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Unanswered Questions

DIET
The dietary causes of stone formation have not been thoroughly researched in the scientific community. Guinea pigs used in research are often given artificial diets that provide basic nutrients but do little to help us understand why stones form in the bladder of our pet guinea pigs. We provide them a healthy diet of grass hay and fresh vegetables in the belief that we are doing the best we can to give them a long and healthy life.

A thorough study would measure calcium loss through both urine and feces, evaluating a wide variety of healthy food stuffs. It would look at overall calcium, the calcium/phosphorus ratio, and how high-oxalate, high-calcium foods are handled. And it would also look closely at the vitamin/mineral supplements routinely added to the food pellets that make up a large part of the diet of many of our pet guinea pigs.

Studies to evaluate the role of exercise, weight, and essential minerals like vitamin D to good urinary tract health could be invaluable.

Until we have answers, we can only continue to provide a sensible, natural diet.

OXALATES
Most stones found in guinea pigs are composed of calcium carbonate.

In humans, oxalates bind with calcium producing calcium oxalate. Leafy greens high in oxalates are often high in calcium. Because they bind together, the calcium in these greens is not as available to humans.

When in the digestive process are the two bound? Does this creation of calcium oxalate make calcium more or less available to a guinea pig's body? Do oxalates help guinea pigs process calcium by binding some of the free calcium? Could this mean that because romaine doesn't have enough oxalates, the free calcium in romaine is more of a problem?

HORMONES
Might a spayed sow be more likely to get stones? In humans, menopause leads to more bone loss and calcium absorption decreases. Calcium may be handled differently in older, spayed sows.

KIDNEY STONES
In humans, kidney stones are most commonly composed of calcium oxalate. Most analyses of guinea pig stones focus on bladder stones, which are usually calcium carbonate.

How might the composition of kidney stones in guinea pigs differ from bladder stones?

Calcium in the Human Diet

Dietary Supplement Fact Sheet: Calcium, provided by the National Institutes of Health, Office of Dietary Supplements, offers a thorough look at calcium in the human diet. It is likely that many of the points covered in the article also apply to guinea pigs.

Find below, selected excerpts from Dietary Supplement Fact Sheet: Calcium

Calcium is required for muscle contraction, blood vessel expansion and contraction, secretion of hormones and enzymes, and transmitting impulses throughout the nervous system [1]. The body strives to maintain constant concentrations of calcium in blood, muscle, and intercellular fluids, though less than 1% of total body calcium is needed to support these functions.

The remaining 99% of the body's calcium supply is stored in the bones and teeth where it supports their structure [2]. Bone itself undergoes continuous remodeling, with constant resorption and deposition of calcium into new bone [1]. The balance between bone resorption and deposition changes with age. Bone formation exceeds resorption in growing children, whereas in early and middle adulthood both processes are relatively equal. In aging adults, particularly among postmenopausal women, bone breakdown exceeds formation, resulting in bone loss that increases the risk of osteoporosis over time [1].

Guidelines for calcium intake in humans:

  210 mg - Birth - 6 mo
  270 mg - 7-12 months
  500 mg - 1-3 years
  800 mg - 4-8 years
1,300 mg - 9-13 years
1,300 mg - 14-18 years
1,000 mg - 19-50 years
1,200 mg - 50+ years

Not all calcium consumed is actually absorbed in the gut. Among the factors that affect its absorption are the following:

    Amount consumed: the efficiency of absorption decreases as the amount of calcium consumed at a meal increases [1].

    Age: net calcium absorption is as high as 60% in infants and young children, who need substantial amounts of the mineral to build bone [1,12]. Absorption decreases to 15%-20% in adulthood and continues to decrease as people age; this explains the higher recommended calcium intakes for ages =51 years [1,12,13].

    Vitamin D intake: this nutrient, obtained from food and produced by skin when exposed to sunlight of sufficient intensity, improves calcium absorption [1].

    Other components in food: phytic acid and oxalic acid, found naturally in some plants, bind to calcium and can inhibit its absorption. Foods with high levels of oxalic acid include spinach, collard greens, sweet potatoes, rhubarb, and beans. Among the foods high in phytic acid are fiber-containing whole-grain products and wheat bran, beans, seeds, nuts, and soy isolates [1]. The extent to which these compounds affect calcium absorption varies. Research shows, for example, that eating spinach and milk at the same time reduces absorption of the calcium in milk [14]. In contrast, wheat products (with the exception of wheat bran) do not appear to have a negative impact on calcium absorption [15]. For people who eat a variety of foods, these interactions probably have little or no nutritional consequence and, furthermore, are accounted for in the overall calcium DRIs, which take absorption into account.

Some absorbed calcium is eliminated from the body in urine, feces, and sweat. This amount is affected by such factors as the following [not included in selected excerpts: caffeine and alcohol]:

    Sodium, potassium, and protein intakes: high intakes of sodium and protein increase calcium excretion [16,17]. Adding more potassium to a high-sodium diet might help decrease calcium excretion, particularly in postmenopausal women [18,19].

    Fruit and vegetable intakes: these foods, when metabolized, shift the acid/base balance of the body towards the alkaline by producing bicarbonate, which reduces calcium loss. Metabolic acids produced by diets high in protein and cereal grains, for example, cause bone to release minerals such as calcium and phosphates and alkaline salts that neutralize the excess acid. In one experiment, women =50 years of age who took supplements of bicarbonate showed significant reductions in calcium excretion, indicating reduced bone resorption [25].

Vegetarians might absorb less calcium than omnivores because they consume more plant products containing oxalic and phytic acids [1]. On the other hand, some vegetarian diets contain less protein than typical omnivore diets, which reduces calcium excretion [2].

Osteoporosis,[is] a disorder characterized by porous and fragile bones....When calcium intake is low or ingested calcium is poorly absorbed, bone breakdown occurs as the body uses its stored calcium to maintain normal biological functions. Bone loss also occurs as part of the normal aging process, particularly in postmenopausal women due to decreased amounts of estrogen. Many factors increase the risk of developing osteoporosis, including being female, thin, inactive, or of advanced age....

[Calcium deficiency] Hypocalcemia results primarily from medical problems or treatments, including renal failure, surgical removal of the stomach, and use of certain medications (such as diuretics). Symptoms of hypocalcemia include numbness and tingling in the fingers, muscle cramps, convulsions, lethargy, poor appetite, and abnormal heart rhythms [2]. If left untreated, calcium deficiency leads to death.

Kidney stones in the urinary tract are most commonly composed of calcium oxalate. In the Women's Health Initiative clinical trial, postmenopausal women who consumed 1,000 mg of supplemental calcium and 400 IU of vitamin D per day for 7 years had a higher risk of kidney stones than subjects taking a placebo [94]. Other studies, however, have found that high dietary calcium intakes decrease this risk [95-97]. For most individuals, other risk factors for kidney stones, such as high intakes of oxalates from food and low intakes of fluid, appear to play a bigger role than calcium [98].

Health Risks from Excessive Calcium
Excessively high levels of calcium in the blood known as hypercalcemia impair kidney function, and lead to reduced absorption of other essential minerals, such as iron, zinc, magnesium, and phosphorus [1,116-119]. However, hypercalcemia rarely results from dietary or supplemental calcium intake and is most commonly associated with hyperparathyroidism, advanced cases of cancer or excessive intakes of vitamin D from supplements at doses of 50,000 IU/day or higher [1].

    See also: http://courses.washington.edu/bonephys/opcalcium.html

Calcium Oxalate Stones in Dogs may be Hereditary
These are the second most common canine bladder stones and are mostly caused by increased levels of calcium in the blood stream. These stones are seldom the result of bladder infections. Instead, they are formed in dogs that have a hereditary condition whereby they lack a calcium-binding glycoprotein called nephrocalcin which inhibits the growth of calcium oxalate crystals in the urinary tract.

Struvites: Struvites are the most common type of bladder stones in dogs. They are composed of magnesium ammonium phosphate. These stones form in an alkaline urine and usually are the result of a bladder infection which raises the urine pH to neutral or alkaline. They are more common in female dogs who are more prone to infections of the bladder and the urinary tract.

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MEDICAL REFERENCE

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    dot Bladder Stones
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    dot Urine Scald
    dot UTI
 

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