confounding factors such as alcohol consumption and BMI.
In the French prospective NutriNet-Santé cohort study, red meat intake was associated with increased overall cancer risk (HRQ5 vs. Q1 = 1.31; 95% CI 1.10, 1.55; ptrend = 0.01) and increased breast cancer risk (HRQ5 vs. Q1 = 1.83; 95% CI 1.33, 2.51; ptrend = 0.002) . We used the NutriNet-Santé cohort to further investigate the relationship between Neu5Gc and anti-Neu5Gc antibodies with meat and dairy intake in a qualitative and quantitative manner.
Evaluating levels of daily Neu5Gc intake from red meat and dairy
The amounts of Neu5Ac and Neu5Gc were accurately quantitated in diverse food items (Additional file 1: Table S1). On average, Neu5Ac content was ~ 3 times greater than Neu5Gc (414 ± 58 nmol/gr versus 149 ± 30 nmol/gr, respectively; mean ± sem). Neu5Gc content was highest in dairy sheep and goat products, moderate in red and processed meat, but rather low in dairy cow (422 ± 10 nmol/gr, 118 ± 17 nmol/gr, 21 ± 1 nmol/gr, respectively).
Yet daily dietary Neu5Gc intake relies on actual amounts of food consumed by individuals (e.g., common beef steak serving size is ~ 225 g/day, while much lower for dairy). To account for individual records, daily Neu5Gc intake was calculated from all available NutriNet-Santé participants enrolled between May 2009 and May 2015 and that had a minimum of six 24-h dietary records (16,149 participants of 19,621 registered; Fig. 2a). Based on these questionnaires and Neu5Gc measurements in food, daily Neu5Gc intake was calculated per participant (Fig. 2b), then quartiles of total dietary Neu5Gc intakes were computed by gender and age (Q1–Q4; Additional file 1: Table S2; Fig. 2a).
For further detailed analysis, 120 representative individuals who provided at least 18 dietary records, and had available blood samples, were randomly selected (Table 1; Fig. 2a). This focused cohort of 120 individuals included 10 men and 10 women aged 45–60 per Neu5Gc intake quartile by gender (Q1–Q4; 80 samples) and 10 men and 10 women aged > 60 per quartile, from the first and fourth quartiles by gender (Q1 and Q4; 40 samples) (Fig. 2a, c, d, Table 1).
Selected men and women were matched for age, education levels, and smoking habits (Table 1), but nutritional habits and intakes were expected to vary across gender [41, 42]. Accordingly, in this cohort, there were statistically significant differences between men and women in energy intake and intake of proteins, animal proteins, lipids, and carbohydrates (Table 1).
Hence, the total daily Neu5Gc intake in this study cohort was first computed by gender and age (Fig. 3a; Additional file 1: Table S2). Generally, daily Neu5Gc intake was largely contributed from cow’s dairy and meat (33% and 25%, respectively), then from pig’s meat (16%), goat’s dairy (13%), sheep’s dairy (11%), and lamb’s meat (2%). Hence, dietary Neu5Gc was also divided into three sub-classes based on the contributing food source (red meat, dairy cow, and dairy sheep or goat; Fig. 3a). Total daily Neu5Gc intake was significantly higher in men versus women aged 45–60, largely contributed due to higher consumption of red meat. Similar trends were found in the > 60 age group, though differences were not statistically significant (Fig. 3a). There were no significant differences in dairy consumption between men and women in both age groups.
Table 1 – General characteristics of the study cohort of 120 representative individuals, each with at least eighteen 24-h dietary records (means ± SD for continuous variables; relative frequencies for qualitative variables)
|Characteristics||Age group (years)|
|Age, years||57.0 ± 4.5||58.0 ± 4.4||0.27||70.3 ± 4.6||69.2 ± 4.7||0.45|
|Primary or less||0||5||0.44||5||15||0.55|
|Energy intake, kcal/day||2411 ± 450||1774 ± 336||0.0001||2247 ± 387||1780 ± 252||0.0001|
|Proteins intake, g/day||94.1 ± 18.6||74.1 ± 16.9||0.0001||93.3 ± 18.4||74.0 ± 12.5||0.0004|
|Animal protein intake, g/day||62.1 ± 16.7||49.3 ± 16.0||0.0008||62.3 ± 18.1||49.5 ± 11.5||0.01|
|Lipids intakes, g/day||102.3 ± 23.0||74.9 ± 16.3||0.0001||92.6 ± 20.3||80.3 ± 18.1||0.051|
|Carbohydrates intakes, g/day||252.5 ± 64.8||189.6 ± 46.1||0.0001||226.2 ± 56.6||174.9 ± 33.9||0.001|
|Number of 24-h dietary records||21.4 ± 2.9||21.6 ± 3.3||0.83||21.7 ± 3.2||22.9 ± 3.4||0.26|
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7510162/
More information: Salam Bashir et al, Association between Neu5Gc carbohydrate and serum antibodies against it provides the molecular link to cancer: French NutriNet-Santé study, BMC Medicine (2020). DOI: 10.1186/s12916-020-01721-8