Two Hormones that Impact Fat Loss and Muscle Growth

Sadly, health/fitness subculture seems to give both cortisol and insulin a bad rap. Truth is, these two hormones are absolutely necessary for improving body composition. Insulin and cortisol are arguably the two most demonized hormones in health/fitness subculture; it’s unfortunate that so few people actually understand how important they are for improving muscle growth and fat loss.


Insulin is a peptide hormone secreted in the pancreas of humans in response to elevations in blood sugar levels (e.g. after carbohydrates are ingested). As insulin increases, glucose- transport proteins (GLUTs) are upregulated in cells and allow for the absorption/passage of carbs from the blood, thus lowering blood sugar levels. In this sense, insulin is a “ storage ” hormone.

Unfortunately, modern-day eating habits have caused a dramatic increase in type-2/insulin-resistant diabetes, and insulin finds itself being castigated as the nemesis of human physiology as we know it. But what people seem to look right over is that insulin is one of the most potent anabolic hormones we produce.1 Basically, if your goal is to build a lean, muscular physique then you’ll be well served to let insulin do its thing and help your body build muscle when the time is right rather than trying to avoid it all costs like so many anti-carb zealots suggest.


Cortisol is one of several glucocorticoids produced in humans that is secreted primarily in response to stress (thus it is referred to as a stress hormone). It should be noted that stress isn’t always a negative thing (negative stress is more properly referred to as distress).

Glucocorticoids are a class of steroid hormones produced in the adrenal glands that regulate metabolism, development, immune function, and cognition/alertness. Therefore, cortisol is an essential hormone necessary to sustain life, but as with insulin, its secretion needs to be controlled for optimal health and body composition.

In normal circadian rhythms, cortisol release is very low in the four hours that surround midnight and rises sharply as dawn approaches peaking around 7 AM and subsequently drifting down over the day but remaining elevated well above the midnight period.

Contrarily to insulin, cortisol is a catabolic hormone, meaning it breaks down substrates to provide energy. For example, when we are undernourished/fasted, cortisol acts to maintain nominal blood sugar level by initiating a process in the liver called gluconeogenesis. For better or for worse, gluconeogenesis may utilize amino acids as a substrate for creating glucose. Obviously this is not ideal when muscle growth is the goal, since amino acids are used to synthesize muscle protein.

Research has demonstrated that cortisol strongly impairs protein synthesis in skeletal muscle and sustained exposure to high cortisol levels leads to muscle tissue atrophy.2,3 It’s also important to note that cortisol and insulin have interplay between each other, which will be discussed below.

Cortisol under some circumstances stimulates catabolism. This isn’t good or bad per se, but rather iterating that cortisol is capable of acting positively or negatively for body composition purposes. Insulin counters cortisol’s catabolic actions, which is often why bodybuilders flock to drink whey protein and fast-acting carbs promptly after training.

But this is where the shortsightedness of that train of thought comes into play. If your goal is to improve body composition, then you should not be solely concerned with breakdown of proteins in the body but also the breakdown of fats. While insulin can preserve the breakdown of protein it also can hinder the breakdown in fats. While cortisol can increase the breakdown of proteins it can also increase or make fat breakdown more sensitive to growth hormone.

So you see, there is a balance that needs to be found between protein breakdown and fat breakdown, as both are obviously significant in the grand scheme of building muscle and burning fat.

Research suggests that during endurance exercise/cardio, the cortisol: insulin ratio in the absence of insulin obviously increases and continues to do so during two hours of exercise at 70% VO2max.4 Introducing glucose during this time and it drops the ratio in favor of insulin and thus cortisol cannot act catabolically (i.e. breakdown fat).

Therefore, if fat loss is the goal during this period, lack of glucose and rising cortisol is good. If the goal is to preserve muscle, then you may consider countering the actions of cortisol with something other than glucose or insulin.

Moreover, another study demonstrated that medium and high doses of synthetic glucocorticoids (such as hydrocortisone and prednisolone) significantly increase visceral and total body fat loss.5

From these points, it would seem like the more cortisol, the better, right (especially for fat loss)? But not so fast, as with most things, cortisol excreted in excess can spell danger for muscle catabolism and overall body composition. Excessive cortisol levels are also likely to induce large amounts of inflammation and impair insulin response.6

Insulin, on the other hand, is great for building muscle but inhibits the fat loss effects of cortisol. So we can see that cortisol (and insulin) are both double-edged swords, particularly for improving body composition. The main thing to take away is that the endocrine system doesn’t just secrete hormones for no reason, and insulin and cortisol are required for both muscle growth and fat loss. Optimizing their secretory patterns is worthwhile, especially for those looking to get lean and be muscular.

1) Jepson, M. M., Bates, P. C., & Millward, D. J. (1988). The role of insulin and thyroid hormones in the regulation of muscle growth and protein turnover in respo nse to dietary protein in the rat. British journal of nutrition, 59 (03), 397- 415.
2) McNurlan, M. A., et al. (1996). “Protein synthesis rates of skeletal muscle, lymphocytes, and albumin with stress hormone infusion in healthy man.” Metabolism 45.11 : 1388 – 1394.
3) Gore, Dennis C., et al. “Acute response of human muscle p rotein to catabolic hormones.” Annals of surgery 218.5 (1993): 679.
4) Fellmann, N., Coudert, J., Jarrige, J. F., Bedu, M., Den is, C., Boucher, D., & Lacour, J. R. (1985). Effects of endurance training on the androgenic response t o exercise in man. International journal of sports medicine, 6 (04), 215- 219.
5) Babikian LG. (1962). Effect of Hydrocortisone, Corticosteron e, and Cortisone on Visceral and Total Body Fat in Albino Mice Physiological Zoology; 35 (4):314- 322.
6) Fraser, R., Ingram, M. C., Anderson, N. H., Morrison, C., D avies, E., & Connell, J. M. (1999). Cortisol effects on body mass, blood pressure, and cholesterol in the general population. Hypertension, 33 (6), 1364 – 1368

Disclaimer: This content is for informational purposes only and is not meant as medical advice, nor is it to diagnose or treat any medical condition. Please consult your physician before starting or changing your diet or exercise program. Any use of this information is at the sole discretion and responsibility of the user.