To be or not to be… with exercising the question is whether to be fasted or not to be fasted – in this is the Answer.

            To understand why the body loses weight (or why not) when exercising, you need to know a few key concepts that revolve around exercising in a fasted or fed state.

Fasted states versus Fed states, what’s the difference?

             “Fasted” states are defined here as not ingesting (drinking or eating) any calories within the previous 6 hours. It’s a state of lowered blood-insulin levels. Low-insulin levels drive the body to access fat stores as its primary fuel source for any activities it takes part in. 

            “Fed” states, in contrast, are defined as having ingested calories that stimulate increased insulin levels in the blood. In opposite to a fasted state, the body uses the immediate calories available in the blood (the sugars that stimulated insulin release) shutting off use of fat stores for its fuel. 

            Conclusion: If you want to use and lose fat in your body, then exercise in a fasted state! 

It’s not about Calories in versus Calories out: 

            For most, the drives to exercise are tied to its impact on weight and are commonly based on the misconception it’s about calories in versus calories out.  This is exactly why most are frustrated, “I exercise and exercise and I am just not losing weight… Why?” 

The unrecognized benefits of being active

            To be clear, the major benefit of frequent activity is the impact on your total health, both physical and psychological. Your heart health (decreased heart attacks, and severity of any attack, improved heart health, supporting independence),  your brain health (decreased risk of strokes, improved brain function), and numerous other body areas (improved ability/strength of will to live life healthiest to the last heart beat, increased independence, psychologically healthier state, etc)  are the big winners independent of any impact on your weight. If your success is primarily tied to a change of weight, then you’re missing the real unrecognized benefits of these major life changes. The body weight decrease is a far second to living with a healthier heart, brain, and metabolism.  

“Insulin” blood levels are at the center of exercise & weight change!           

            From the 10,000-foot perspective, high blood insulin levels shut off your body’s ability to use fat during activity. In contrast, low blood insulin levels turn on your body to primarily use fat stores during activity (and at rest).  Simply, that is how you burn fat. Low insulin levels. 

Burning Fat or Sugar, it depends on your Insulin blood level

            You have only two fuel sources to use for activity: Sugar (if it is available) or Fat  (which for most of us is more than readily available – especially with me the last few years). 

            Which fuel source you use when being active is determined by if you choose to trigger insulin to be released in the body.

·         High blood insulin levels, shut off fat use and focus on using sugar from the blood, but not fat! (the high insulin is usually a result of your recent intake of sugar that immediately increases insulin)

·         Low blood insulin levels turn on and drive the muscle cells to burn fat.  

            So it’s simple—when exercising to burn fat you need to be active in a low insulin state. Hold it… how do you do this?? 

How to achieve to exercise in a low insulin level so you can burn fat? 

            Easy! It’s what you put in your mouth in the hours before you exercise that triggers your body to release insulin or to not, which puts you in a high insulin state or a low insulin state.. that is, a “fed state” (high insulin), or “fasted state” (low insulin). 

A few points: 

How is sugar intake handled in the body?

·         It’s either stored or it’s used immediately if the body is active and requires energy.

·         Sugar introduced through your mouth is stored if it’s not immediately needed for an activity. That is, increasing your fat stores.

·         When stored, it’s stored as glycogen in the liver and/or as fat in adipose (fat) cells/tissue. This is a necessary prehistoric adaptation for two different situations:

1.       Fight or Flight Emergencies: where the body needs high, immediate energy sources (sugar) for high-intensity activities, where readily-accessible sugar is needed to sustain immediate high-energy actions, the body immediately accesses the glycogen stores where sugar is available.

2.      Periods Without Food: having to travel very long distances searching for our next meal is when fat stores are used as the primary energy source because the body needs to save the glycogen that is the only energy source that can be used for very fast fight-or-flight activities in emergencies. 

Ingesting carbohydrates and proteins triggers insulin release

Refined foods dramatically increase insulin blood levels!! 

In essence, there are only two sources of fuel for your body:

1.       External sources - what food you eat.

2.      Internal sources, from body sources -glycogen in the liver or fat from body fat (adipose tissue) 

For simplicity’s sake you can only use one of these two fuel sources at a time.

·         Your blood insulin levels determine which fuel source the body will choose to use.

·         But your blood insulin level is determined by two factors: what you had ingested (that is “what” you put in your mouth- an immediate trigger), and the length of time since you last ingested foods that trigger insulin release.   

Can you control your insulin blood levels? 

For the most part, you bet you can.  

Blood insulin level = Type of food ingested x When you eat  

            It’s all about the messages you send to your body through your mouth.

You do this through what you eat (type of food) AND when you last ate. With respect to what you eat, if you eat refined foods, or sugars your insulin levels jump dramatically higher (not good) than with intake of proteins. With respect to when you last ate, if you ate recently your insulin levels will be higher than if you have fasted and last ate 6 hours ago. Plain and simple!  

So insulin is the switch that determines if you will burn fat or burn sugar. If you want to burn fat during an activity you need to be in a low insulin (fasted) state. 

            Even more important, sustained weight loss is not about calories in versus calories out. Sustained weight change is about reducing Insulin blood levels.  

            So, if you are working on your weight, one of the worst things you can do before you exercise is have sugar gels or drinks or even eat within a few hours before you exercise (if you have a normal metabolism). All these increase your blood insulin levels and block your body’s ability to use your body fat stores. Fasted exercise is better for sustained weight loss. 


1. J Nutr Metab. 2016; 2016: 1984198.

Published online 2016 Sep 21. doi: 10.1155/2016/1984198

PMCID: PMC5050386

PMID: 27738523

Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults

Jessica L. Bachman, 1 , * Ronald W. Deitrick, 1 and Angela R. Hillman 2


2.  Feeding Influences Adipose Tissue Responses to Exercise in Overweight Men

Yung-Chih Chen, Rebecca L Travers, Jean-Philippe WalhinJavier T GonzalezFrancoise KoumanovJames A BettsDylan Thompson

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