White adipose tissue and skeletal muscle are essential in energy metabolism and they represent the largest organs in the human body. Both tissues have endocrine functions and are able to secrete hundreds of bioactive molecules, many of which are called adipokines and myokines. These terms should be reserved for proteins and molecules secreted from adipocytes, whereas corresponding molecules from skeletal muscle should be derived from myocytes.
During the last decades it has been observed that very few proteins are exclusively derived from either adipose tissue or skeletal muscle but rather produced and released from both cell types; these bioactive molecules expressed in both adipose tissue and skeletal muscle should be named adipo-myokines.
It is well known that diet may extensively influence adipokine expression, transcription and secretion, as exemplified with leptin. Indirectly and directly, exercise can also affect adipokine expression for leptin and myostatin, respectively. It is likely that many adipo-myokines may be influenced by exercise and dietary changes, and provide mechanistic explanations for benefits of exercise and healthy diets.
So far we have little knowledge of how diet may influence expression of myokines in skeletal muscle although it is likely that diet has smaller effect on myokine expression in skeletal muscle as compared to the effect of exercise on adipokines.
Recently, we discovered that physical activity can reduce plasma concentration of the hepatokine fetuin A (protein mainly secreted from the liver). The change in plasma fetuin A and free fatty acids (FFAs) interacted to predict change in glucose infusion rate (GIR) after exercise.
With another approach we discovered 161 candidate secretory transcripts in skeletal muscle biopsies that were upregulated after acute exercise, and 99 that where increased after 12 weeks exercise training. Furthermore, we focused on 17 exercise-responsive transcripts encoding new secretory proteins from skeletal muscle. One of these candidate myokines were identified as colony stimulating factor 1 (CSF1), which was secreted from cultured muscle cells and upregulated in vitro as well as in plasma after exercise. Thus, plasma CSF1 might be a marker of physical activity for short- and long-term although this has to be examined in much larger studies.
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