I love kimchi, and this year during Spring Festival I went to South Korea to try the real deal. But when I got there my friend living in Seoul informed me that kimchi is (a) not vegan and (b) may be linked to stomach cancer. What!?
我非常喜欢吃韩国泡菜。今年春节时，我去了韩国品尝这道佳肴。但是到了那儿之后，一位首尔的朋友却对我说，(1) 泡菜不是素食食品，(2) 可能导致胃癌。什么情况！？
Kimchi has been previously rated one the world’s top 5 healthiest foods. Originally meant to keep people well nourished during long Korean winters, it contains high levels of vitamin C, calcium, and iron. Since kimchi is fermented it contains plentiful friendly bacterial strains, so it’s a good alternative to dairy-based probiotics like yogurt.
Kimchi and Additives 泡菜与添加剂
Back in California a friend taught me to make kimchi by combining cabbage, garlic, ginger, chili peppers, and salt in a jar. Apparently most varieties served in restaurants use the fish sauce or shrimp instead of salt, albeit in small amounts. I can’t really taste the difference between the two recipes.
More concerning is the presence of additives in some types of commercially prepared kimchi. Korea has blocked imported kimchi from China over safety concerns about artificial sweeteners and banned colorings 1 and Japan invented fake processed food kimchi. They used citric acid to replace the flavor produced by the fermentation process, which shortens production time but of course eliminates the probiotic benefits. 2 For kimchi fans like me, the best option is probably making it at home (very easy), or finding restaurants that prepare it fresh.
Kimchi and Cancer 泡菜与癌症
More recently, some studies have drawn links between high stomach cancer rates in South Korea and kimchi, citing kimchi as a risk factor. As with most nutritional studies, the relationship is complex. Let’s break it down:
Note: If your eyes start to glaze over, skip to end for conclusions
Gastric (stomach) cancer is most prevalent type of cancer in Korea 3. Koreans also have a far higher rate of stomach cancer than Korean Americans, suggesting that environmental rather than genetic factors are at play. 4
According to the World Institute of Kimchi, Koreans eat 50-100 g/d of kimchi in the summer and 150-200 g/d per day in the winter, accounting for 12.5% of total daily food intake. Kimchi contain relatively high levels of nitrates, a known precursor to carcinogenic compounds 5 6
However, it’s unlikely the nitrates in plant foods are actually harmful. One study done in Mexico City showed that it was the total level of nitrites or nitrates/nitrities from animal sources that increased risk, while total intake nitrates or nitrites/nitrates from fruits and vegetables decreased risk. 7 In another study done in Korea, it wasn’t absolute nitrate intake but a higher ratio of nitrate to antioxidant and vitamin intake (i.e. fruit and vegetable) that was posed a risk for gastric cancer. 8
The one study that analyzed kimchi and reported high levels of N-nitroso (the carcinogenic compound) found this after the kimchi was mixed with nitrites in “simulated human stomach conditions.” This at best is an imperfect proxy for how much N-nitroso is truly formed in the body after eating kimchi 9.
Another element of kimchi that may contribute is high salt levels. In addition to the high prevalence of gastric cancer, Koreans have some of the highest 24-hr sodium excretion levels (a proxy for sodium intake) in the world 10 A study examining the role of dietary factors in Korea and stomach cancer concluded that a high salt diet was correlated with stomach cancer. 11 In 2003, another study found that high intakes of salt-fermented fish and kimchi were associated with gastric cancer. The same study reported that patients with both heliobacter pylori infection and high salty preference had a 10 fold higher risk of early gastric cancer risk compared to those with no infection and a low-salty preference. 12
So what does it all mean?
In sum, I think if you stick to low-salt kimchi, there’s not much of a risk. If you are truly worried about gastric cancer, get tested for H. Pylori infection, reduce your salt intake, eat more fresh fruits and vegetables, and reduce known animal sources of nitrites and nitrates in your diet, such as processed meats.
So where did the hype come from? In 2005 a study entitled “Kimchi and Soybean Pastes are risk factors for Gastric Cancer” came out in the World of Gastroenterology, in which they concluded that the heavy kimchi eaters have a ~50% higher risk of gastric cancer. 13 (Note: This was a case-control study, so they were comparing people who already had gastric cancer with another group of statistically age-gender similar people without gastric cancer. Of the people with gastric cancer, 60% reported eating lots of kimchi in the previous year, 40% reported eating not that much kimchi in the previous year. In the controls, this number was split 50/50 between high and low kimchi. This is how they concluded that there was a 50% higher risk of gastric cancer associated with “high kimchi” eating. Check out this link on the odds ratio if you want to know how they came up with this number).
Naturally, after reading the title and results you would assume that kimchi and soybean paste = bad. But the study doesn’t mention other potential risk factors like processed meat consumption or H. pylori infection. Moreover, they also concede that the high salt level in kimchi and soybean paste may be one factor accounting for the relationship. In the future, I think a study that includes high and low salt kimchi varieties, animal nitrates and nitrites, and ratio of these foods with fresh fruits and vegetables would be illuminating on this topic.
As with most nutrition research, the interpretation generally focuses myopically on one food item. But the categorization of one food or another as a “risk factor” is really a distraction from the main picture. Forget about kimchi: stop smoking, start exercising, sleep well, eat fresh fruits and vegetables, and ignore the latest nutrition headlines.
- Safety Concerns of China-made kimchi. The Standard. Hong Kong, 10/5/2008. ↩
- Sandor Ellix Katz. Wild Fermantation, the flavor, nutrition, and craft of live-culture foods, pg 46. ↩
- Shin HR, Ahn YO, Bae JM, ShinMH, Lee DH, et al.: Cancer incidence in Korea. Cancer Res Treat 34, 405–408, 2002 ↩
- Gomez SL, LE GM, Clarke CA, et al. Cancer incidence patterns in Koreans in the US and in Kangwha, South Korea. Cancer Causes Control. 2003. Mar, 14,2: 167-74. ↩
- After ingestion, the body converts nitrates into nitrites and then into N-nitroso carcinogenic compounds. ↩
- Knekt P, Jarvinen R, Dich J, Hakulinen T. Risk of colorectal and other gastro-intestinal cancers after exposure to nitrate, nitrite and N-nitroso compounds: a follow-up study. Int J Cancer 1999;80:852–6. ↩
- Hernandez-Ramirez RU, Galvan-Portillo MV, Ward MH, et al. Dietary Intake of Polyphenols, Nitrate and Nitrite, and Gastric Cancer Risk in Mexico City. Int J Cancer. 2009. Sep 15. 125,6:1424-30. ↩
- Ja KH, Lee SS, Choi BY, Kim MK. Nitrate intake relative to antioxidant vitamin intake affects gastric cancer risk: a case-control study in Korea. Nutr Cancer 2007;59:185–91. ↩
- N-nitroso compounds in two nitrosated food products in southwest Korea. Food Chem Toxicol. 1994. Dec. 32,12:1117-23 ↩
- Joossens JV, Hill MJ, Elliott P, Stamler R, Lesaffre E, Dyer A, Nichols R, Kesteloot H. Dietary salt, nitrate and stomach cancer mortality in 24 countries. European Cancer Prevention (ECP) and the INTERSALT Cooperative Research Group. Int J Epidemiol 1996; 25: 494-504. ↩
- Lee JK, Park BJ, Yoo KY, Ahn YO. Dietary factor and stomach cancer: a case study in Koreaa. Int J Epidemiol. 1995. Feb, 24,1: 33-41. ↩
- Lee SA, Kang D, Shim KN, et al. Effect of diet and Helicobacter pylori infection to the risk of early gastric cancer. J Epidemiol. 2003 May. 13,3:162-8. ↩
- Hong-Mei N, Jin-Woo P, Young-Jin S, et al. Kimchi and soybean pastes are risk factors of gastric cancer. World J Gastroneterol. 2005. 11,21:3175-81. ↩