Food item
Measure
Sodium per measure (mg)
Table salt
1 teaspoon
2,325
Salted fish (cod, mackerel), canned anchovy
1 oz
1,030–1,970
Pizza, frozen entrees/meals
1 slice, 1 meal
300–1,660
Soy sauce, fish sauce
1 tablespoon
1,000–1,415
Pretzel twists
10 twists
1,000
Cold cuts, luncheon meats
2 oz
600–1,000
Canned vegetables, bottled/canned tomato sauce
½ cup
500–960
Cured meats (ham, sausage, kielbasa, hot dog)
2 oz
400–800
Salted pumpkin seeds
1 oz
710
Bacon, cooked
2 slices
600
Breads, rolls, bagels, muffins
1 slice
120–500
Condiments (ketchup, barbecue sauce)
1 tablespoon
190–450
Cheeses (American process, Blue, cottage)
1 oz
265–420
Certain ready-to-consume breakfast cereals
¾ to 1 cup
250–350
Salad dressings
1 tablespoon
120–240
Cheeses (Swiss, parmesan, Monterey Jack, ricotta, mozzarella, cheddar, brie)
1 oz
55–180
In the case of the acid-base balance of the diet, a diet high in acid ash may cause calcium to be released from bone, which is then excreted in urine. Foods with an acid ash are those that have sulfur in their amino acid structure, specifically from the amino acids methionine and cysteine. These amino acids are mostly found in foods of animal and marine origin but are also richly expressed in some plant foods. While these amino acids are essential – needed for growth and maintenance of health – if not balanced with a sufficient amount of foods with an alkaline ash, the diet becomes high in acid load. If urinary calcium excretion is high, and if a diet with a high acid load is deemed to be contributory, then reducing the acid load of your diet may help to decrease the amount of calcium in your urine. If urinary calcium excretion is not high, or if it is high for some other reason, then reducing the acid load of your diet may not influence your calcium oxalate stone risk. Table 27.2 lists some of the foods most likely to contribute to high dietary acid load if consumed in excess and/or without a sufficient amount of foods to counter the acid load, i.e., those that confer an alkaline load (Table 27.2).
Table 27.2
Food groups and their average potential renal acid load (PRAL)
Food group | Serving size used for PRAL calculation | Average calculated PRAL/serving (mEq) |
|---|---|---|
Meat, poultry | About 4 oz | 9.5 |
Fish | About 4 oz | 7.9 |
Flour, from various grains | 1 cup, dry | 7.0 |
Spaghetti, noodles | About ¾ cup, cooked | 6.7 |
Cheeses with high protein contenta | 1 oz | 5.9 |
Breads, bagels, muffins, rolls | About 3½ ounces | 3.5 |
Cheeses with lower protein contentb | 1 oz | 2.0 |
Yogurt, milk | 1 cup | 1.0–2.4 |
Vegetables | About ½ cup | −2.8 |
Fruits | 1 piece or about ¾ cup | −3.1 |
Another potential contributor to high urinary calcium excretion, though less common, is excessive calcium intake, usually from calcium supplements. Calcium intake, even in individuals with premature bone loss, is usually not required to be more than 1,500 mg per day. Unfortunately, many over-the-counter calcium supplements are formulated to provide a high amount of calcium, including 1,000 and 2,000 mg per day. A person who takes this much calcium from a supplement is very likely to have a high calcium intake as their diet, even without eating dairy foods, also provides some calcium. Too much calcium in the diet and/or from supplements may increase the amount of calcium in the urine, contributing to calcium oxalate stone disease. Adults typically need between 1,000 and 1,500 mg of calcium daily. A registered dietitian may be helpful in identifying how much calcium you usually get from your diet – from foods and beverages – and then the dietician can suggest the appropriate amount of supplemental calcium to take, if needed. Ideally, calcium intake should be from foods and beverages alone. Vegetarians who do not use dairy and people with lactose-intolerance can very easily achieve their ideal calcium intake from foods and beverages alone, as many calcium-fortified products are available. Additionally, there are some foods that are naturally rich in calcium that are non-dairy. Table 27.3 lists foods that are rich in calcium.
Table 27.3
Foods rich in calciuma, b
Food item | Measure | Calcium per measure (mg) |
|---|---|---|
Calcium-fortified non-dairy milks (soy, rice, coconut, almond) | 8 fluid ounces | 350–450 |
Tofu, prepared with calcium sulfate | ½ cup | 430 |
Calcium-fortified orange juice | 8 fluid ounces | 350 |
Buttermilk | 8 fluid ounces | 350 |
Yogurt (brands vary greatly) | 6 oz | 200–350 |
Eggnog | 8 fluid ounces | 330 |
Mustard spinach, raw, chopped | 1 cup | 315 |
Calcium-fortified vegetable juice | 8 fluid ounces | 300 |
Milk | 8 fluid ounces | 300 |
Sesame seeds, whole, roasted | 1 oz | 280 |
Almonds, whole | ½ cup | 190 |
Sesame seeds, whole, roasted | 1 oz | 280 |
Collards, chopped, cooked | 1 cup | 270 |
Spinach, turnip greens, cooked | 1 cup | 250 |
Salmon, canned, with bones | 3 oz | 240 |
Figs, dried | 1 cup | 240 |
High Urinary Oxalate Excretion
It is estimated that only about 20 % of people who form calcium oxalate stones have high urinary oxalate excretion (hyperoxaluria). In other words, for the vast majority of calcium oxalate stone formers, high urinary oxalate excretion is not the cause. Yet, many people with calcium oxalate stones have been advised to avoid long lists of healthy foods, purportedly because of their oxalate content. The truth about high urinary oxalate excretion has more to do with factors other than oxalate intake. There are two sources of urine oxalate: endogenous, referring to oxalate that the human body makes; and exogenous, referring to oxalate that is absorbed from the digestive tract. The relative contribution of these sources to urine oxalate varies greatly between individuals for a variety of reasons both genetic and nutritional. Individuals with an overly high production of oxalate are usually those with an underlying disorder having nothing to do with their diet. Primary hyperoxaluria, a genetic disorder that results in extremely high oxalate production in the body, will thus not be addressed in this chapter (see section “Special medical situations” for a brief discussion of this).
How Does Oxalate Get into the Digestive Tract?
Oxalate is produced by many plants and is also picked up by plants from the soil. Most all plant foods contain some oxalate; some more than others. When we eat these plants, we eat their oxalate. It is difficult, if not impossible, to follow recommendations for a healthy, plant-based diet without consuming some oxalate. In fact, oxalate restriction is controversial as (a) oxalate restriction also requires a calcium restriction in order to maintain a suitably low oxalate-to-calcium ratio in the urine; (b) high-oxalate foods are frequently those that provide stone inhibitors, such as fiber, magnesium, and phytate; and (c) high-oxalate foods are mostly fruits, vegetables, and whole grains, which are recommended in high amounts for cancer prevention, cardiovascular disease prevention, and overall good health. It is worth noting that vegetarians, whose oxalate intake can be as much as three times higher than the typical omnivore’s, appear to have a lower incidence of calcium oxalate kidney stones (See Chap. 28).
What Happens to Oxalate in the Digestive Tract?
Oxalate in the digestive tract has three possible fates: (1) it can either be complexed with calcium or another mineral and excreted harmlessly in the stool; (2) it can be eaten (degraded and destroyed) by bacteria that live in the human digestive tract; or (3) it can be absorbed into the bloodstream. Once absorbed, oxalate must be excreted into the urine by the kidneys, as there is no use for oxalate by humans.
A low or sub-optimal calcium intake is a major contributor for high oxalate absorption in the digestive tract. When there is too little calcium within the digestive tract, especially to match with the amount of oxalate in the digestive tract, oxalate is freely absorbed. This knowledge has led to cessation of calcium restrictions in people with high urinary oxalate, instead focusing on normalizing calcium intake. Most recommendations to control high urinary oxalate excretion are to include calcium-containing foods or beverages at each meal in order to provide sufficient calcium in the digestive tract to bind any oxalate that has come from the meal. Magnesium is also capable of complexing oxalate in the digestive tract and thereby reducing its absorption, but our need for magnesium is about three-fold lower than for calcium. Thus, the use of magnesium as a therapy for binding oxalate is not as clinically relevant as the use of calcium. Usually, foods and beverages providing about 300 mg of calcium at each meal will be sufficient. People with certain bowel disorders, such as malabsorption, may need supra-physiologic doses of calcium at meals to bind oxalate and prevent its absorption. Table 27.3 lists foods and beverages rich in calcium.
A low or sub-optimal amount of healthy bacteria in the digestive tract, commonly referred to as the “gut microbiome,” may contribute to high oxalate absorption from the digestive tract and, thus, high urinary oxalate excretion. This is an area of current research, so evidence-based information to guide therapy is not sufficient. However, studies show that diets low in fruits and vegetables, which provide food (known as prebiotics) for good bacteria (known as probiotics), lead to a different profile of the gut microbiome. This could potentially lead to a reduced number of bacteria that can degrade oxalate and remove it from the digestive tract, thereby reducing it’s absorption. Aside from not eating enough prebiotic material to grow and sustain a healthy gut microbiome, antibiotic therapy may also alter the gut bacterial profile. Research in this area is increasing, and there is a strong potential for evidence to guide clinical therapy for individuals suspected to have low or sub-optimal bacterial oxalate degradation capacity in their digestive tracts.
Finally, certain amino acids we eat, such as from animal flesh, can provide ingredients (precursors) in the body to form oxalate. Another way people get oxalate is from high doses of vitamin C (ascorbic acid), which, when consumed in amounts greater than the body can use, is broken down or metabolized to oxalate. There is some thought, too, that fructose in high amounts – such as from drinking a lot of soda or fruit juice or from eating foods processed with high amounts of fructose – can contribute to the formation of oxalate in the body. Once oxalate is formed in the body, by whatever means, it is excreted into urine by the kidneys where it can bind with calcium to form stones. Table 27.4 lists some of the foods highest in oxalate per portion.
Table 27.4
Foods high in oxalate per serving
Food item | Measure | Oxalate per measure (mg) |
|---|---|---|
Spinach | 1 C if raw; ½ C if cooked | 340–755 |
Rhubarb, fresh | ½ C | 200–540 |
Almonds | 1 oz. or about 22 kernels | 120–130 |
Potato, baked, with skin
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