Cancer Patients and Hyponatremia: Understanding the Risks of Low-Sodium Diets

Hyponatremia is a common electrolyte imbalance in clinical practice and is one of the common complications in cancer patients. The normal serum sodium level is 135-145mmol/L, and when the blood sodium level is below 135mmol/L, it is referred to as hyponatremia. The incidence rate among adults is approximately 5%, rising to about 20% in individuals over 65 years old, 35% among hospitalized patients, 30% among those with heart failure, and even up to 50% among cancer or cirrhosis patients [1].
Why are cancer patients prone to hyponatremia?
There are mainly two factors leading to hyponatremia: inappropriate lifestyle and disease-related factors. In cancer patients, hyponatremia often occurs due to the tumor itself or as a side effect of cancer treatment.
Common causes include:
1. Improper diet: Some patients and their families mistakenly believe that cancer patients should have a bland diet and restrict their salt intake. While cancer patients may avoid oily foods, it's crucial to ensure adequate salt intake. Some patients already have low blood sodium levels due to the tumor, and further restricting salt intake can lead to severe hyponatremia. Additionally, some patients experience decreased appetite after anticancer treatment and may only consume foods like plain congee, which lacks salt, resulting in hyponatremia and further reduced appetite in a vicious cycle.
2. Disease effects: Tumor cells can release antidiuretic hormone (ADH), causing increased reabsorption of water in the renal tubules, resulting in water retention and dilutional hyponatremia. This is most common in small cell lung cancer, followed by head and neck tumors, especially those in the oral and pharyngeal regions [2]. Tumors can also lead to pituitary anterior lobe dysfunction, disrupting the balance of ADH/ adrenocorticotropic hormone (ACTH) levels, resulting in dilutional hyponatremia. Additionally, conditions such as heart failure, ascites, and hypoalbuminemia can lead to water retention from excessive fluid intake, causing dilutional hyponatremia.
3. Effects of chemotherapy drugs: Some chemotherapy drugs such as vinorelbine, platinum-based agents, alkylating agents, and cyclophosphamide can stimulate ADH secretion, causing syndrome of inappropriate antidiuretic hormone secretion (SIADH) and directly impairing sodium reabsorption in renal tubular epithelial cells, leading to hyponatremia [3]. Elderly patients with comorbidities such as hypertension and diabetes are particularly susceptible to tumor-related hyponatremia (TRHN). Additionally, opioid drugs, nonsteroidal anti-inflammatory drugs, antidepressants, antipsychotics, antiepileptic drugs, and sulfonylureas used in palliative care for cancer patients can also stimulate abnormal ADH secretion, contributing to TRHN.
4. Other factors: Cerebral salt-wasting syndrome (CSWS) occurs in patients with brain metastases, where brain damage disrupts hypothalamic-pituitary-renal regulation, resulting in excessive renal excretion of water and sodium. Moreover, cancer patients are prone to fever and sweating, leading to sodium loss, which, if not promptly replenished, can also cause hyponatremia.

What are the symptoms of hyponatremia?
Hyponatremia is classified as mild when serum sodium is 130-135mmol/L, moderate when it is 125-129mmol/L, and severe when it is below 125mmol/L.
Mild hyponatremia is generally asymptomatic or presents with nonspecific symptoms such as appetite loss, weakness, difficulty concentrating, and mood changes.
Moderate hyponatremia can cause digestive symptoms such as nausea and vomiting, as well as neurological symptoms such as disorientation, instability, falls, and even altered mental status.
Severe hyponatremia can lead to neurological symptoms due to cerebral cell swelling, such as seizures, coma, respiratory distress, or even death.
How should cancer patients with hyponatremia eat?
1. For hyponatremia caused by sodium deficiency, dehydration, or reduced volume, sodium and fluid replacement therapy should be administered. Some patients may have pseudohyponatremia, such as those with high blood glucose, excessive mannitol, hyperlipidemia, or hyperglobulinemia, which can elevate blood sodium levels. Therefore, whether patients need additional salt supplementation should be based on the advice of doctors and clinical nutritionists.
2. For patients who do require sodium supplementation, it is advisable to avoid overly bland flavors in daily meals. Salted sauces such as soy sauce, bean paste, and vinegar can be added appropriately during cooking, or fresh vegetables can be dipped in sauce to increase sodium intake. The saltiness of dishes should be moderate to avoid affecting the patient's appetite and enjoyment.
3. High-sodium fruits such as papaya, cantaloupe, tomatoes, and bananas, sodium-rich vegetables such as spinach, celery, and carrots, and high-sodium foods such as refined noodles, high-sodium biscuits, dried shrimp skin, salted duck eggs, cheese, sea cucumbers, and wolfberries can be included in the daily diet.
4. Some hyponatremia patients may need to restrict water intake, such as those with early dilutional hyponatremia. It is important to consult a doctor to determine if water intake should be controlled.
5. For chronic hyponatremia patients, sodium capsules can be taken orally to gradually increase serum sodium levels and prevent neurological damage [4]. It is not advisable to consume high-salt foods such as luncheon meat, sausages, and pickles for an extended period as they contain excessive additives such as nitrites and colorants, which are harmful to the patient's body.
6. Patients with cardiovascular diseases and hypertension should avoid excessive salt intake and high-sodium foods.
Hyponatremia has a high incidence rate in cancer patients, with complex pathogenesis and strong concealment, often affecting tumor treatment and even increasing the risk of death. Therefore, patients and their families should pay attention to it, striving for early detection and treatment to avoid serious consequences.

[1] Adrogué HJ, Tucker BM, Madias NE. Diagnosis and Management of Hyponatremia: A Review. JAMA. 2022 Jul 19;328(3):280-291.
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