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March 6, 2014

Have you ever had a kidney stone? I did, in college. Kidney stone pain is very similar to labor, but passing a kidney stone is continuously miserable, whereas the pain of labor comes and goes. Needless to say, I don't want another one.

With a personal history of kidney stones, I started to get concerned that my morning smoothie might be putting me at risk. Smoothies have a high concentration of oxalates, and about about 80% of kidney stones are made up of calcium oxalate. Oxalates are naturally present in our bodies. They are also present in varying levels in certain plant foods. Low-oxalate diets are sometimes recommended to decrease the kidney stone development.

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There are nine foods that are shown to significantly increase urinary concentrations of oxalates:

  • Parsley
  • Spinach
  • Rhubarb
  • Beets
  • Nuts
  • Chocolate
  • Tea
  • Wheat bran
  • Strawberries

This is a big surprise, since many other vegetables are high in oxalates. (Check out the content of your fave veggie here.)

Why do some foods that are high in oxalates not increase the amount of oxalate in the urine?

It's probably because greens, fruits, and vegetables are very high in calcium, too. Calcium binds the oxalates in the food while it's still in the intestine, limiting the absorption and allowing the potentially harmful oxalates to be excreted from your precious, beautiful body.

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Why do vegetarians have half the risk of kidney stones compared to omnivores?

There are two good reasons for this finding:

1. Meat consumption results in increased acidity in the urine.

The body responds to the increased acidity by adding calcium to the urine, much in the same way my mom takes a Tums when her stomach hurts. The increased calcium in the acidic urine binds the oxalate and whammy! A stone is formed. Keeping the urine alkaline with proteins (from grains and beans) reduces the calcium in the urine and minimizes the risk of calcium-oxalate stones.

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2. Collagen in meat supplies the amino acid hydroxyproline, which is metabolized to oxalate.

The daily turnover of collagen from your own body is a major source of hyroxyproline. Just turning over your own collagen accounts for 5 to 20% of the urinary oxalate daily. Dr. John Knight of Wake Forest University School of Medicine in Winston-Salem, N.C. compared oxalate levels in the urine after feeding people diets loaded with gelatin (made from the cow skeleton and loaded with collagen) versus whey, which is a protein by-product of the cheese industry. Those who ate gelatin had much higher levels of urinary oxalate for the next 24 hours.

Just eating 5 to 10 grams of gelatin significantly increased oxalates in the urine for six hours.

How much collagen is in meat? Samples of lean meats contain from 2.5 to 5% collagen by weight and hamburgers are up to 7.1%. Hot dogs and sausages, made from scraps and pieces, are up to 19% collagen by weight. Meat that contains 7% collagen will have 7 grams per serving and 10 grams per about one third of a pound. Meat servings in the range of a hot dog or quarter pounder will increase urinary oxalates for an entire day after consumption.

Several dietary supplements promoted for skin, bone, and joint health also contain gelatin at 10 grams per recommended dose.

What should a person who wants to limit their dietary oxalates do?

Urinary oxalate is predominantly caused by the oxalate created in your own body from collagen breakdown, but it may also be affected by dietary intake of oxalate and calcium.

Although oxalate-rich foods enhanced excretion of urinary oxalate in normal volunteers, the increase was not proportional to the oxalate content of the food. Fruits and veggies that are high in oxalates are often high in calcium too, which prevents the absorption of the oxalate by binding it up in the intestine.

Increased dietary calcium intake may reduce urinary oxalate excretion by binding more oxalate in the gut.

Collagen in meats break down into hydroxyproline, which then breaks down into oxalates. Meat represents a significant source for oxalates in omnivores. One serving of meat increases urinary oxalate levels for 24 hours.

Decreasing meats and processed dairy cuts the risk of kidney stone by 40%. Check out your supplements and reconsider gelatin-containing products. Avoid gelatin-containing foods and desserts.

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Mary Clifton, M.D.
Mary Clifton, M.D.

Mary Clifton, M.D., has been an Internal Medicine doctor for almost twenty years. She specializes in weight loss, osteoporosis and menopause, disease prevention, management, and reversal. She regularly speaks at health and inspirational seminars, medical and heath conferences, corporate wellness events, Universities, and for private groups. Mary is the author of the best-selling book, Waist Away, co-author of the book Get Waisted: 100 Addictively Delicious Plant-Based Entrees, and co-founder of the healthy weight loss program Get Waisted.