“Low sodium” on a food label means the product contains 140 milligrams of sodium or less per serving. That is a specific number set by the U.S. Food and Drug Administration, not a loose marketing phrase. Low sodium in your blood is a different thing entirely — a measured lab value, usually below 135 mmol/L, that has nothing to do with what you ate for lunch. The same two words describe a food claim and a medical condition, and mixing them up causes real confusion.
Here is the short version. A low sodium diet label is a regulated claim about a packaged food. A low sodium blood level, called hyponatremia, is a clinical finding your doctor measures with a blood test. Eating low sodium foods does not usually cause low blood sodium. In fact, most people with hyponatremia have a problem with water balance or hormones, not a salt shortage from their diet.
What Does Low Sodium Mean on a Food Label?
The FDA regulates sodium claims on food packaging, and each term has a fixed definition. “Low sodium” means 140 mg or less per serving. “Very low sodium” means 35 mg or less. “Sodium free” or “salt free” means less than 5 mg per serving. These are not vague promises. They are legal definitions a manufacturer must meet to print the claim.
A few related terms work differently. “Reduced sodium” means the product has at least 25 percent less sodium than the regular version — but it can still be high in sodium overall. “Light in sodium” also means at least 50 percent less sodium than the standard product. A reduced-sodium soy sauce, for example, might still deliver a substantial amount of sodium per tablespoon.
This is where label reading gets practical. A product marked “reduced sodium” is lower than its original, but it is not necessarily low in sodium. Only “low sodium,” “very low sodium,” and “sodium free” carry the strict per-serving limits.
How much sodium is that in context?
Federal dietary guidelines recommend keeping sodium under 2,300 mg per day for most adults, which is about one teaspoon of salt. That number includes all sodium from food, not just what you shake from a shaker. A single “low sodium” serving at 140 mg is a small fraction of that daily limit. A “reduced sodium” product could still contain several hundred milligrams.
Most sodium in the American diet does not come from the salt shaker. It comes from packaged foods, restaurant meals, and processed items like bread, deli meat, canned soup, and cheese. That is why the label claim matters more than the seasoning at the table.
What Does Low Sodium Mean in Your Blood?
Low sodium in the blood is called hyponatremia. It is defined as a blood sodium concentration below 135 mmol/L. Normal is generally 135 to 145 mmol/L. This is a measurement of sodium dissolved in your blood, and it reflects how your body balances water and electrolytes — not how much salt you ate that day.
The counterintuitive part: hyponatremia is usually a water problem, not a salt problem. When the amount of water in your blood rises relative to the sodium, the concentration falls. You can develop low blood sodium while eating a normal amount of salt if your body is holding onto too much water.
Sodium is the main electrolyte outside your cells. It controls fluid balance, nerve signaling, and muscle function. When blood sodium drops, water shifts into cells, including brain cells, which is why symptoms can affect thinking and consciousness.
Can a Low Sodium Diet Cause Low Blood Sodium?
In healthy people, cutting sodium in the diet rarely causes hyponatremia. Your kidneys are good at conserving sodium when intake drops. The body regulates sodium tightly through hormones and the kidneys, so a low sodium diet alone is an unlikely cause of low blood sodium.
The exception is drinking very large amounts of water, especially during endurance exercise. This dilutes blood sodium and is a well-documented cause of exercise-associated hyponatremia in marathon runners and other athletes. Drinking excessive fluid without replacing electrolytes is the mechanism, not a low sodium diet.
Certain medications are a more common trigger. Thiazide diuretics, some antidepressants, and certain seizure medications can lower blood sodium. So can medical conditions like heart failure, kidney disease, liver disease, and hormonal disorders such as SIADH (syndrome of inappropriate antidiuretic hormone). Older adults are at higher risk because kidney function and hormone regulation change with age.
What Are the Symptoms of Low Blood Sodium?
Mild hyponatremia often causes no symptoms at all and is found on a routine blood test. When symptoms do appear, they tend to be vague — headache, nausea, fatigue, confusion, or muscle cramps. Because these symptoms are common to many conditions, they do not point clearly to low sodium on their own.
More serious drops can cause vomiting, seizures, or loss of consciousness. How severe the symptoms are depends partly on how fast the sodium falls. A rapid drop is more dangerous than a gradual one, because the brain has less time to adjust.
This is a condition diagnosed by a blood test, not by symptoms alone. If you have persistent confusion, severe headache, or vomiting — particularly with heavy water intake or a new medication — that warrants medical evaluation rather than self-diagnosis.
Low Sodium Labels vs. Low Blood Sodium: How They Differ
The two concepts sit at opposite ends of the same word. One is a food claim you read on a package. The other is a lab value your doctor measures. The table below lays out the difference clearly.
| Feature | Low Sodium Diet Label | Low Blood Sodium (Hyponatremia) |
|---|---|---|
| What it describes | A regulated claim on packaged food | A measured blood concentration |
| Definition | 140 mg or less sodium per serving | Below 135 mmol/L in blood |
| How it is measured | Food package label | Blood test |
| Main cause | Product formulation | Water imbalance, medications, or disease |
| Who it concerns | People watching dietary sodium | Anyone, especially older adults |
Who Should Watch Dietary Sodium?
Reducing sodium is a common recommendation for people with high blood pressure, heart failure, and kidney disease. The evidence that cutting sodium lowers blood pressure is well established, though the size of the effect varies from person to person. Some people are more salt-sensitive than others.
That said, the relationship between sodium and health is more debated than headlines suggest. The strongest evidence supports reducing very high sodium intake. Whether pushing sodium extremely low benefits everyone is less clear, and some research has raised questions about very low intakes. The honest position: aim for moderation within established guidelines rather than chasing the lowest possible number.
Anyone with a diagnosed sodium disorder should follow their clinician’s specific advice. General label guidance does not apply to people managing a medical condition, and self-adjusting sodium intake based on a label claim is not a substitute for medical care.
How Do You Read a Sodium Label Correctly?
Start with the serving size. The milligram number on the label applies to one serving, and packages often contain more than one. A can of soup labeled “low sodium” at 140 mg per serving may hold two servings, doubling the total if you eat the whole can.
Check the percent daily value column. Five percent daily value or less per serving is considered low. Twenty percent daily value or more is considered high. This gives you a quick read without memorizing milligram limits.
- Look at serving size first — the numbers depend on it.
- Treat “reduced sodium” as lower than the original, not necessarily low.
- Use the percent daily value for a fast low-versus-high check.
- Remember that bread, cheese, and deli meat add up even when they do not taste salty.
One clarification worth keeping in mind: a food being low in sodium says nothing about your blood sodium. They are separate measurements in separate systems. A label cannot tell you anything about hyponatremia, and a blood test cannot tell you whether your soup was low sodium.
When Should You Ask a Doctor About Sodium?
Ask about your blood sodium if you take a diuretic or another medication known to affect sodium, if you have heart, kidney, or liver disease, or if you have unexplained confusion, fatigue, or nausea. These are reasons to get a blood test, not to change your diet on your own.
Do not try to correct a suspected sodium imbalance by eating more salt or drinking less water without medical guidance. Both can be harmful depending on the cause. Hyponatremia has many possible causes, and treatment depends on identifying which one applies.
The bottom line is simple. “Low sodium” on a label is a fixed food claim of 140 mg or less per serving. Low sodium in your blood is a medical finding below 135 mmol/L that usually reflects water balance, medications, or disease — not your diet. Keeping the two straight helps you read labels well and know when a symptom deserves a doctor’s attention.
Frequently Asked Questions
What does low sodium mean on a nutrition label?
It means the food contains 140 milligrams of sodium or less per serving, as defined by the FDA. This is a regulated claim, not a marketing phrase.
Can eating a low sodium diet cause low blood sodium?
In healthy people, a low sodium diet alone rarely causes low blood sodium because the kidneys conserve sodium well. Hyponatremia is more often caused by excess water intake, certain medications, or medical conditions.
What is a dangerously low sodium level in blood?
Hyponatremia is defined as below 135 mmol/L, and severe cases can drop much lower and cause seizures or confusion. How dangerous a level is depends on how fast it falls and the person’s health, so a blood test and medical evaluation are needed.
Does low sodium on a label mean it is healthy?
Not necessarily. A low sodium claim only describes the sodium content, and a product can still be high in added sugar, saturated fat, or calories. Read the whole label rather than relying on one claim.
