Inflammaging is associated with an increased risk of chronic disease. Monocytes are the principal immune cells for the production of inflammatory cytokines and contribute to inflammaging in the elderly. However, the underlying mechanisms remain largely unknown. Here, we found that monocytes from aged individuals contained high levels of lipid droplets (LDs), and this increase was correlated with impaired fatty acid oxidation. Downregulated peroxisome proliferator-activated receptor (PPAR)-a may be responsible for the pro-inflammatory phenotype of monocytes in aged individuals, as it was positively correlated with LD accumulation and increasing TNF-a concentration. Interestingly, interventions that result in PPAR-a upregulation, such as fenofibrate treatment, TNF-a neutralization, or calorie restriction, reversed the effect of aging on monocytes. Thus the downregulation of PPAR-a and LD levels in monocytes represents a novel biomarker for inflammaging. Furthermore, PPAR-a activation in the elderly may also alleviate long-term inflammaging, preventing the development of life-limiting chronic diseases.Recent progress in the study of immunometabolism has indicated that macrophage lipid metabolism and inflammatory activation are closely intertwined. For example, saturated fatty acids (SFAs) activate Toll-like receptor (TLR) signaling in both human and murine macrophages to elicit inflammatory responses (Milanski et al., 2009;Rocha et al., 2016). Mechanistically, SFAs induce endoplasmic reticulum (ER) stress by activating the c-Jun N-terminal kinase (JNK) and nuclear factor-kB signaling pathways, which can lead to inflammation (Hu et al., 2006;Urano et al., 2000). Conversely, fatty acid (FA) catabolism, including fatty acid oxidation (FAO), sustains anti-inflammatory macrophage differentiation and attenuates ER stress and inflammatory responses in mice and humans (Namgaladze and Brü ne, 2016). Therefore, maintaining an optimal FA level is necessary to ensure efficient mitochondrial respiration and prevent macrophage-mediated inflammation through mitochondrial dysfunction (Jana et al., 2019). In addition, aging is accompanied by modifications to nutrient sensing and metabolic pathways (Saare et al., 2020). These findings have led many to consider whether lipid metabolism reprogramming might alleviate inflammaging by altering lipid metabolism in aging monocytes.The steroid receptor superfamily of peroxisome proliferator-activated receptors (PPARs) has emerged as a link between lipid metabolism and innate immunity. For example, PPAR-a agonists inhibit interferon-g