What are kale's side effects?

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Written by

Tina Carter
Published: December 01, 2025
Updated: December 01, 2025

While kale is incredibly nutritious, there are some side effects to be aware of if you're sensitive. Kale contains compounds that can affect thyroid function and digestion, and understanding these factors can help you consume kale wisely and reap its health benefits.

Thyroid Function

  • Goitrogens may interfere with iodine absorption
  • Cooking reduces compounds by 30-50%
  • Space intake 3-4 hours before/after thyroid medication

Digestive Impact

  • High fiber content causes bloating if eaten excessively
  • Introduce gradually starting with 1 cup raw (67g) daily
  • Massage leaves to break down cellulose fibers

Kidney Concerns

  • Oxalates concentrate in stems and mature leaves
  • Blanching reduces oxalates by 40-60%
  • Limit to 2 cups cooked (130g) daily for susceptible individuals
Compound Levels in Kale Preparations
Preparation MethodRawGoitrogen Reduction
0%
Oxalate Reduction
0%
Preparation MethodSteamed 5 minGoitrogen Reduction
30%
Oxalate Reduction
45%
Preparation MethodBoiled 7 minGoitrogen Reduction
50%
Oxalate Reduction
60%
Preparation MethodFermentedGoitrogen Reduction
65%
Oxalate Reduction
55%
Based on USDA nutrient retention studies

Preparation methods. Cooking helps reduce harmful compounds in these veggies. Steaming or boiling them decreases the goitrogens and oxalates they contain. I make sure to eat my kale with iodine-rich foods, such as fish and seafood, as a counterbalance to the negative effects of the goitrogenic properties. Fermenting them as kimchi or sauerkraut both aids in digestibility and enhances their nutrient content.

Moderation guidelines: prevent digestive discomfort. Most adults should aim for no more than 2-3 cups (134-201g) of raw kale per week. Space out servings over the week rather than consuming daily. Those with existing gastrointestinal conditions should consult their healthcare provider for personalized advice.

Combining kale with other vegetables adds nutritional variety. Try alternating with low-oxalate greens such as cabbage and lettuce. Consume kale with vitamin C-containing foods to improve iron absorption. This ensures maximum benefit and minimal downside.

What are the side effects of kale? Possible considerations include: Thyroid function: Compounds may affect iodine absorption. Digestion: High fiber content can cause bloating if consumed excessively. Kidney stones: Oxalate content indicates individuals susceptible to kidney stones may need to limit consumption.

Read the full article: How to Grow Kale: Ultimate Guide for Home Gardeners

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