Like its cousin isoleucine, leucine is one of the most common amino acids known in human anatomy and physiology. In fact, leucine is only second to glycine in terms of amino acid concentrations found in proteins. L-leucine’s chemical composition is identical to that of isoleucine, but yields dissimilar properties due to the different arrangement of atoms. In humans, leucine remains nutritionally essential. This nutrient is incapable being synthesized in mammalian tissues.
Leucine continues to work in sync with the amino acids isoleucine and valine, to protect muscle and act as fuel. These three critical amino acids (isoleucine, leucine, and valine) form the branched-chain amino acids; paramount in promoting “the healing of bones, skin, and muscular tissue.” 
Serving sizes below are based upon 100 grams of a given food source and are expressed in milligrams, representing the amount of leucine contained. The most complete natural sources of leucine include; brown rice, whole wheat, nuts, beans, soy flour, and meats.
|3057||Leavening agents, yeast, baker’s, active dry|
|Vegetables and Vegetable Products|
|4947||Seaweed, spirulina, dried|
|Nut and Seed Products|
|3841||Seeds, sesame flour, low-fat|
|3501||Seeds, sunflower seed flour, partially defatted|
|3404||Seeds, cottonseed flour, low fat (glandless)|
|Legumes and Legume Products|
|6782||Soy protein isolate|
|6782||Soy protein isolate, potassium type|
|3790||Soy flour, low-fat|
|3660||Soy meal, defatted, raw|
|3644||Tofu, dried-frozen (koyadofu)|
|3384||Peanut flour, defatted|
|3223||Soybeans, mature seeds, dry roasted|
|Finfish and Shellfish Products|
|5106||Fish, cod, Atlantic, dried and salted|
|3008||Pork, cured, bacon, cooked, microwaved|
|2985||Pork, cured, bacon, cooked, broiled, pan-fried or roasted|
|2985||Pork, cured, bacon, cooked, pan-fried|
|Dairy and Egg Products|
|6837||Egg, white, dried|
|4046||Egg, whole, dried|
|4013||Cheese, parmesan, shredded|
|3542||Milk, dry, nonfat, regular, with added vitamin A|
|3453||Cheese, parmesan, hard|
|3360||Milk, buttermilk, dried|
|3009||Egg, yolk, dried|
|3221||Snacks, pork skins, barbecue-flavor|
The dietary supplementation of L-leucine for potential benefit in various health conditions remains specific, and varies depending on individual need. Today, leucine is found in a variety of supplements and protein powders sold at health food stores. This availability has spawned an increased usage among persons involved in athletics which require resistance/aerobic training (i.e. bodybuilding). Although it has not been proven to enhance or produce significant changes in body composition, the usage among this demographic for blood sugar regulation and muscular recovery, remains quite popular.
Leucine, 1-3 grams daily, may assist the body in completing many specialized processes.  Increasing the body’s supply and availability of leucine is beneficial in the growth and repair of vital tissues in the body, growth hormone production, and energy regulation. Higher levels of leucine may also inhibit muscle wasting that often times result due to periods of high stress and/or severe trauma.
Leucine is also instrumental in treating assorted, chronic conditions. In study, increases in leucine supplementation have been shown to assist individuals suffering from phenylketonuria. This is a condition in which the body cannot metabolize the amino acid phenylalanine.
Persons suffering from Parkinson's disease have also showed improvement from supplemental leucine; especially when its administration reached levels of nearly 10 grams daily. 
Although 0.2 grams per kilogram of body weight proved ineffective at treating muscular dystrophy over the course of 1 year, initial findings from current research may approve leucine as a potential treatment for this disease. 
The established Recommended Daily Allowance (RDA) for leucine is listed below. Additionally, the U.S. National Academy of Sciences recommends that healthy people achieve .36 grams of highly bioavailable protein for each pound of bodyweight - equaling 0.8 grams of protein, per kilogram of bodyweight.
|Requirement - mg. per kg. of body weight|
|Amino acid||Infant 3 - 6 mo.||Child 10 - 12 yr.||Adults|
|Histidine||33||not known||not known|
|S-containing amino acids||45||22||10|
|Aromatic amino acids||132||22||16|
The average daily intake of leucine is approximately 16 milligrams per kilogram of bodyweight. It is recommended that adequate amounts of isoleucine and valine be taken in conjunction with the dietary supplementation of leucine. A supplemented ratio of 2 milligrams of leucine and valine, for each 1 milligram of isoleucine (2:1), has proven more beneficial than either of these amino acids taken alone.
A deficiency of leucine is extremely rare. Those individuals most at risk for onset include vegetarians with inadequate protein sources, and persons suffering from kidney or liver disease. Symptoms of a deficiency may include dizziness, fatigue, irritability, and headache; all of which resemble a hypoglycemic condition.
The excessive supplementation of leucine has not been directly linked to leucine toxicity. However, it has been theorized that higher levels of leucine taken over a prolonged period of time may contribute to pellagra by inhibiting the absorption of a specific vitamin and amino acid. Pellagra is defined as a disease that results from a dietary deficiency of niacin (a B-vitamin compound) and tryptophan (an essential amino acid).
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2. Balch, Phyllis A., James F. “Amino Acids.” Prescription for Nutritional Healing. Ed. Amy C. Tecklenberg. New York, NY: Penguin Putnam Inc., 3rd Ed. 2000. 42-53.
3. Liniger S (ed): The Natural Pharmacy. Prima Health Publishing, Rocklin, CA. 1998.
4. Werbach MR: Nutritional influences on illness: a sourcebook of clinical research. Keats Publishing, Inc. New Canaan, Connecticut, 1987.
5. Mendell JR, Griggs RC, Moxley RT III, et al. Clinical investigation in Duchenne muscular dystrophy: IV. Double-blind controlled trial of leucine. Muscle Nerve. 1984;7:535, 541.
6. Zest for life information page. “RDA of amino acids.” (1999-2003) http://www.pdrhealth.com/drug_info/nmdrugprofiles/herbaldrugs/101840.shtml (14 Sept. 2004).