The Truth About Oxalate

Answers to Frequently Asked Questions

By Michael Liebman, Ph.D.

Reprinted from Number 22, Summer/Fall 2002

What is calcium oxalate? What is the difference between calcium oxalate and “oxalate?” What is oxalic acid?

Oxalic acid (chemical formula HOOC-COOH) is a strong organic acid which is widely distributed in nature in both plants and animals. However, plants contain more than animals. The name comes from the plant Oxalis (wood sorrel) from which it was first isolated.

Oxalic acid has the ability to form a strong bond with various minerals, such as sodium, potassium, magnesium, and calcium. When this occurs, the compounds formed are usually referred to as oxalate salts. Thus, “oxalate” usually refers to a salt of oxalic acid, one of which is calcium oxalate.

Although both sodium and potassium oxalate salts are water soluble, calcium oxalate is practically insoluble, which is why calcium oxalate, when present in high enough levels, has the propensity to precipitate (or solidify) in the kidneys or in the urinary tract to form calcium oxalate crystals.

Calcium oxalate crystals, in turn, contribute to the formation of kidney stones. Approximately 75% of all kidney stones are composed predominantly of calcium oxalate. For reasons not yet fully understood, women have a much lower incidence of kidney stones than men.

Dr. Michael Liebman, Ph.D.

Michael Liebman, Ph.D.

Is oxalate found in most foods?

Most foods do not contain significant amounts of oxalate. The primary sources of dietary oxalate are plants and plant products. Although the physiological role of oxalate in plants is not clearly understood, it is well established that a number of plants have the ability to synthesize oxalate. Seeds and leafy plants related to spinach and rhubarb contain the most oxalate.

Many plant foods contain very low levels of oxalate. Foods of animal origin contain negligible amounts of oxalate.

Does drinking water (tap water, spring water, distilled water) contain oxalate?

Water is not a dietary source of oxalate unless the water has come in contact with high-oxalate plants (e.g., water that high-oxalate plants have been cooked in).

Hard water (which contains calcium and magnesium) is likely to exert a beneficial effect with respect to oxalate absorption because these minerals will tie up much of the oxalate consumed in the diet within the gastrointestinal tract, thereby decreasing oxalate absorption. Less oxalate absorption translates to less oxalate gaining access to various tissues within the body.

Does a person following the low oxalate diet need to completely eliminate all oxalate for the diet to be effective?

Foods with only low levels of oxalate do not need to be completely eliminated. The propensity for the consumption of a specific food to increase urinary oxalate is a function of both the amount of oxalate in that food and the amount of oxalate which can be absorbed from that food. Since dietary oxalate is not efficiently absorbed into the body, the consumption of foods which provide low levels of oxalate will not have a significant effect on the amount of dietary oxalate which enters the body through the gastrointestinal tract or on the amount of oxalate which is excreted via the urine.

Total Dietary
Fiber in
Grams
Oxalate
Content
Group
Fruits
1 large apple with skin
1 banana
1 orange
1 pear
5 prunes, drained
3.7
2.8
3.1
4.0
3.0
Medium
Low
Medium
Low
Medium
Vegetables
½ cup raw broccoli
½ cup brussel sprouts, cooked
½ cup raw carrots
½ cup cooked lentils
1 cup iceberg lettuce, shredded
½ cup green peas, canned
½ cup split peas, cooked
½ cup potatoes, boiled
1.3
2.0
1.8
7.8
0.8
3.5
8.1
1.6
Low
Low
Medium
Medium
Medium
Low
Low
Medium
Grains
1 slice white wheat bread
¾ cup oatmeal
1 cup brown rice, cooked
1 rye crispbread wafer
0.6
3.0
3.5
1.7
Medium
Medium
Medium
Low
Other
1 piece apple pie 1.9 Medium
Total fiber in all Low Oxalate
foods in this chart
23.4 Low
Average fiber per Medium
food in this chart
2.8 Medium
Total fiber in 7 Low foods
and 2 Medium foods
29.0 Combined

Do all women have oxalates in their urine?

This is true of all individuals (male or female) because the human body synthesizes oxalate from a number of compounds which are precursors to oxalate. Thus, even when dietary oxalate is reduced to almost zero via careful food selection, the body will continue to synthesize and excrete via urine a certain amount of oxalate.

Is it true that the amount of oxalate in urine varies largely based on the foods eaten approximately twelve hours beforehand?

It is true that the amount of oxalate excreted via the urine is partially dictated by the amount of oxalate absorbed from foods consumed. Our research suggests that the majority of the urinary oxalate derived from food oxalate absorption will appear in the urine 2 to 6 hours after the consumption of the oxalate-containing food rather than 12 hours post food ingestion.

Is the low oxalate diet difficult to follow, or impossible to stay on?

The list of foods that have been tested for levels of oxalate is limited, though growing. However, with careful food selection, a low oxalate diet which provides all the needed nutrients and is reasonably varied can be followed.

Does the low oxalate diet cause weight loss?

Foods restricted on the low oxalate diet include certain vegetables, fruits, whole grains, legumes, and nuts. With the exception of nuts, these foods are not calorically dense (i.e., not high in fat). Thus, omitting high-oxalate foods should not necessarily lead to a reduction in caloric intake and subsequent weight loss. However, there is evidence that some people will consume less calories when put on a restrictive diet due to a “monotony” factor. Thus, the degree to which oxalate-containing foods are restricted could partially dictate whether a low oxalate diet is lower in calories than a diet not restricted in oxalate-containing foods.

Does the low oxalate diet cause weight gain?

This would only be true if an individual compensates for not eating certain fruits and vegetables, whole grains, and legumes by eating significantly larger quantities than normal of animal products such as meat, fish, poultry, eggs, and dairy products. Otherwise, the low oxalate diet should not lead to weight gain.

Is a low oxalate diet nutritionally inadequate?

There are certain nutrient intakes which could be compromised in a low oxalate diet. For example, low oxalate diets may provide lower levels of nutrients which are found primarily in plant products, such as vitamin C and folate. However, with wise food selections, an individual can maintain a low oxalate diet and still consume recommended levels of all key nutrients.

Does a low oxalate diet cause constipation?

There may be a possibility of constipation with a low oxalate diet. Whole grains, legumes, fruits, and vegetables are the best sources of fiber, an important dietary component that aids regularity.

Careful selection of foods on a low oxalate diet can provide adequate fiber. Any food which provides at least 3 to 4 grams of fiber per serving would be considered an excellent source (see chart).

To put the numbers in perspective, the recommendation for total intake of dietary fiber is 25 to 35 grams per day.


Michael Liebman, Ph.D., Professor of Human Nutrition, University of Wyoming (see page one) is filling in for the Scientific Research Committee in this issue of the as a guest consultant. Dietary oxalate has been studied by research scientists throughout the world since the 1930s. It is one of Dr. Liebman’s research specialties.



Return to Home