Americans' cholesterol levels are heading in the right direction, a new study finds.

In the United States, average cholesterol levels have decreased significantly from 1999-2000 to 2013-2014, according to the study, published today (Nov. 30) in a research letter in the journal JAMA Cardiology.

Cholesterol is a waxy substance that can build up in blood vessels and increase a person's risk for heart disease, according to the Centers for Disease Control and Prevention. But the body needs some cholesterol to function. For example, HDL (high-density lipoprotein) cholesterol, or "good" cholesterol, carries cholesterol to the liver so that it can be flushed from the body, the CDC says. [Heart of the Matter: 7 Things to Know About Your Ticker]

In the study, the researchers focused on three cholesterol measurements: LDL (low-density lipoprotein) cholesterol, or "bad" cholesterol; triglycerides, which are a type of fat; and total cholesterol. Total cholesterol includes triglycerides, LDL cholesterol and HDL cholesterol.

During the study period, total cholesterol levels in U.S. adults decreased from an average of 204 milligrams per deciliter (mg/dL) in 1999-2000 to an average of 189 mg/dL in 2013-2014, the researchers found. Adults should aim for total cholesterol levels of less than 200 mg/dL, the CDC says.

Triglycerides also decreased during the study period, from an average of 123 mg/dL in 1999-2000 to an average of 97 mg/dL in 2013-2014, according to the study. A healthy triglyceride level is less than 150 mg/dL, the CDC says.

For LDL cholesterol, there was a decrease from an average of 126 mg/dL in 1999-2000 to an average of 111 mg/dL in 2013-2014, according to the study. A healthy LDL cholesterol level is less than 100 mg/dL, according to the CDC.

The researchers noted that the decreases in cholesterol levels were similar in people who were taking cholesterol-lowering medications and those who were not.

The declines in cholesterol levels over the study period may be due to efforts to remove trans fats from foods, the researchers, led by Asher Rosinger, an epidemic intelligence service officer at the CDC, wrote in the research letter.

Trans-fat consumption has been shown to increase people's levels of bad cholesterol and decrease their levels of good cholesterol — two changes that can increase a person's risk for heart disease.

Although the Food and Drug Administration did not officially ban trans fats until 2013, by that point, many food companies and fast-food restaurants had already begun to reduce or remove trans fats from their products. Indeed, the FDA estimates that between 2003 and 2012, trans-fat consumption in the U.S. declined by 78 percent.

In the study, the researchers looked at data on cholesterol levels collected over eight years of the National Health and Nutrition Examination Survey, a government survey that looks at the health and nutrition of Americans. In addition to answering questions, survey participants undergo a physical exam, including giving a blood sample.

Nearly 40,000 adults in the study had their total cholesterol levels measured, more than 17,000 adults had their triglyceride levels measured and about 17,000 adults had their LDL cholesterol levels measured, according to the study.

Originally published on Live Science.