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How to know when you are Fat Adapted


A question I get asked all the time. The “science only” response is to get you RER (Respiratory Exchange Ratio) or RQ (Respiratory Quotient) tested which can be done in any sports physiology lab. Pretty straightforward test and it measures the rate at which you burn fat and glucose as the exercise intensity increases. However, this is done in the lab. It measures 2 variables, oxygen intake and carbon dioxide output. Now I’m not going to get into it here, but there is a lot more to this whole fat adaptation thing than just that.

So, based on 6 years of what I have studied and tested (in the field), I have come up with a few practical ways which you can do yourself that will “arguably” tell you a lot more about what’s going on inside your cells. Here we go:

  1. You are not hungry in the morning

If you wake up starving, there are possibly two main things going wrong. One is that you had a bad sleep thereby messing with your circadian rhythm. This is your own internal biological clock that looks after the timing of everything. It can do this by regulating hormone release. If you sleep poorly, melatonin is not released, cortisol is elevated and prolactin is suppressed. This combination increases appetite or offsets your normal eating patterns.

The other thing that causes hunger in the morning is hypoglycaemia whereby your blood sugar levels are low. This can happen if you have some problems with insulin and tolerance of carbohydrates. In other words, you are insulin resistant and eat a high carb diet.

So sleep and being fat adapted…. A) Controls you appetite hormones and B) your blood sugar levels

2. You can train “fasted”

Following on from No. 1, if you do not feel hungry, the next self test is to do whatever your typical training session is without breakfast. Now I know this can be anything from a 30min yoga session to a 6hr hilly bike ride. So I can’t talk specifically. All I will say is this:

If you can do whatever one of your tougher/longer training sessions is in the morning, without breakfast, and do it as you would have normally, then it’s a pretty good sign you are on the right path.

Example: morning 60-90min steady pace run or 2-3hr cycle or 60min swim sets

No breakfast, no eating during, no hunger during, normal or better performance = on the right path to fat adaptation

Next step up… is if you can do your longest/hardest session of the week, without breakfast.

Example: 4-5hr bike ride or 3-4hr mountain run

No breakfast, minimal eating during, water only, no hunger during, normal or better performance = pretty sure sign you are fat adapted and upregulating your fat adaptation.

3. No need for snacks

If you are hungry between meals, you are likely to have drops in blood sugar levels. This is caused by insulin insensitivity and/or a high carbohydrate diet. Hunger between meals can also simply be a habitual thing… its 11 o’clock in work and everyone is drinking tea/coffee and eating biscuits. So changing the habit is the first step. However, not feeling the need to eat and simply not being hungry between meals, means that your blood sugar levels are steady and insulin is not misbehaving. Becoming fat adapted allows this.


Example: you eat some meat + avocado salad for lunch at 1pm. You do not eat again until 7pm and feel no hunger in between.


4. You can intermittent fast without chewing your arm off

So intermittent fasting is becoming a trend. Like most things today in the diet and fitness industry… they try to package it and then sell it. You’ve got 5:2, 16/8 and various other “protocols”. Here’s all I’ll say:

If you have dinner at say 8pm. Get up the next morning and not feel any hunger. Go to work, school or whatever and not feel hungry during the morning. Get to lunch time without counting down the clock and then eat at an unplanned time. Eat a satisfying meal… not a horse… just a decent meal… and not slosh back half a loaf of bread and 3 slices of cake. Then and only then… can you consider you fast to be beneficial and for it to indicate that you’re fat burning mechanisms are intact. You should be comfortably able to skip breakfast and also lunch…. And just eat dinner without any major effort. This would be a much better sign of fat adaptation.

5. You are not hungry after training

I’ve saved the best till last as its 1. The hardest and 2. IMO, the best indicator.

So you get up and train. Instead of coming back and necking a bowl of cereal, 2 flap jacks and a protein smoothie… you eat nothing. You may have to drive somewhere or you are just showering and distracted with other stuff at home… whatever… but you feel no major hunger and you are not chewing your leg off since you already chewed your arm off during an intermittent fast.

Now I don’t mean a 30min workout in the gym or a yoga class. I mean something where you had to push it and expend a good deal of energy. A long run or bike or hard swim set…. Or a weights/cardio long session in the gym. Something where you out for a while and burnt a decent bit of fuel.

Example: wake up, no breakfast, or just some eggs

Train 9-11am

Drive home 30mins. Shower/change 30mins.

Cook/Eat: 12.30 – 1pm

If at the end of that session you do not feel ravenous or feel no rush to eat… then this is a major sign that you have increased the rate at which you can burn fat as a fuel. Why? As it means you have increased the rate at which you can oxidise fatty acids and convert them to ATP. It means you have increased the storage depot of fatty acids in the muscle called IMTG’s (intramuscular triglycerides).  It also possibly means you have increased gluconeogenesis, which is your own glucose manufacturing process. It may also mean you are producing ketones as an addition fuel source. In addition, you may have lowered your overall inflammatory response to the exercise.

All this occurs when you are highly fat adapted. 

  1. Dave04-24-2016

    What is the authors take on someone who is not fat adapted but who ticks all of the boxes above!?

    Exceptional metabolism is not reserved for those in ketosis only. Arguably the inverse!?

    • Barry Murray04-26-2016

      that depends…

  2. Robby Hanna07-27-2016

    I find these accurate representations of early signs of fat adaptation. One challenge is training in the keto-adapted state long and hard enough in order to increase strength, power and endurance… and in my direct experience this takes both determined commitment over elapsed with applied time in training while in the fully fat adapted state. I haven’t seen any studies actually test these merits as many simply cover a several week period – not nearly long enough for fat adapted newbies to advance their peak performance.

    Because of cultural conventions towards HF diets, it’s way harder than it seems if you live a regular life of balanced social commitments and not the monastic worship of improving the final 2% that professional athletes can indulge.

    Something I hope we all explore more fully and share going forward…

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