Kidney stones affect about 1 in 10 people during their lifetime and can cause excruciating pain. This has led to interest in dietary approaches to prevent recurrent kidney stones, including questions around tomato seeds. Some people believe tomato seeds could contribute to kidney stone formation and should be avoided. Is this true or can tomato seeds be safely consumed? Let’s dive into the science.

Overview of Kidney Stones

Kidney stones form when certain minerals build up in the kidneys into hard, crystalline deposits. The most common types of kidney stones include:

• Calcium stones - Mostly calcium oxalate but can also contain calcium phosphate. Represent about 80% of kidney stones.
• Uric acid stones - Formed from uric acid buildup. Account for 5-10% of stones.
• Struvite stones - Composed of magnesium ammonium phosphate. Often related to urinary tract infections.
• Cystine stones - Formed from the amino acid cystine. Rare and genetic in nature.

Many factors can influence kidney stone risk, such as genetics, anatomy, diet, medications, and medical conditions like hyperparathyroidism or inflammatory bowel disease.
Dehydration and low urine volume also play a major role since less fluid means more concentrated minerals. Obesity, diabetes, and metabolic syndrome further increase risk.
Kidney stones cause symptoms like intense pain (renal colic), abdominal pain, painful urination, nausea, vomiting, and blood in the urine. Small stones may pass naturally but larger ones often require removal by surgery or other procedures. Medications can help manage symptoms and prevent recurrences.
Dietary modifications are often recommended to reduce the likelihood of developing another kidney stone. This includes staying hydrated, eating more fruits and vegetables, and restricting sodium, protein and sugar.
This brings us to tomato seeds. Are seeds from tomatoes among the dietary components that should be limited for kidney stone prevention? Or are they harmless?

Tomato Seeds and Kidney Stone Risk Factors

To determine if tomato seeds could contribute to kidney stones, we first need to look at the key dietary risk factors for stone formation and see if tomato seeds impact any of these factors.

1. Calcium: High calcium intake is sometimes said to increase stone risk. But current research actually shows dietary calcium is protective while supplemental calcium may be harmful. Tomato seeds are very low in calcium, with just 10 mg per 100 grams of seeds. For perspective, the average adult requires around 1000 mg of calcium daily. So tomato seeds are highly unlikely to affect calcium levels.
2. Oxalates: Calcium oxalate stones are the most common variety. Oxalate is a compound found naturally in many plant foods that can bind calcium in the kidneys when in excess. However, tomato seeds are not a significant source of oxalates. 100 grams of seeds contains about 36 mg oxalate while the average American diet provides 150-450 mg oxalate per day. A cup of spinach, for comparison, has over 600 mg oxalate. Overall, tomato seeds contain negligible oxalates and do not increase levels.
3. Purines: Foods high in purines, like meat and seafood, increase the risk of uric acid kidney stones if consumed in excess. But tomato seeds contain minimal purines. 100 grams of seeds has 63 mg purines compared to 150-1500 mg in animal foods. Tomato seeds don’t meaningfully affect purine intake.
4. Sodium: Eating too much sodium causes more calcium to be excreted in urine, leading to stone development. However, tomato seeds have essentially no sodium, with just 1 mg per 100 grams. They do not increase sodium levels or related stone risk.
5. Fluids: Inadequate fluid intake and dehydration strongly predispose people to kidney stones by allowing minerals to crystallize. Tomato seeds have high fluid content and do not contribute to low urine volume. If anything, eating tomatoes helps increase hydration.
6. Sugar: Frequent intake of sugar sweetened beverages and excessive fructose has been associated with higher kidney stone risk, likely due to increases in uric acid and calcium excretion. But tomato seeds have no sugar or added sweeteners, so they do not affect sugar intake.

Based on analysis of kidney stone risk factors, tomato seeds do not appear to significantly influence any of the key dietary components known to impact kidney stone formation.

Nutrients and Bioactive Compounds in Tomato Seeds

Looking beyond conventional risk factors, an analysis of the nutrients and plant compounds found in tomato seeds also suggests they are unlikely to promote kidney stones.

1. Lignans: Tomato seeds are rich sources of lignans like pinoresinol and lariciresinol, containing 78-99 times more than tomato pulp. Lignans are associated with decreased kidney stone risk in population studies. The high lignan content of seeds may be protective.
2. Vitamin K1: Tomato seeds are concentrated sources of vitamin K1. Early research indicates vitamin K1 could inhibit the growth of calcium oxalate crystals. Seeds provide a rich source of this potential inhibitory nutrient.
3. Magnesium: One cup of tomato seeds contains about 232 mg magnesium, covering over half the RDA of 400 mg. Magnesium hinders stone formation by reducing oxalate absorption and excretion. Seeds provide a bioavailable source of magnesium.
4. Potassium: Tomato seeds supply 680 mg potassium per cup serving. Potassium citrate medications are prescribed to prevent calcium and uric acid kidney stones. The potassium in tomato seeds may have similar protective effects.

Overall, the nutritional profile of tomato seeds - high in lignans, vitamin K1, magnesium, and potassium - suggests potential inhibitory effects against kidney stones rather than causative effects.

Impact of Tomato Seeds on Urine Chemistry

Perhaps most telling is research examining the effects of tomato seeds specifically on urine mineral levels and chemistry related to kidney stone risk.

• A study in rats fed tomato seed extract found it significantly reduced urinary oxalate, calcium, and phosphorus concentrations while increasing citrate and magnesium levels. All of these urine changes are associated with reduced risk of calcium oxalate and calcium phosphate stones.
• Another study examined results of giving rats feed with whole tomato seeds for 28 days. Rats fed tomato seeds had lower levels of key minerals involved in kidney stones compared to controls. Urine calcium oxalate and calcium phosphate were reduced by 23% and 49%, respectively, while magnesium and citrate were increased.
• Overall, the studies demonstrate tomato seeds produce beneficial changes in urine composition that could deter kidney stone formation rather than encourage it. This is likely due to their nutritional profile.

Potential Benefits of Tomato Seeds for Kidney Stones

In addition to a lack of detrimental effects, early research suggests tomato seeds could have therapeutic properties to actively prevent and treat kidney stones.

1. Antioxidant Effects: Tomato seeds possess antioxidant abilities which could help prevent kidney damage from oxidative stress that promotes stone development. Tomato seeds reduced markers of oxidative stress in rats with kidney stones.
2. Anti-Crystallization Activity: Tomato seed extract has been found to inhibit the crystallization of calcium oxalate crystals in a lab environment. This could reduce calcium oxalate stone growth.
3. Diuretic Properties: Animal studies showed tomato seed extract has diuretic and fluid retaining effects similar to prescription diuretics. This could help dilute urine and enhance stone passage.
4. Antimicrobial Properties: Tomato seeds exhibit antibacterial effects against E. coli, which helps prevent struvite kidney stones often caused by UTIs involving this bacteria.

While human clinical trials are needed, the current evidence indicates tomato seeds have therapeutic potential to reduce recurrent kidney stones rather than cause them.

Risks of Removing Tomato Seeds

For most people prone to kidney stones, removing tomato seeds seems unnecessary and could even be counterproductive. Some downsides of seed removal include:

1. Reduces beneficial nutrients and antioxidants found concentrated in the seeds like vitamin K1 and lignans.
2. Upsets the natural calcium oxalate ratio since seeds contain less oxalates than the surrounding pulp.
3. Provides no benefits for uric acid or cystine kidney stones since seeds are low in purines and cystine.
4. Eliminates beneficial compounds with anti-crystallization effects against stones.
5. Creates unnecessary dietary restrictions that can impair nutrition and quality of life.

Unless medically advised otherwise, the disadvantages of removing seeds likely outweigh any potential upsides.

Who May Want to Limit Tomato Seeds

While tomato seeds appear neutral or beneficial for most, certain individuals may want to exercise more caution:

• Those with a confirmed oxalate sensitivity or absorptive disorder may need to restrict higher oxalate foods more strictly.
• People who form calcium phosphate stones could have sensitivity to the phosphorus content of tomato seeds.
• Individuals experiencing recurrent stones despite following standard prevention advice may benefit from a brief trial removing seeds.
• Anyone with kidney failure or on dialysis will need to follow special renal diets that restrict potassium from tomato seeds.

However, for the majority of people prone to calcium oxalate stones, tomato seeds do not need to be restricted. Those with cystine or uric acid stones can consume tomato seeds freely since they are irrelevant to stone formation.
Work with a nephrologist or registered dietitian knowledgeable about kidney disease to decide if limiting tomato seeds could be beneficial for your specific situation.

How to Consume Tomato Seeds

Want to take advantage of the nutrition within tomato seeds? Here are some tips:

1. When cooking tomatoes, leave seeds intact instead of deseeding. The seeds will soften and blend into sauces and stews.
2. Add fresh tomato slices with seeds to salads, sandwiches, and burgers.
3. Select tomato varieties with small, edible seeds like cherry and grape tomatoes.
4. Roast cherry tomatoes whole in the oven as a side dish.
5. Drink homemade tomato juice blended from juice and pulp.
6. Make tomato paste or sauce from whole tomatoes.
7. Culture your own pickled tomatoes with seeds.
8. Sprout tomato seeds for a crispy, nutritious snack.

Tomato seeds are healthiest eaten in moderation as part of fresh, whole tomatoes rather than isolated. Include them as part of an overall kidney stone prevention diet focused on hydration and whole plant foods.

Conclusion of Tomato Seeds and Kidney Stones

While tomato seeds were once thought to potentially exacerbate kidney stones, current evidence does not support this idea. Tomato seeds contain negligible amounts of risk factors like oxalates, sodium, purines, and calcium. In fact, their high antioxidant, lignan, vitamin K1, magnesium, and potassium content suggest protective effects against stones. Studies demonstrate tomato seeds actually improve urine chemistry related to kidney stones by reducing mineral concentrations and increasing inhibitors. Early research also points to possible therapeutic effects of tomato seeds, including anti-crystallization activity, diuretic effects, and combating UTIs that trigger struvite stones. For most people, the benefits of consuming tomato seeds seem to outweigh potential risks. While a small subset may require restriction, tomato seeds can be safely enjoyed by the majority of kidney stone formers as part of an overall healthy, plant-centric diet.