• Allison Fahrenbach

Is too much HIIT making you fat? HIIT, Hormones, Adrenal Health & Weight Loss

The title of this post is a mess, I know but hopefully you will walk away from reading this with a sound understanding of not only what HIIT is, but how it can work FOR or AGAINST you.


Years ago, HIIT was the trend. The internet was saturated with articles railing against low intensity cardio and advocating high intensity interval training as being the superior form of cardio.

Gyms across the country introduced bootcamp classes.The continued popularity of Crossfit and the rise of trendy workouts like Orange Theory has only furthered the lure of metabolic conditioning and HIIT styled workouts as being effective methods for weight management and fat loss.


HIIT can (in the simplest of terms) be described as a workout that alternates between intense bursts of activity and fixed periods of less-intense activity or even complete rest.

HIIT workouts can be performed using a variety of exercise modalities, anything from sprinting to cycling, to swimming, bodyweight training, cross training, even strength movements.


One of the reasons for HIIT’s popularity is the post-exercise calorie burn which makes it (according to many "experts") more efficient than traditional steady state exercise. This post-exercise period is called “EPOC”, which stands for excess postexercise oxygen consumption.

This is generally about a 2-hour period after exercise where the body is restoring itself to pre-exercise levels, and thus using more energy. The vigorous contractile nature of HIIT workouts makes the EPOC greater, adding about 5-15% more calories to the overall workout energy expenditure. In essence, you continue to burn more calories AFTER a HIIT workout than you would with a traditional steady state cardio session.

Beyond this, studies have shown HIIT results in an increase in metabolism, improved cardiovascular fitness, improved muscle quality, and that it helps with weight management even fat loss.

With so many benefits, I understand the popularity of HIIT and why so many individuals are making room for it in their weekly training routine.

However, nothing in fitness is black and white. There is no "one size fits all".

Is it ALWAYS beneficial to do HIIT?

Or are there cases where it can do more harm then good?

How much HIIT is best?

What is too much?


Several month ago I began working with a client who came to me utterly frustrated with how she looked and felt. Despite training regularly, she had little to no muscular definition and was steadily gaining weight and fat. She also told me she was constantly exhausted, had a low sex drive, and was sleeping poorly. "I train hard, I never miss a workout and I constantly watch what I eat, how am I not making progress?”

When i began to delve into her history here's what I found:

1) She owned her own business and was constantly overwhelmed with managing her company, her employees, etc. She went nonstop from the moment she woke up until she went to bed.

2) She was a passionate mother with two kids that she was responsible for

3) She did a truckload of poorly programmed HIIT (or as I affectionatelly call it... SHIIT) Not only did she take hour long rigorous bootcamp classes, but she also ran, a lot.

4) She was constantly under-eating, particularly protein and carbs. Her typical daily intake didn't even surpass her BMR, so not only was her body stressed from her lifestyle and her training, it was also stressed from insufficient caloric intake.

5) She struggled to fall asleep and claimed she constantly felt "wired"

In this case, based on her busy life and her many responsibilities, I felt her addiction to bootcamp styled HIIT workouts were HURTING, not helping her. Simply put- her body was so stressed that she was seeing diminishing returns for her efforts; low energy, increased visceral (abdominal fat) soft muscles and constant fatigue. This isn't surprising. High-intensity workouts, particularly ones that are longer in duration (10:00 or longer) place more stress on the body and cortisol, the hormone released in response to stress, is both fat increasing and muscle wasting. In essence, too much cortisol makes you softer and fatter.

I did two things initially 1) I increased her calories 2) We went to three full body strength training workouts a week, with 3 days of 30 minutes low intensity walking (outdoors) and only one day of running, her choice of time and distance.

In less then one month of making these changes, she not only lost bodyfat, but she immediately began noticing an overall improvement in her wellbeing. She felt better, she had more energy and she finally began getting 7-8 hours of sleep instead of the 6 she struggled to get before.

This is one instance where HIIT was HURTING and not HELPING.


I'm currently working with a client, who, when we began together was largely sedentary. She worked an office job in which she neither stood nor moved much, and aside from her nightly gym training and her post weights elliptical workout, she did not lead an active life.

Despite her focus on healthy eating, she could never seem to really get the weight off, nor see the muscular definition she was after. "I train in the gym all the time and I do cardio and I just don't ever lose the fat, especially in my midsection."

When I began to delve into her history here's what I found:

1) Her job, while sedentary, was VERY low stress. She told me it was a very "brainless job". She was also not the primary earner in her household, so she did not have any stress in terms of having to provide for her family in that regard.

2) She had no children

3) Overall she led a very low stress life.

4) She was eating "healthfully" but eating too much, particularly carbohydrates. Given that her lifestyle wasn't very active, I didn't feel she had a need to continue consuming a carbohydrate intake that high.

I did two things initially 1) I pulled BACK on her calories 2) She trained a push/pull cycle, with four days of weight training. On three of those days, we followed up her weight training with HIIT workouts, using :20-:30 ALL OUT EFFORTS followed by 2x the rest (:40-:60 seconds). She did this for 8-10 rounds. On the days she did not train she did 40 minutes low intensity walking on the treadmill.

In a few weeks, we began to see the results of her efforts. Her midsection pulled in and the inches in her waist began falling off. She also changed NOTICEABLY in her muscle tone and definition, looking leaner and more defined. She also realized she had more energy then she had previously.

This is one instance where adding in HIIT HELPED the client get their desired results.

I'm using these two client examples so that you can see that sometimes HIIT can help, and sometimes it can hurt. Everyone is different and responds to exercise differently so whether or not HIIT is going to hurt or help will depend upon a variety of factors.


My client in the first study had a very chaotic, busy and stressful life. Whenever I determine whether or not HIIT is appropriate (and if so how much) I first look at the collective stress present in a clients life. When stress is high, it takes a toll on the adrenal glands, and adding intense exercise into the mix is only going to make that worse.

Cortisol is released by the adrenal glands under conditions of high mental and physical stress. In small amounts, this is fine. However, individuals who train intensively too frequently tend to have a baseline cortisol level that’s very high. These consistently high cortisol levels are associated with poor sleep, poor mood, weight gain, depression, suppressed thyroid function, digestive issues, chronic fatigue, and brain fog. Studies have also found that excess cortisol can stimulate visceral fat gain and reduce overall lean muscle mass. In other words, a person might end up gaining weight and looking worse, which is counterintuitive and opposite of the desired objective of training.

The client from case study #1 was also not consuming enough food. This isn't uncommon. Like many women she was both addicted to training and addicted to dieting. But a low calorie/low carb diet is an additional factor in cortisol rise. Blood sugar level drops with low carb meals and the body needs cortisol to take glucose from muscles and liver. This results in a rise in cortisol, which can, in turn result in poor sleep and stimulate fat storage enzymes.

In this client's case, her cortisol was already elevated due to non-exercised induced stresses. And since she was stressed due to daily life, HIIT was only making matters worse.

BUT HIIT HELPED IN CASE STUDY #2 because in this case, my client had a very low stress life. She wasn't overly active, had very few obligations and didn't work a high stress job. Adding HIIT into her training not only helped increase her activity level, but the added stress from training was well received by her body because she wasn't under stress in other areas of her life. Understand that cortisol is a good thing in small, short lived bursts as it helps the body repair, adapt and grow stronger. Since my case stud #2 client had virtually NO stress in her life, she was able to reap the positive benefits of HIIT, such as the release of endorphins, testosterone, growth hormone and insulin. HIIT training helped increase her energy (which dwindled during the day due to the fact that her job wasn't very mentally stimulating), allowed her cells to take up more glucose (which lowered insulin), and helped stimulate her mind and elevate her motivation.

The bottom line in all of this is that YES, when done correctly, HIIT provides numerous health benefits for a number of people. However, to maximize its benefits, it needs to be done within context, implemented the right way and used in moderation. I don’t want to discourage people from doing HIT, or doing exercise they like but I do want to educate them on what is ideal and make them aware of the potential dangers in doing too much high intensity activity.


  • HIIT is transformative for many because it pushes your heart rate into high training zones for short periods of time. But you need to look at how much time you spend "stressed" in your day-to-day life when determining whether or not to add in HIIT

  • Short term increases in cortisol helps the body to repair, adapt and grow stronger. Long term increases in cortisol can cause fatigue, joint pain, weight gain, sleep disruption and mood disturbance.

  • If your life is low stress, collectively, then typically 1-2 HIIT workouts can help, not hurt.

  • All HIIT workouts should be kept SHORT. Including a warm up and a cooldown no more than 20 minutes. Avoid long duration high intensity training classes and bootcamps that have you training at all out intensity for 30+ minutes.

  • Be aware of the symptoms that you may be doing too much high intensity training: A plateau or drop in training progress; An increase in musculoskeletal injuries; Increased frequency of illness; Feeling exhausted, rather than energized, after exercise; Disrupted sleep; Moodiness, a short fuse, or depression; A soft appearance or looking less defined; weight gain, particularly in the midsection.


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