菸鹼酸 – 維生素B3

Recommended dosage

Niacin is measured in milligrams (mg) of niacin equivalents (NE). 1 NE is equivalent to 1 mg of niacin or 60 mg of tryptophan. The recommended dietary allowance (RDA) for adults over 19 years of age is: 16 mg NE for men, 14 mg NE for women, 18 mg NE for pregnant women, and 17 mg NE for breastfeeding women.

The tolerable upper intake limit is the maximum daily intake that is unlikely to cause harmful effects on health. The UL for niacin for all adults 19 years and older is 35 mg.

Niacin and health

Cardiovascular diseases

For more than 40 years, patients have been taking niacin in its form to treat dyslipidemia, a major risk factor for cardiovascular disease (CVD) such as coronary artery disease, heart attack, and stroke. Blood tests in people with dyslipidemia may show higher levels of total and LDL "bad" cholesterol, lower levels of HDL "good" cholesterol, and elevated triglycerides.


Niacin supplements contain large amounts of niacin, with daily intakes of up to 1,000-2,000 mg. Research shows they can increase HDL cholesterol and lower LDL cholesterol and triglycerides. However, supplements are often associated with side effects (skin flushing, stomach upset, diarrhea), leading to poor patient compliance. Clinical trials have not consistently shown that niacin reduces cardiovascular events or cardiovascular disease deaths.

Although early clinical trials showed that niacin treatment reduced CVD events and deaths, two recent large clinical trials on CVD outcomes have reached different conclusions. Both trials were large randomized placebo-controlled trials that followed participants for up to 4 years. Niacin supplements (1,500-2,000 mg per day) were taken alone in one trial and with statins in another trial. Two trials concluded that taking niacin supplements did not show any benefit. Despite significant improvements in HDL cholesterol and triglyceride levels, the niacin/statin trial did not show a reduction in stroke, heart disease, or cardiovascular disease deaths. Trials of niacin supplementation alone found no reduction in CVD events and a significant increase in adverse effects compared with placebo, including an increased incidence of type 2 diabetes, gastrointestinal bleeding and ulceration, and diarrhea.
A review of 23 randomized controlled trials of niacin supplements to prevent CVD events found that supplements did not reduce overall death, CVD death, heart disease, and stroke, and were associated with negative side effects.
The results of these and other cardiovascular outcome trials led the U.S. Food and Drug Administration (FDA) to conclude that "the scientific evidence no longer supports the conclusion that drugs induce reductions in triglyceride levels and/or increases in HDL cholesterol levels." High" patients receiving statin therapy may have a reduced risk of cardiovascular events. ” Based on this conclusion, the FDA stated that the benefits of taking niacin extended-release tablets and fenofibric acid capsules with statins no longer outweigh the risks and should not be recommended.

cognitive health

Severe niacin deficiency is associated with cognitive decline, such as memory loss and dementia. Niacin is thought to protect brain cells from stress and damage. But it's unclear whether smaller changes in dietary niacin intake can negatively impact brain function.

A large prospective study followed 3,718 men and women aged 65 and older for 6 years, using dietary questionnaires and cognitive assessments. When comparing the highest and lowest intakes of niacin, protective effects against Alzheimer's disease and cognitive decline were found.
The Coronary Risk Development in Young Adults study followed 3,136 men and women aged 18-30 years for 25 years. The study measured dietary and supplemental B vitamin intake and cognitive function. Higher intakes of B vitamins, especially niacin, throughout young adulthood were associated with better cognitive function scores in midlife. However, cognitive function was only assessed at the end of the study, so changes in cognitive function over time are unknown.
Research in this area is limited, but several ongoing clinical trials may shed further light on niacin's effects on brain health.

food source

Niacin deficiency is rare because it is found in many animal and plant foods.

  • Red meat: beef, beef liver, pork
  • poultry
  • fish
  • brown rice
  • Fortified cereals and breads
  • nuts, seeds
  • beans
  • banana

supplements

Niacin is available as a supplement in the form of niacin or niacinamide. Sometimes supplements contain far more than the recommended daily allowance, causing unpleasant flushing side effects. Niacin supplements are also available as a prescription treatment for high cholesterol. It usually comes in the extended-release form of niacin, which is absorbed more slowly and gradually without causing flushing. Because the required dose of niacin is very high, up to 2,000 mg per day, this supplement should only be used under the supervision of a doctor.

Signs of deficiency and toxicity

insufficient

Niacin deficiency is rare in the United States and other industrialized countries because niacin is well absorbed by most foods (except certain grains, where niacin binds to their fiber, reducing absorption) and is added into many foods and beverages. Various vitamins. Severe niacin deficiency can cause pellagra, a condition that causes a dark, sometimes scaly rash on areas of skin exposed to the sun; a red tongue; and constipation/diarrhea. Other symptoms of severe niacin deficiency include:

  • frustrated
  • Headache
  • fatigue
  • memory loss
  • hallucination

Groups at risk of deficiencies

  • Restrict your diet. People whose diets include limited types and amounts of food, such as those who live in poverty or are seriously ill and do not eat a balanced diet, are at greater risk. Developing countries whose main food source is maize or maize are at risk of pellagra because these foods have lower levels of absorbable niacin and tryptophan.
  • Chronic alcoholism. Excessive drinking can reduce the absorption of many nutrients, especially water-soluble vitamins including B complex.
  • Carcinoid syndrome. This is a disease in which intestinal cancer cells grow slowly and release a chemical called serotonin. This syndrome causes dietary tryptophan to be converted into serotonin instead of niacin, increasing the risk of niacin loss.

toxicity

Toxicity from consuming foods containing niacin is rare, but toxicity may occur with long-term use of high-dose supplements. Red, itchy, or tingling skin on the face, arms, and chest are common symptoms. Flushing occurs primarily when taking high-dose niacin supplements (rather than niacinamide). Taking high doses of niacin supplements may also increase uric acid levels, a risk factor for gout.

Other signs:

  • Dizziness
  • hypotension
  • fatigue
  • Headache
  • upset stomach
  • nausea
  • blurry vision
  • In severe cases, impaired glucose tolerance and liver inflammation may occur (at very high doses of 3,000-9,000 mg daily for months/years)

Other information

  • Many B vitamins are thought to help increase energy, including niacin. Because niacin is water-soluble (with less risk of building up to toxic levels in the body), many people don't hesitate to take supplements that may contain 100 times the RDA for the vitamin. Although niacin helps multiple enzymes convert food into ATP (a form of energy), taking doses well above the RDA will not produce a specific boost in energy levels. To get the energy-boosting benefits of niacin, eating a balanced diet with a variety of foods is usually all you need.
  • Corn is naturally rich in niacin, but it binds to carbohydrates, making it difficult for the body to absorb. However, when corn is nixtamalized (a traditional process of tortilla making in which corn is treated with calcium hydroxide, cooked and ground), the niacin becomes absorbable due to the calcium hydroxide treatment.

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