This is an open label trial of a supplement called HMB in patients with ALS. The researchers are evaluating its safety and tolerability, as well as its ability to lower insulin-like growth-factor binding protein 7 (IGFBP7) and Neurofilament light chain levels (NFL) and to slow ALS Functional Rating Scale, Revised (ALSFRS-R) progression.

This will be a widely inclusive, three-center, open-label pilot trial enrolling 100 people living with ALS. Duke (25 participants) and Temple (25 participants) will be traditional sites that consent, screen and follow participants in person, Everything ALS (50 participants) will be a decentralized site providing virtual consenting, screening and follow up. The total time commitment will be 9 months. All participants who pass screening will provide demographics, disease characteristics, co-morbidities, and concomitant medications. They will have a baseline ALSFRS-R score obtained, a baseline slope of Listener Effort Prediction Model (LEPM)-predicted Listener Effort (LE) progression, and Motor Movement Exam (MME) score. For those who enroll at Duke or Temple, blood will be drawn for neurofilament light chain (NfL) and IGFBP7 levels. For patients at Duke or Temple who consent to lumbar puncture, CSF will be drawn for NfL and IGFBP7 levels as well. Each month after baseline, participants will be contacted by phone by each site's study coordinator to review adverse events, new co-morbidities, and concomitant medications, and to generate a new clinician-administered ALSFRS-R score. For Duke and Temple participants, at month 1, 3 and 6 they will be asked to return for in person blood draws for NfL and IGFBP7 levels. Those who sign the additional consent for lumbar punctures will have CSF drawn as well and this will be checked for NfL and IGFBP7 levels at month 3, too. LSFRS-R is routinely measured as part of clinical care. All of the described blood tests and investigational treatments are being performed exclusively for research purposes. HMB will be given at a dose of 3g daily.

Inclusion Criteria:

1. Male or female aged at least 18 years.
2. Sporadic or familial ALS diagnosed as per Gold Coast Criteria (37).
3. Patient is able to understand and express informed consent (in the opinion of the site investigator).
4. Patient is able to read and write English.
5. Patient is expected to survive for the duration of the trial.
6. Women must not be pregnant (will have evidence of a negative pregnancy test obtained by study team at baseline, or by local physician within past 7 days or be post-menopausal)
7. Women must not be able to become pregnant (e.g., post-menopausal, surgically sterile, or using adequate birth control methods) for the duration of the study and three months after study completion. Adequate contraception includes abstinence, hormonal contraception (oral contraception, implanted contraception, injected contraception, or other hormonal contraception, for example patch or contraceptive ring), intrauterine device (IUD) in place for ≥ 3 months, barrier method in conjunction with spermicide, or another adequate method.

Exclusion Criteria:

1. Actively or recently (within past 30 days) participating in another intervention trial.
2. Currently or recently (within 30 days) taking HMB
3. Prior side effects from HMB deemed to be significant by the investigator
4. Patient has a medical or psychiatric illness that could in the investigator's opinion interfere with the patient's ability to participate in this study.
5. Pregnant women or women currently breastfeeding.
6. Elevated serum calcium or vitamin D levels.
7. Life expectancy shorter than the duration of the trial.