Why is star fruit dangerous for kidney patients?

There is a star-shaped fruit, better known as "star fruit" or "carambolo. It is common in countries in Asia and Latin America, although it is also present in many other parts of the world. In fact, it stands out for its flavor and for its medicinal properties.

However, there is evidence that negatively relates it to acute kidney disease (ARD) and chronic kidney disease (CKD). For example, Núñez and Moreno, through the ALERTA magazine, refer to it as a nephrotoxic fruit due to its high content of oxalic acid.

The acute kidney damage induced by oxalates from this food is poorly understood in people with normal kidney function. Still, medical case studies reveal otherwise. In diagnosed kidney patients, the toxicity of star fruit consumption is documented in the journal BMC Research Notes.

Obstruction caused by oxalate crystals and cell death of the kidney tubules are listed as possible causes of acute kidney disease. Do you want to know more about this fruit and its nephrotoxigenic potential? Next, we will tell you all about it.

What is star fruit?

The star fruit (averrhoa carambola) or carambola is produced by a tropical shrub belonging to the family Oxalidaceae. We find it in various regions of Central America, South America and in Asia, where it is originally known as the "star fruit." Similarly, the plant has other popular names, such as the following:

  • Carambolo.
  • Chinese tamarind.
  • Cool tamarind.
  • Cucumber tree,.
  • Carambolera.
  • Carambolero.
  • Chinese fruit.

In countries of Europe and North America it is considered an exotic fruit, coveted for its flavor and its particular shape. It has a yellow-orange color that grows in clusters on the stems and trunk. Its pulp is juicy and fibrous, and it has a flavor that varies between sour and sweet.

It contains large amounts of oxalate salts, ranging from 80 to 730 mg / 100 g of fruit. It is often eaten fresh, juiced, cooked, or as a vinaigrette-type dressing. It also has a wide variety of medicinal uses, among which the following stand out:

  • Decreased headache.
  • Relief of arthralgia symptoms.
  • Prevention and relief of coughs, flu and colds.
  • Decrease in fever.
  • Adjuvant against type 2 diabetes.

The star fruit is known for its flavor and medicinal properties. However, it is contraindicated for kidney patients.

Also read: Diet in type 2 diabetes: how should it be?

Nutritional benefits of carambola

In addition to its medicinal applications, the star fruit is a low-seeded berry that contains more than 90% water and provides almost no calories. It does not contain fat and its few carbohydrates are fast-digesting sugars, such as glucose, fructose, and sucrose.

It provides almost 3% of insoluble and soluble fiber, which regulates intestinal disorders, such as constipation. These, in turn, They help to maintain stable blood lipid, blood glucose and cholesterol levels. The slow intestinal absorption of star fruit is beneficial for type 2 diabetes.

Among other things, it is a natural source of antioxidants, since it provides around 34 mg of vitamin C. This is reinforced with its beta-carotene content with provitamin A and polyphenols activity. These antioxidants have the ability to inhibit the activity of free radicals that oxidize tissue cells.

They are also coadjuvants in the optimal functioning of the immune system to prevent infectious diseases. Vitamin C improves collagen synthesis and is key to maintaining healthy skin.

Another outstanding micronutrient of this fruit is potassium, which facilitates the regulation of the nervous impulse and normal muscular and cardiac activity. Phosphorus, on the other hand, is in a good quantity to help in the formation of bones and teeth.

At the same time, this mineral allows the proper use of fats and carbohydrates, by regulating your metabolism and storing energy in the body in the form of adenosine triphosphate (ATP).

Why should kidney patients avoid star fruit?

To talk about the star fruit in kidney patients, it is necessary to comment on oxalic acid. This substance is found in many foods and is considered a chelating agent in the world of chemistry. Traps various minerals, with which it forms salts or insoluble precipitates.

The most common is calcium oxalate, the main constituent of kidney stones. However, the most harmful form for kidney patients is the soluble form of oxalate.

Read also: Foods that help improve kidney function

Mechanism of action of star fruit in kidney damage

According to Núñez and Moreno, the high content of oxalic acid in the star fruit binds calcium in the gastrointestinal tract. Then, the oxalate salts are formed, which are insoluble and not reabsorbed. Meanwhile, the soluble form of oxalic acid is reabsorbed in the intestine, passes into the blood, and then binds to calcium circulating in the kidney's tubules.

These precipitates cause injury and acute obstruction of the kidney. As pointed out by ALERTA magazine, andn experimental animals cause renal epithelial cell death. On the other hand, star fruit causes harmful effects at the brain level, attributed to a nitrogenous metabolite known as «caramboxin».

People without kidney damage pass them without problems, but kidney patients accumulate them over time. Consequently, toxicity occurs in the brain. Specifically, it can produce the following:

  • Excess brain excitability.
  • Vomiting
  • Mental confusion.
  • Weakness.
  • Epileptic attacks.
  • Eat and death.

The high content of oxalic acid in star fruit is associated with kidney complications, especially in patients with diagnosed diseases.

Can kidney damage from carambola also occur in normal people?

Few people in the world have experienced ARD for the first time when ingesting large amounts of star fruit. However, a post in Indian Journal of Nephrology reports 5 cases of nephrotoxicity in normal people after eating the fruit or ingesting its juice.

The patients had abdominal pain, decreased urine output, and very high creatinine, which in some cases required dialysis. All patients recovered and the biopsy revealed the presence of calcium oxalate and tubular necrosis.

In the study of BMC Research Notes they presented themselves 2 cases of diabetic people, without kidney complications, who developed acute kidney disease (ARD) and chronic kidney disease (CKD) after consuming large amounts of star fruit. They quickly recovered without the need for dialysis.

Another 3 cases of patients with type 2 diabetes who developed ARD when ingesting star fruit were reported through the journal BMC Nephrology.

How much carambola can cause kidney damage?

The maximum amount of "star fruit juice" or fresh fruit has not yet been determined in kidney patients. However, there are cases of consumption of 300 ml of concentrated juice on an empty stomach, which have subsequently developed kidney failure.

The amount of fruits that can cause toxicity is minimal. 25 g of fruit or half of a star fruit are indicated. The frequency of consumption is also important. A diabetic patient developed CKD after 4 days of consumption.

The following situations are considered risk factors for poisoning:

  • Consumption in fasting period.
  • Ingestion during dehydration processes.
  • Consumption of fruit juice in very large volumes.
  • Ingestion in the presence of previous kidney lesions.

In general, before consuming the star fruit, it is best to make sure that the renal excretion function is in good condition. Patients with type 2 diabetes mellitus should be more careful when consuming it. In fact, the ideal is to consult with the doctor.