*Review* **Utilization of Human Samples for Assessment of Mitochondrial Bioenergetics: Gold Standards, Limitations, and Future Perspectives**

**Rebeca Acin-Perez 1,2, \*, Cristiane Benincá 1,2 , Byourak Shabane 1,2 , Orian S. Shirihai 1,2,3,4 and Linsey Stiles 1,2,3, \***

	- <sup>4</sup> Molecular Biology Institute, University of California, Los Angeles, CA 90095, USA
	- **\*** Correspondence: RAcinPerez@mednet.ucla.edu (R.A.-P.); LStiles@mednet.ucla.edu (L.S.)

**Abstract:** Mitochondrial bioenergetic function is a central component of cellular metabolism in health and disease. Mitochondrial oxidative phosphorylation is critical for maintaining energetic homeostasis, and impairment of mitochondrial function underlies the development and progression of metabolic diseases and aging. However, measurement of mitochondrial bioenergetic function can be challenging in human samples due to limitations in the size of the collected sample. Furthermore, the collection of samples from human cohorts is often spread over multiple days and locations, which makes immediate sample processing and bioenergetics analysis challenging. Therefore, sample selection and choice of tests should be carefully considered. Basic research, clinical trials, and mitochondrial disease diagnosis rely primarily on skeletal muscle samples. However, obtaining skeletal muscle biopsies requires an appropriate clinical setting and specialized personnel, making skeletal muscle a less suitable tissue for certain research studies. Circulating white blood cells and platelets offer a promising primary tissue alternative to biopsies for the study of mitochondrial bioenergetics. Recent advances in frozen respirometry protocols combined with the utilization of minimally invasive and non-invasive samples may provide promise for future mitochondrial research studies in humans. Here we review the human samples commonly used for the measurement of mitochondrial bioenergetics with a focus on the advantages and limitations of each sample.

**Keywords:** bioenergetics; fibroblasts; frozen tissue; leukocytes; mitochondria; oxygen consumption; platelets; respirometry; skeletal muscle
