Common and Rare Side Effects of Eating Nuts

November 24, 2025 •

3 min read

According to the National Institutes of Health, while nuts offer numerous benefits, some individuals, especially those with pre-existing conditions, may experience negative reactions. Understanding the potential side effects of eating nuts is crucial for incorporating these nutrient-dense foods into your diet safely.

Quick Summary

This article explores the various health issues associated with consuming nuts, from common problems like digestive distress and weight gain to serious risks such as allergies, mineral deficiencies, and nut-specific toxicities from excessive intake.

Key Points

Digestive Distress: Overeating nuts, particularly high-fiber types, can lead to bloating, gas, and stomach upset.
Severe Allergies: Nut allergies can trigger reactions from mild itching to life-threatening anaphylaxis in susceptible individuals.
Brazil Nut Toxicity: Consuming more than a couple of Brazil nuts per day can lead to selenium poisoning, causing fatigue and brittle hair.
Mineral Absorption Inhibitors: Nuts contain phytic acid, an 'anti-nutrient' that can inhibit the absorption of minerals like iron and zinc from your meals.
Weight Gain Risk: Due to their high-calorie density, excessive consumption of nuts without adjusting other dietary intake can lead to weight gain.
Aflatoxin Contamination: Poorly stored nuts can be contaminated with aflatoxins, a carcinogenic mold, highlighting the importance of proper sourcing and storage.
Kidney Stone Concern: For people prone to kidney stones, certain nuts high in oxalates like almonds and cashews may increase risk.
Medication Interference: Certain nuts, including walnuts, may interact with some medications, such as thyroid hormone drugs, by hindering absorption.

Common Digestive Problems from Nuts

For many, consuming nuts in moderation poses no issues, but overindulging can lead to notable digestive distress. This is often due to their high fiber and fat content, which can be challenging for the body to process in large quantities.

Bloating and Gas: Nuts contain compounds like phytic acid and tannins, which can be difficult to digest for some people. This leads to increased gas production and bloating. Macadamia nuts are an exception, as they are lower in these anti-nutrients.
Diarrhea: The high fat content in some nuts can have a laxative effect if consumed in excess, potentially causing diarrhea.
Stomach Pain and Cramps: The combination of high fat and fiber can cause stomach cramps and overall abdominal discomfort.

Serious Allergic Reactions and Intolerances

Nut allergies are among the most serious side effects, affecting millions worldwide. Reactions can range from mild discomfort to life-threatening anaphylaxis.

Food Allergies: An immune system response to specific nut proteins can trigger a release of chemicals like histamine, causing a range of symptoms. Common signs of a mild to moderate reaction include hives, swelling, and tingling of the mouth.
Anaphylaxis: This is a severe, life-threatening allergic reaction. Symptoms include a constricted throat, rapid pulse, shock, and difficulty breathing. Immediate medical attention is required.
Food Intolerance: Distinct from an allergy, an intolerance is a less severe inflammatory response, with symptoms like bloating, stomach pain, and skin issues appearing up to 72 hours later.

Mineral Absorption and Anti-Nutrients

Nuts contain natural compounds known as anti-nutrients, most notably phytic acid, which can bind to essential minerals and inhibit their absorption.

Phytic Acid: Found in the outer layer of seeds and nuts, phytic acid can reduce the body’s ability to absorb iron, zinc, and calcium during the same meal. While this is not a concern for most with a balanced diet, it can lead to deficiencies over time for those consuming large amounts of nuts or on certain diets.

Specific Nut Risks: Toxicity and Oxalates

Certain nuts carry specific risks that require careful moderation, beyond general overconsumption issues.

Brazil Nut Selenium Toxicity: Brazil nuts are extremely rich in selenium. Eating more than one or two per day can cause selenosis, a toxic overdose. Symptoms include bad breath, brittle hair and nails, fatigue, and nerve pain; in rare cases, high levels can be fatal.
Almond Cyanide: Bitter almonds contain hydrocyanic acid, a form of cyanide. While regulated, consuming excessive amounts could lead to toxicity, and they should be avoided entirely by pregnant women.
Kidney Stones from Oxalates: Nuts like almonds and cashews are high in oxalates, compounds that can contribute to the formation of calcium oxalate kidney stones in susceptible individuals.
Aflatoxin Contamination: Nuts can be contaminated with aflatoxins, a type of carcinogenic fungus that can lead to liver cancer, especially in nuts from certain regions. Salmonella and other bacteria are also occasional contaminants.

Comparison of Common Nut Risks


Nut Type	Primary Overconsumption Risk	Key Symptoms	Vulnerable Individuals
Brazil Nuts	Selenium toxicity (selenosis)	Brittle hair, fatigue, nerve pain	Anyone, due to high selenium content
Almonds & Cashews	Oxalate overload, digestive issues	Kidney stones, bloating, gas	Those with a history of kidney stones or digestive sensitivity
Peanuts	Allergies, aflatoxin contamination	Severe allergic reactions, potential illness	Individuals with allergies or those consuming imported products
Walnuts	Digestive issues, nutrient absorption	Bloating, diarrhea, blocked mineral absorption	Those with sensitive digestion or mineral deficiencies

Conclusion

While nuts are a heart-healthy and nutrient-rich addition to most diets, being mindful of their potential downsides is essential. Portion control is key to avoiding issues like weight gain due to their high calorie density. For most, a small handful (around 30g) is the recommended daily amount, which minimizes risks while providing benefits. Special care is required with Brazil nuts to prevent selenium toxicity and for individuals with nut allergies, where exposure can be life-threatening. Choosing raw or dry-roasted, unsalted options can further reduce health risks. For those with digestive sensitivities, conditions like kidney stones, or concerns about nutrient absorption, varying nut consumption and consulting a doctor can help manage risks effectively.

Center for Science in the Public Interest: How to pick the healthiest nuts & seeds

Frequently Asked Questions

Yes, eating too many nuts can cause digestive issues like bloating, gas, and diarrhea. This is often due to the high fiber and fat content, which can be hard for the digestive system to handle in large amounts.

Brazil nuts are exceptionally high in selenium. Consuming more than one or two per day can lead to selenium toxicity, known as selenosis, with symptoms such as brittle hair, nail loss, fatigue, and nerve damage.

Yes, it is possible to gain weight from eating nuts, especially if overconsumed. Nuts are very calorie-dense due to their high fat content, so portion control is crucial for managing weight.

Yes, nuts contain phytic acid, an anti-nutrient that can inhibit the absorption of minerals like iron, zinc, and calcium from the same meal. However, this is generally not a concern with a balanced diet.

A nut allergy is an immune system response that can cause a severe, immediate reaction like anaphylaxis. An intolerance is a less severe inflammatory response, often delayed, causing milder symptoms like digestive upset or skin problems.

Yes, consuming excessive amounts of bitter almonds can lead to toxicity from hydrocyanic acid. Furthermore, certain nuts can be contaminated with dangerous fungi (aflatoxins) or bacteria, posing a risk of foodborne illness.

For individuals susceptible to kidney stones, nuts high in oxalates, such as almonds and cashews, may increase the risk of forming calcium oxalate stones.