If Americans cut their salt intake by just half a teaspoon per day, it would produce public health benefits on par with reducing high cholesterol, smoking, or obesity, a new study has found.

A study, published in the journal Neurology, is the largest to look at the link between high blood pressure and memory problems.

Blood pressure is the measure of the force of blood as it pushes against the arteries. It’s expressed in two numbers. The first (top) number is the systolic pressure. It’s the force measured while the heart is beating, or pumping blood to the body. The second measure is the diastolic pressure and represents the amount of pressure between beats (while the heart is at rest).

Normal blood pressure is defined as a systolic pressure of less than 120 and a diastolic pressure of less than 90. Hypertension, or high blood pressure is defines as a systolic pressure of 140 or higher or a diastolic pressure of 90 or higher. Patients with a systolic pressure of 120 to 130 or a diastolic pressure of 80 to 89 are said to have pre-hypertension.

No one knows why African-Americans tend to have a higher risk for hypertension. One controversial hypothesis, known as the Middle Passage theory, is that black people who survived the harrowing trip across the Atlantic Ocean from Africa in the holds of ships did so because their bodies were better able to preserve salt, allowing them to avoid death by dehydration. Therefore, the theory goes, some African-Americans have a higher prevalence of “salt-sensitive hypertension”: Their blood pressure increases by unusually high amounts in response to salt.  Isolation of the MYH9 gene does not necessarily refute the theory

About 65 million Americans (roughly one in three adults) have high blood pressure.

Salt-sensitive patients are over three times more likely to experience a heart attack or stroke than those who are not salt sensitive.

Even people with “normal” blood pressure can be salt sensitive.

Researchers are looking for specific genetic links to salt sensitivity that may help them predict which patients are at risk for the problem.