Can Fermented Foods Be Harmful For My Body?


5 Min Read

While fermented foods are largely beneficial, there are a few scenarios where they can be harmful. Some fermented foods may contain high levels of carcinogenic aldehydes. Incomplete fermentation can leave traces of toxins behind. Lack of hygiene or non ideal fermentation conditions can cause contamination.

Fermentation of food is a simple means of food preservation. Through fermentation, food is exposed to fermenting microorganisms (bacteria and yeasts), either via inoculation or naturally through the air. Traditional fermented foods are mostly, naturally made with fresh, organic ingredients. Fermented foods form an integral part of the daily diet of humans across the world as they offer some wonderful and unparalleled benefits in addition to preservation – beneficial probiotics, enzymes, vitamins, minerals and different tastes and textures.

Some Popular Fermented Foods And Their Benefits

Other than the staples – bread, cheese, wine and tea, here are some popular fermented foods from across the world:

  • Yoghurt is rich in pro-biotic bacteria, which can help reduce cholesterol levels
  • Kefir is rich in beneficial micro-organisms that contribute to a healthy immune system. It has been used in the treatment of tuberculosis and cancer
  • Koumiss is known for preventing pulmonary tuberculosis
  • Natto is a popular dish in Japan, made from fermented Soybeans. Natto may help prevent people from having brain hemorrhages
  • Douchi is a popular dish in China and helps lowers high blood pressure
  • Korean specialty Kimchi prevents constipation, colon cancer and reduces serum cholesterol
  • Pulque, one of the oldest alcoholic beverages prepared from the juices of cactus plants in Mexico, is rich in vitamins such as thiamine, riboflavin, niacin and biotin

Can Fermented Foods Be Harmful?

While fermented foods come with a multitude of benefits, there are a few scenarios where they can be harmful as well.

Aldehyde Poisoning

Aldehydes are intermediate products created during fermentation. They are highly reactive and are therefore toxic. Acetaldehyde is classified as a probable human carcinogen linked to nose and throat irritation and cancer. Acetaldehyde is also a toxicant to the neurological (neurotoxin), respiratory, endocrine and immune systems. Animal research shows that this chemical crosses the placental barrier, causing skeletal deformities, reduced birth weights and infant deaths [1].

Some reports claim that fermented foods such as Kombucha tea and some pickles may contain high levels of aldehydes. However, other reports counter these claims, suggesting that any fermented product always contains significantly higher amount of the acids and/or alcohols that are the outputs of the fermentation process and small amounts of the intermediate products such as aldehydes. Kombucha tea, when fermented in the typical manner, predominantly contains acetic acid and small amounts of ethanol (and even smaller amounts of acetaldehyde). Indeed, even within our bodies, when energy is produced from sugars, aldehydes are generated as intermediate products.

Aldehyde poisoning is realistically only possible, if for some reason, the intermediate aldehydes are not converted to the eventual end product of fermentation. This could be due to incomplete fermentation.


Cereals, legumes, and tubers that are used for the production of fermented foods may contain significant amounts of antinutritional or toxic components (such as phytates, tannins, cyanogenic glycosides, oxalates, saponins and lectins) and inhibitors of enzymes (such as alpha-amylase, trypsin and chymotrypsin). These substances reduce the nutritional value of foods by interfering with the mineral bioavailability and digestibility of proteins and carbohydrates.

Fermentation actually helps to reduce or eliminate these toxins and increases nutritional value [2]. Incomplete fermentation can leave traces of toxins behind. For example, soybeans contains significant amount of toxins but Natto, Tempeh and Miso, are fermented soybean products with plenty of benefits.

Fermented soy may offer health benefits due to the fermentation process. However, it may have adverse effects on gastric cancer risk due to high levels of nitrate or nitrite, large amounts of salt, and the loss of key nutrients under acidic and oxygenic conditions. A meta-analysis study found that a high intake of fermented soy foods (such as miso soup and soybean paste) was significantly associated with an increased risk of gastric cancer, while a diet high in non-fermented soy foods (such as soy milk, tofu, soynuts and soybeans) was associated with a decreased risk of gastric cancer [3].


When working with fermented food, proper food-safety precautions (clean hands and clean surfaces) need to be taken. Contamination can happen either before or after fermentation.

It is important to ensure that the raw food is not contaminated in the first place. Vegetables that have been in contact with manure or compost are prone to contamination by pathogens such as E. coli or Salmonella.

Contamination can happen when fermented food is brought in contact with raw foods; this can happen during storage or even during food preparation (fermented foods and raw salads on the same kitchen surface).

Non Ideal Fermentation Conditions

Proper fermentation (and storage of fermented foods) needs somewhat ideal conditions to be maintained. An ideal temperature is essential for eliminate and prevent build up of harmful bacteria.

The conditions tend to be specific for each fermented food. For example, fermented tofu needs to be stored below 41 degrees F in closed containers. Home-fermented tofu and other fermented bean products are the leading cause of botulism poisoning in China.

Processed Fermented Food

Commercial off-the-shelf fermented food typically contains additives and preservatives and is pre-processed before fermentation (to add to shelf life) in a manner that strips the food of most essential minerals, vitamins and micro-organisms that contribute to the nutritious value of fermented food. Also, fermented food that is sold at supermarkets is often sold in packages and cans lined with BPA (an endocrine disruptor).


1. Acetaldehyde. U.S. Environmental Protection Agency.

2. Reddy, N. R., and M. D. Pierson. “Reduction in antinutritional and toxic components in plant foods by fermentation.” Food Research International 27.3 (1994): 281-290.

3. KIM, Jeongseon, et al. “Fermented and non-fermented soy food consumption and gastric cancer in Japanese and Korean populations: A meta-analysis of observational studies.” Cancer science 102.1 (2011): 231-244.


CureJoy Editorial

The CureJoy Editorial team digs up credible information from multiple sources, both academic and experiential, to stitch a holistic health perspective on topics that pique our readers' interest.

CureJoy Editorial

The CureJoy Editorial team digs up credible information from multiple sources, both academic and experiential, to stitch a holistic health perspective on topics that pique our readers' interest.