The goal of this intervention study is to determine the effects of indispensable amino acid (IAA) supplementation on children of either sex, aged 18 to 36 months. The main questions it aims to answer are:

• Does IAA supplementation affect biomarkers of Environmental Enteric Dysfunction (EED), through the Dual Sugar Lactulose/Rhamnose (L/R) test, plasma intestinal fatty acid binding protein, plasma lipopolysaccharide binding protein, fecal myeloperoxidase, fecal neopterin, and gut microbiota?
• Does IAA supplementation impact amino acid digestion, as assessed by a dual-stable isotope tracer?
• What is the weight gain and the changes in length-for-Age z-scores (LAZ) and body composition?

The researchers will compare the effects of consumption of standard complementary food with added IAA against a control group that has no added IAA through the assessment of the various biomarkers.

Participants will:

• answer questions regarding personal, demographic, anthropometric, and food intake information.
• provide biological samples, including breath, urine, stool, blood, and saliva.
• consume solutions or meals that contain sugars and/or stable isotope tracers.
• visit the FNRI and receive house visits by study staff.
• consume complementary food with or without supplement daily for 28 days.

Protocol treatment will start within two weeks of enrollment to the study. Following the baseline study assessments, each participant will receive the standard complementary foods (in the control group) or standard complementary foods supplemented with IAAs (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) (in the intervention group).

Standard complementary foods will be comprised of ingredients mainly of rice and legumes. Standard complementary foods are foods that meet the dietary needs of children 6-36 months of age.

At least three observed (controlled) feeding sessions per week will be conducted over the 4 weeks of the study to confirm that the entire supplement was consumed or confirm intake. The observed feeding will take place at the child's home or at the barangay center. Uptake on non-observed days will be based on the caregiver's report. On each intervention monitoring day, a questionnaire will also be administered to assess child morbidity that day and on the day prior. A pediatrician or an alternate medical practitioner will also be invited during the data collection where in stable isotopes will be administered. The duties and responsibilities of the physician include: medical assessment of the participant, assistance in the collection of blood samples, assessment of adverse events (if any), and provision of first aid (if needed).

In the absence of treatment delays due to adverse events, the intervention will continue for 1 month or until one of the following criteria apply:

• Child is not able to tolerate the supplement (throws up)
• Severe acute malnutrition (SAM)
• Inter-current illness that prevents further administration of treatment
• Unacceptable adverse event(s)
• Study participant voluntarily withdraws from treatment OR
• Other general or specific changes in the study participant's condition render the study participant unacceptable for further treatment in the judgment of the investigator.

For each participant, tests to be conducted at study enrolment will be spread over three days. At the end of the study, endline tests will be spread over 2 days. Baseline visits for breath, blood, urine, and fecal collection should be conducted within -3 days prior to study treatment (day 3) while endline visits should be conducted within +7 days of study treatment (day 31). The intervention will be provided in coded sachets, distributed as per convenient for local custom to best ensure compliance with a minimum of three supervised feeds per week to ensure supplement is consumed at least periodically by the child and also to assess compliance.

Inclusion Criteria:

• 18 to 36 months old (of either sex)
• Stunted (LAZ of less than -2)
• Parent, caregiver or guardian able and willing to give written, informed consent

Exclusion Criteria:

• Wasted (WHZ of less than -2)
• Overweight (WHZ of more than 2)
• Have had diarrhea (by self-report) in the preceding month
• Sibling or twin of another participating study child (2 children of different families in the same household are both eligible)
• Have taken antibiotics in the preceding month
• Have food allergies/intolerance
• Had a serious illness within the last three (3) months or have any underlying condition which in the opinion of the pediatrician would put the subject at undue risk of failing study completion or would interfere with analysis of study results