Leucine (Leu or L) is one of the branched amino acids (BCAAs) essential for growth and development in cats and dogs. Valine and isoleucine are the other BCAAs.
It is highly ketogenic (undergoes oxidation to form ketones for energy production and a molecule of coenzyme A (CoA) as well as being glucogenic (produces glucose when oxidized).
Like the other BCAAs, its intestinal absorption occurs in the small intestine’s jejunum by the help of neutral amino acid transporters while reabsorption inside the kidney occurs at proximal tubule actively.
Foods rich in leucine
Dietary sources of leucine include chicken, beef, pork, fish, dairy, legumes, soybeans, oats, lentils, peanuts, wheat germ, pumpkin seeds, among other foods.
Note that this BCAA is abundant in most natural foods (not limiting). This is the same case with valine and isoleucine.
- It is involved in protein biosynthesis (part of protein building unit).
- It enhances directly the synthesis of protein and it can do so indirectly by increasing the level of insulin in blood plasma and suppresses the degradation of protein in skeletal muscles making it vital in ensuring all amino acids are efficiently utilized in growth, reproduction or maintenance.
- It regulates all BCAAs catabolism or degradation. For instance, its excess amount will promote the catabolism of all the three BCAAs. This role ensures it prevents instances of BCAAs toxicity in cats and dogs.
Based on dry matter, a minimum of 1.28% is required by cats for growth and reproduction and for adult maintenance, the minimum amount is 1.24%. The respective amount based on 1000 kcal ME are 3.20g and 3.10g 9 (AAFCO -2014).
These amounts are enough to meet your feline nutritional needs and you do not have to supplement it.
In kittens, the only clinical sign that was noted is weight loss. No clinical signs have been noted in adult kitties.
There have been no clinical signs of toxicity due to feeding cats with high doses of leucine. On the contrary, weight gain was noted [NRC]. Since it regulates BCAAs catabolism, it keeps in check their levels.
However, there is a source that seems to link “high doses of leucine can result in decreased plasma concentrations of other plasma amino acids and cause depression of growth rate in kittens.”  This could be due to other impairments that make it’s regulation not possible.