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Cortisol is known as “the stress hormone.”

It helps fuel our “fight or flight” response to stressful situations, such as attacks or threats to survival.

Though many of us are not often faced with life-or-death situations, we still experience cortisol fluctuations in response to modern stressors, such as work stress or lack of sleep.

High cortisol can lead to high blood pressure and blood sugar, insulin resistance, reduced libido, and even chronic diseases like metabolic syndrome and type 2 diabetes.

On the other hand, we don’t want cortisol levels to be too low, either. Low cortisol can lead to general fatigue, loss of appetite, and low blood pressure.

How Do You Keep Cortisol in Check?

To keep cortisol in check, we must keep stress in check. This can be difficult when dieting since dieting itself is a stressor.

A study conducted in 2010 titled "Low Calorie Dieting Increases Cortisol"¹ prompted a plethora of media publications blaming cortisol for weight plateaus and rebounds and lack of successful dieting. However, dieting is complex and not all dieters are the same.

An effective diet is any diet that puts you into a caloric deficit, so it makes sense to consider the impact of calorie restriction on cortisol when thinking about the effects of dieting on cortisol levels.

However, it’s also important to consider one’s current body weight and level of body fat, both of which can also affect cortisol level through activation of the body’s stress response. Finally, we must consider the impact of “diet mentality” on cortisol levels. The simple act of deciding to diet can also be a stressor.

Calorie Restriction and Cortisol

Although there was prior evidence that low-calorie diets increase cortisol levels, a 2016 systematic review found that fasting increased serum cholesterol but that low-calorie diets did not significantly increase serum cholesterol².

While this conclusion conflicts with previous findings, it is important to note that this review focused on an acute response, not a response to months or even years of calorie restriction. One study showed that a 2-year calorie restriction did not increase cortisol levels³, but more research is needed on the impact of long-term calorie restriction on cortisol levels.

For instance, reducing calories by a small percentage for a short period of time impacts the body differently as compared with severely restricting calories for months or years, an act that is not uncommon in many athletes, including bodybuilders.

Despite these limitations, there is a substantial lack of evidence that calorie restriction increases cortisol. In fact, mild caloric restriction has been found to help with inflammation⁴ and decrease levels of markers of inflammation⁵ like CRP and TNF-alpha⁶. Further research is needed that teases out what exactly “mild” calorie restriction means and investigates how the duration of caloric restriction relates to an inflammatory response.

In addition, calorie restriction has been shown to have positive health outcomes, such as increased markers of longevity⁷ and enhanced quality control in skeletal muscle⁸. When we look at calorie restriction by itself, it seems like dieting might not be so stressful on the body after all.

Body Weight, Body Fat, and Cortisol

While research suggests that there are health benefits to calorie restriction, low body weight and low body fat, which are both typical outcomes of dieting, are another story.

There is evidence that overweight individuals actually have the lowest cortisol levels, while obese and underweight individuals have higher levels (with underweight individuals having the highest levels)⁹. Underweight individuals have also shown enhanced cortisol response to waking from sleep¹⁰.

These findings suggest that, even if dieting doesn’t increase cortisol levels because of calorie restriction, dieting might increase cortisol levels if body fat and body weight are too low.

It turns out that the activation of the hypothalamic-pituitary-adrenal (HPA) axis, our central stress response system, is related to low body weight and low percent body fat¹¹. The inverse relationship between body weight/fat and stress response at least partially explains why we feel so drained after dieting for a long time. If it isn’t the calorie restriction that’s giving us problems, it’s the low body weight or percent body fat.

The important takeaway here is to consider your current body weight and composition before you decide to put yourself in a caloric deficit. While someone with a high percent body fat might benefit from dieting, someone who is already extremely lean might end up exacerbating an already-present stress response in his/her body, leading to undesirable health outcomes.

“Diet Mentality” and Cortisol

There is more to dieting than restricting calories or seeking a low body weight or low percent body fat. Someone might say he/she is "dieting" and but not be doing either one of these.

The "diet mentality" or perception of dieting can be a huge stressor regardless of whether someone is actually following an effective diet. For example, repeatedly dieting is associated with emotional stress¹². The mindset that we take on when we choose to diet can, therefore, increase cortisol levels by setting off the body's stress response that we discussed.

When considering the impact on cortisol levels of adhering to a “diet mentality,” we should also take into account sex differences, as dieting is especially common in females¹³, and males and females have different cortisol responses to stress¹⁴. It may correspondingly be the case that adopting a “diet mentality” has different effects in males and females.

There is much more to be explored with regard to the stresses of dieting, but for now, what is important to remember is that dieting can impact our body's stress response not only through our actions and our physiology but also through our perceptions and our psychology.

A useful tool when dieting might be to have a laidback attitude as much as possible to reduce the physiological and psychological stresses involved. We can choose to approach each meal and workout with an open mind and try not to let any speed bumps upset us.

Takeaway Message

Dieting may or may not improve your overall health depending on your current situation. Consider your body weight, percent body fat, dieting history, and overall health status, and definitely consult with your doctor or another health professional before making significant changes to your diet.

Cycles of bulking and cutting can take a toll on the body. Evidence suggests that negative health consequences of repeated dieting, such as above-normal values for blood pressure, heart rate, blood glucose, lipids, and insulin, are more common in normal weight dieters as compared with those who are overweight or obese dieters¹⁵.

Again, the most important takeaway is that dieting when you are already at a low body weight or already very lean can put unnecessary stress on your body.

Recent estimates suggest that a striking 42% of adults worldwide are trying to lose weight¹⁶. Dieting might not lead to negative health outcomes in the beginning, but dieting for long periods of time can put a lot of stress on your body.

To keep cortisol levels and overall stress in check, eat a balanced diet, get plenty of exercise but also plenty of rest, spend time with family and friends, enjoy the outdoors when you can, and make time for activities that make you feel good.

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