Leucine

Studies in people

• Open question

  Muscle strength and growth²

  Leucine is one of three branched-chain amino acids that may help muscle growth, but studies disagree.

• Benefit

  Muscle mass in older adults³

  In older people with weak muscles, leucine with whey protein and vitamin D increased muscle mass.

• Benefit

  Muscle loss with age⁴

  In older adults, leucine-enriched whey protein plus resistance training increased lean muscle mass, strength and physical performance.

• Benefit

  Muscle health in older adults⁵

  In people, leucine seems to help older muscles build protein, but no long studies prove it works.

• Benefit

  Muscle loss while dieting⁶

  Leucine may help people keep muscle while losing weight on a low-calorie diet.

• Benefit

  Muscle strength in older adults⁷

  In older adults, a supplement with leucine and vitamin D improved muscle mass and leg function over 13 weeks.

• Concern

  Heart and blood vessel risk⁹

  In people, leucine from protein-heavy meals switched on a cell signal tied to heart and blood vessel disease.

• Open question

  Muscle growth with training¹⁰

  In men lifting weights, leucine added to whey protein built muscle and strength the same as HMB.

• Open question

  Muscle loss in critical illness¹¹

  Scientists do not yet know whether leucine helps muscle wasting in very sick people, as studies so far show no clear effect.

• Open question

  Blood sugar and muscle¹²

  Leucine may help control blood sugar and build muscle, but long studies do not yet confirm these benefits.

• Benefit

  Muscle building after eating¹³

  In healthy young men, leucine helped muscles start building protein after it was eaten.

Animal and lab studies only

• Concern

  Muscle wasting in cancer¹⁸

  In animal studies, leucine-rich diets helped preserve muscle, but some studies found leucine may speed tumor growth.

• Benefit

  Fat and energy use¹⁹

  In lab studies, leucine may help break down fat and support energy use, but more research is needed.

• Benefit

  Memory and thinking²⁰

  In animal studies, cutting leucine in the diet improved memory and thinking problems caused by a high-fat diet.

• Concern

  Heart and blood vessel health²¹

  In animal studies, leucine in food set off a body signal tied to hardened arteries, but only above a certain amount.

Study type not stated

• Benefit

  Obesity and fat inflammation¹⁶

  Leucine supplements may help with weight loss and cut inflammation in body fat, but scientists do not yet know for sure.

• Concern

  Muscle wasting and insulin resistance¹⁷

  Leucine may help muscle, but too much leucine or with fatty foods may cause insulin resistance.

How it’s made and used

• Muscle growth and strength⁸

  In men doing weights, leucine metabolite supplements like HMB and α-HICA gave no extra muscle growth or strength.

• Immune cell fat handling¹⁴

  In people, mice and lab studies, leucine changed how immune cells handle cholesterol and fat.

• Sprint speed in athletes¹⁵

  Basketball players who took leucine for 28 days sprinted faster, but other skills barely changed.

The signal at the top counts the findings in people and in animal and lab studies. A concern in studies of people outranks animal and lab studies, and “No concerns” needs at least one finding in people. When most findings in people point one way, the page leans that way (“Mostly”), and every finding is still shown. Findings whose study type is not stated are shown but not counted. This page is a reference, not medical advice. It describes what regulators and published research say. Talk to a doctor or dietitian about your own diet.