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One of the latest health-promoting diets to gain attention is a low-sulfur diet due to a suspected sulfur intolerance. Early reports of sulfur intolerance largely stem from food-based additives (e.g., sulfites) associated with asthmatic reactions, diarrhea, hypotension, dermatitis, hives, and even anaphylaxis. While it is understandable that some individuals may want to pinpoint their unusual symptoms related to food, it is important to consider how much validity there is to the science behind any new diet and take a moment to weigh the benefits against the risks.

Unlike some potential triggers in diets, such as lactose or gluten, sulfur is an essential mineral the body needs to perform certain functions, leading some to think it’s impossible to be intolerant to it.

However, there are several sulfur-containing substances to which people may have an allergy, intolerance, or another type of negative reaction:

• Sulfide—a class of minerals that have sulfide, or two sulfide ions, as the major anion.
• Sulfate—a salt that is the reaction between sulfuric acid and another chemical; synthetic sulfates are found in sodium lauryl sulfate and sodium laureth sulfate, which are often used in cleaning and personal care products.
• Sulfite—commonly used in food processing to enhance or preserve food, compounds in this class contain a sulfite ion and are salts of sulfurous acid.
• Sulfa drugs—sulfa may sound like sulfur, but this ingredient found in certain antibiotics and medications is not the same as sulfur or sulfites.

While these additive compounds may contribute to an intolerance to sulfur, this blog will only cover an intolerance to sulfur as it is presented naturally in food and used in the body.

Sources and Roles of Sulfur

A major source of sulfur in the human body is sulfur-containing amino acids, specifically homocysteine, taurine, methionine, and cysteine. These amino acids play many key roles in the body, including the immune system, oxidation reactions, metabolism, and protein structures. However, methionine and cysteine are the only two used in proteins. Methionine is a precursor to SAM (S-adenosylmethionine), while cysteine is a precursor to glutathione and taurine. SAM is an antioxidant, and it is the methyl donor for most methyltransferases that make changes to DNA, RNA, and proteins. Thus, sulfur plays a role in the cell’s methylation processes. Glutathione is the body’s most abundant antioxidant, and it has roles in detoxification from xenobiotics and protection from reactive oxygen species.

Methionine is an essential amino acid, which means it cannot be synthesized within the human body and must be consumed from the diet, whereas cysteine is a conditionally essential amino acid, meaning there may be some situations when the body’s need for the amino acid outweighs its ability to create it. Sulfur amino acids are also resistant to oxidation because they are sensitive to reactive oxygen species (ROS), and the oxidation is reversible. This means they can combine with any ROS (i.e., the potentially destructive free radicals) to reduce the potential for damage.

Sulfur from sulfur-containing amino acids, namely cysteine, is also incorporated into tRNAs known as tRNA thiolation. Any imbalances or alterations in tRNA thiolation could lead to downstream problems, such as insulin signaling impairment. Numerous sulfur-containing cofactors play a role in many different biochemical reactions in the body. One type of these is iron-sulfur clusters, which are believed to be some of the oldest cofactors and are found in almost all living organisms. These are used in various functions, including in the electron transfer chain and DNA repair. These metalloproteins may also play a role in maintaining copper homeostasis since they act as a binding target for copper.

Sulfur is not only found in amino acids; sulforaphanes are phytochemical compounds in cruciferous vegetables and other foods. These compounds provide protective benefits, including antioxidant, anti-inflammatory, and nuclear factor erythroid 2-related factor 2 (NRF2) induction properties. Regular consumption of sulforaphane-rich foods has been associated with the prevention of cancer, diabetes, gastric ulcers, cardiomyopathies, and mental health disorders. Sulforaphane may also support the proper detoxification of drugs and other xenobiotics, largely due to its activation of NRF2.

Methionine-rich foods include:

• Chicken (dark meat)
• Turkey
• Lamb
• Beef
• Tuna
• Pork
• Grouper
• Salmon

Cysteine-rich foods include:

• Chicken (dark meat)
• Lamb
• Pork
• Oats
• Goose
• Beef
• Soybeans
• Buckwheat

Additional foods high in sulfur include:

• Allium vegetables (garlic, leeks, chives, onions, etc.)
• Cheese, especially cheddar and parmesan
• Cruciferous vegetables (broccoli, Brussels sprouts, cauliflower, cabbage, kale, etc.)
• Legumes
• Dried fruits

In addition to food sources, some people may have sulfur in their water, depending on the treatment process and the pipe materials. While some levels are considered normal and harmless, higher amounts of sulfur in drinking water may have an unpleasant odor and laxative effect. Routine water testing and purification techniques can help mitigate unwanted effects of sulfur in water.

Potential Health Benefits of Sulfur Reduction

Despite the importance of sulfur amino acids (SAA), there are some benefits to reducing them in the diet if they are often consumed in excess. High-SAA diets may increase a person’s risk of developing cardiometabolic diseases. For example, high levels have been linked with obesity and obesity-related illnesses such as diabetes, renal failure, and liver disease. This may be due to the link between higher meat consumption in those with higher levels of SAA. Increasing plant-based food consumption may lead to lower SAA levels. One study investigated the effects of low (~2 g/day) or high (~5.6 g/day) SAA plant-based diets in overweight or obese individuals. Compared to controls, SAA restriction caused greater weight loss success, reduced serum leptin, and increased ketones. Additionally, restriction contributed to changes in white adipose tissue gene expression that may become therapeutic targets in future research.

SAA may also play a role in atherosclerosis and lipid metabolism. In a 2017 mouse study, researchers found that mice consuming an atherogenic diet altered the metabolism of SAA and lipids. Notably, there was an increase in the metabolism of SAA in the heart, which also increased the oxidative stress in the heart. An imbalance of SAA (i.e., high consumption) may also contribute to fatty liver disease. The connection between SAA and lipid metabolism may be an influence on the expression of SCD-1 (stearoyl-CoA desaturase), an enzyme that plays a role in the synthesis of monounsaturated fatty acids found mainly in adipose tissue and the liver. Interestingly, suppressing SCD-1 may improve insulin sensitivity, reduce fat deposits, and increase basal energy expenditure, suggesting its connection to lower SAA consumption and lipid metabolism.

While there is evidence to suggest that reducing sulfur in the diet may improve health, it’s necessary to consider the overall diet of an individual. Consuming an animal-based, atherogenic diet may explain these health effects more than sulfur itself. Restricting all sources of sulfur may have detrimental effects on important processes that require sulfur.

Sulfur in Detoxification

Sulfur and sulfur-containing amino acids play an important role in phase II detoxification. During this stage, also known as conjugation, molecules combine with the metabolites from the initial phase of detoxification. Sulfur is used in several phase II processes, including sulfation, glucuronidation, and glutathione-S-transferase, three of the possible routes for conjugation. Methionine is used in methyltransferase and methylation, another possible route. Thus, if a person has a sulfur deficiency, they may not have the capacity to process potential toxins, medications, or metabolites for elimination.

Methionine and cysteine play roles in glutathione synthesis and recycling. Glutathione is the body’s major intracellular antioxidant, countering oxidative stress, reducing inflammation, and keeping the immune system strong. It is also part of the detoxification process. Cysteine is a precursor of glutathione, while methionine can synthesize glutathione through the transsulfuration pathway, which can also use homocysteine to create glutathione.

With such an integral role in the body’s management of toxins, one may assume that consuming as much sulfur as possible is the best measure for supporting these processes. However, as with many nutrients, there is a particular balance to maintain, and it may be possible to have a negative reaction when consuming more sulfur than the body can handle.

So, Is Sulfur Causing Issues for Some People?

If humans are meant to have sulfur and need it for the above functions, how can so many people have issues with sulfur? Is it the sulfur causing the problem, or is it something else?

Symptoms of sulfur intolerance are unclear, however, sulfite allergy symptoms have been well-documented:

• Asthma
• Fatigue
• Flushing
• Headaches
• Hives
• Itchiness
• Hypotension
• Diarrhea

Possible Causes of Sulfur Intolerance

Research on sulfur intolerance or allergy is limited. However, there are potential underlying issues that could affect one’s ability to tolerate sulfur-rich foods, which may present as an intolerance to sulfur-containing foods.

Some of the current hypotheses about possible causes include:

• SIBO, dysbiosis, or gut disorders
• Methylation issues, perhaps related to aging
• Issues in the metabolism of sulfur amino acids
• Polymorphisms or genetic disorders

Small Intestinal Bowel Overgrowth (SIBO) and Dysbiosis

Certain bacteria convert sulfur into hydrogen sulfide gas, which is what causes gas to smell like rotten eggs. This gas can be beneficial and protective, but like many things, excess levels can become toxic. When there is excess growth of these hydrogen sulfide-producing bacteria, it can lead to SIBO and/or dysbiosis, creating issues such as inflammatory bowel disease or irritable bowel syndrome. For example, one rat study found that an abundance of hydrogen sulfide-producing species (e.g., Fusobacterium and Desulfovibrio) in the gut contributed to diarrhea-predominant irritable bowel syndrome. Thus, some of the symptoms and issues may not come directly from consuming sulfur-rich foods but instead stem from digestive difficulties, or the food may lead to a higher amount of hydrogen sulfide gas.

A breath test is one non-invasive way to test for hydrogen sulfide SIBO, but these tests are not a reliable way to diagnose this kind of SIBO. Stool tests are another way to identify if a person has this type of dysbiosis. Treatment for sulfur-associated SIBO and/or dysbiosis would be similar to that of treating other types of SIBO or dysbiosis, although consuming a low-sulfur diet may help. Talk to a doctor, nutritionist, or dietitian about personalizing the diet if sulfur intolerance is a potential concern.

Other Considerations

Could a higher amount of homocysteine or another metabolite of sulfur-rich amino acids contribute to temporary symptoms as well? Hyperhomocysteinemia “symptoms” are likely more related to underlying vitamin deficiencies. However, high homocysteine may trigger acute changes that, over time, develop into chronic disease, especially in sensitive individuals. Although there are many reasons an individual may experience a high level of homocysteine, consuming more methionine than what the body can handle in the methylation cycle may contribute to problems. Roughly 20% of the methionine consumed is processed in the gut, which not only can fuel sulfur-consuming bacteria but can also create excess levels of homocysteine in the gastrointestinal tract that can act locally. This may trigger an inflammatory response, contributing to symptoms. It also can reduce the levels of glutathione, which could affect the gut’s antioxidant capacity and thus increase the risk of the development of diseases, such as inflammatory bowel disease.

However, there is another gene more directly associated with sulfur intolerance: cystathionine beta-synthase (CBS). This gene regulates an enzyme that plays a role in the conversion of methionine to cysteine, which is the initial step of the transsulfuration pathway. Specifically, it converts homocysteine into cystathionine by removing the sulfur-containing amino acids. Polymorphisms or defects (i.e., CBS deficiency) in this gene have the potential to interrupt this cycle, leading to downstream problems and symptoms, including homocystinuria. While rare, homocystinuria is characterized by elevated homocysteine and methionine levels, which can cause whole-body complications related to vascular health, bone mineral density, neurological health, development, and eyesight. Treatment often includes methionine restriction in the diet. A medical diagnosis is required to confirm this CBS deficiency.

What to Do If Sulfur Intolerance Is Suspected

One of the main ways to help a suspected sulfur intolerance is to consume a diet low in sulfur. A trial low-sulfur diet for a few weeks may shed some light on whether the problems experienced are due to sulfur-containing foods or something else. Then, an individual can start to treat any potential underlying reasons for the issue, such as dysbiosis or methylation issues.

Additionally, certain supplements may help process sulfur to reduce or eliminate the symptoms. Some individuals may benefit from molybdenum supplementation. Molybdenum is a trace mineral that plays a role in the synthesis of sulfite oxidase, which is used in the final step of the degradation of cysteine and helps detoxify sulfite. A molybdenum deficiency (i.e., molybdenum cofactor deficiency) impairs the ability to metabolize sulfite, potentially leading to seizures and encephalopathy. While molybdenum deficiency is rare, genetic mutations may cause an underlying inability to properly metabolize sulfur compounds. Emerging research suggests the human gut microbiome may play a role in producing molybdenum cofactors, especially from dietary sources. More research is needed. Nonetheless, supplementing with molybdenum may also cause flatulence and the excess production of colonic sulfides, so this approach should be done under the guidance of a qualified medical professional.

Other supplements shown to support the methylation process include choline and the B vitamins, especially folate and vitamin B12. Suspected deficiencies should be assessed and diagnosed by a medical professional.

As always, making diet, supplement, and lifestyle changes should be done under the guidance of a doctor, nutritionist, dietitian, or healthcare practitioner. An expert can help create a well-balanced diet lower in sulfur-rich foods. Health practitioners can also help identify and treat underlying issues and guide supplement recommendations that will help individuals better process sulfur.

Conclusion

So, is there such a thing as sulfur intolerance? There is no definitive answer in the literature about whether sulfur intolerance is real. Anecdotally, many individuals feel better when consuming a low-sulfur diet, possibly due to the underlying causes mentioned above. Thus, sulfur intolerance most likely is not due to the sulfur itself per se but an inability to keep the right sulfur balance, which may throw off homeostasis and disrupt physiological processes.

There are imbalances in the body that might be exacerbated or triggered by consuming high levels of sulfur-rich food and be relieved through a reduction in sulfur consumption. Does this qualify as sulfur intolerance? Possibly not in the same way that gluten intolerance or lactose intolerance are primary intolerances triggered specifically by the foods themselves.

However, there seems to be a balance required for sulfur, so it may be that a person could have an underlying reason to react negatively toward sulfur-containing foods, making it a secondary problem as opposed to a primary issue. As more research is conducted in this area, researchers may find more evidence for a primary sulfur intolerance.