Irisin is a myokine, a hormone released by muscle cells (myocytes) in response to physical activity. This substance acts as a messenger between muscle and other organs, signaling changes related to energy metabolism throughout the body. Understanding which exercises increase irisin helps harness its beneficial effects on metabolic health and energy expenditure.
Understanding Irisin’s Role in Metabolism
Once released into the bloodstream, irisin primarily targets adipose tissue (body fat). Its most celebrated function is promoting the “browning” of white adipose tissue (WAT), the typical energy-storing fat. This process transforms white fat cells into metabolically active beige or “brite” fat cells, which share characteristics with brown adipose tissue (BAT).
These brown and beige fat cells contain a high number of mitochondria and express uncoupling protein 1 (UCP1), allowing them to burn energy to generate heat instead of storing it. By inducing this conversion, irisin increases the body’s overall energy expenditure and thermogenesis. Irisin is also linked to improvements in insulin sensitivity, helping cells manage blood sugar levels more effectively. This dual action makes irisin a target for combating conditions like obesity and type 2 diabetes.
The Physiological Link Between Muscle Activity and Irisin
Irisin production begins within the muscle cell and is directly tied to the stress of muscle contraction during exercise. Muscle activity triggers events leading to the expression of the gene for fibronectin type III domain containing protein 5 (FNDC5), the precursor protein to irisin. The primary regulator in this process is Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a transcriptional coactivator upregulated in muscle by exercise.
The mechanical and metabolic stress of contraction, specifically the increase in intracellular calcium ions (\(\text{Ca}^{2+}\)) and the activation of the AMPK signaling pathway, stimulates PGC-1α. PGC-1α promotes the transcription of the FNDC5 gene. The FNDC5 protein is then cleaved and modified, releasing the soluble myokine, irisin, into the circulation. The amount of irisin released is highly dependent on the degree of PGC-1α activation, which is directly influenced by the intensity and duration of the exercise performed.
High-Intensity and Interval Training Protocols
High-intensity exercise protocols are particularly effective at causing a rapid and substantial increase in irisin release. Workouts like High-Intensity Interval Training (HIIT) place an acute, intense metabolic demand on muscle tissue, maximizing the activation of the PGC-1α pathway. The brief, maximal efforts followed by short recovery periods create significant muscle stress and energy depletion, which powerfully stimulate irisin secretion.
The Tabata method is a classic example, involving twenty seconds of near-maximal effort followed by ten seconds of rest, repeated for a short duration, often four minutes. Work-to-rest ratios of 1:1, such as one minute of high-intensity effort followed by one minute of active recovery, are also common and effective in driving irisin spikes. Compound movements engaging large muscle groups are ideal for these efforts, including exercises like:
- Burpees
- Kettlebell swings
- Box jumps
- All-out sprints
Studies suggest that HIIT results in a more pronounced elevation of irisin levels compared to other training types.
The Impact of Long-Duration Endurance and Strength Training
While high-intensity work causes an acute spike, other training modalities influence irisin levels differently. Long-duration endurance activities, such as running, cycling, or swimming, generally lead to a more sustained, moderate elevation of irisin. These continuous, sub-maximal efforts maintain a heightened metabolic state for a longer period, resulting in a prolonged myokine release. Endurance training performed at a moderate intensity (50% to 75% of maximum heart rate) has been shown in long-term studies to increase circulating irisin levels.
Strength or resistance training also influences irisin, depending on the intensity and volume. Heavy strength training, involving high loads, causes a transient increase in irisin concentrations immediately after a session. Although this training primarily promotes muscle hypertrophy, the contraction and mechanical tension activate the PGC-1α pathway. Research suggests that high-intensity resistance training may be more effective than endurance training at increasing irisin levels over several weeks.
Optimizing Exercise Frequency and Intensity
To maintain the beneficial metabolic effects of irisin, consistency and strategic intensity variation are important in a weekly routine. Since irisin levels increase acutely after a single bout of exercise, performing high-intensity workouts several times a week is beneficial. For maximizing acute spikes, incorporating two to three sessions of high-intensity interval training (HIIT) or high-intensity resistance training (HIRT) into the weekly schedule is a practical approach.
These high-intensity sessions should aim for a level of perceived exertion (RPE) of 7 to 9 on a 10-point scale, meaning the effort is very hard to near-maximal. On other days, moderate-intensity, long-duration aerobic exercise helps sustain the overall elevated metabolic environment. A balanced weekly plan combines high-intensity efforts on non-consecutive days with longer sessions of moderate-intensity cardio, ensuring the muscle tissue receives both the acute, high-stress stimulus and the prolonged, sustained stimulus necessary for optimal irisin release.