What is fatigue?

Are you often exhausted, and not just after a workout? Do you lack spark during the day? Do you wake up tired and feeling rundown? You are likely suffering from fatigue – a feeling of weariness, tiredness, and lack of energy that doesn’t usually go away when you rest. Your lethargy, which can be lingering, exhaustive, and unproductive, could be caused by a number of factors.

How energy is made within the cells

To understand why you might be tired, it is important to know the basics of how cellular energy is produced. There are three energy systems that your body uses to create and maintain a steady supply of energy to get you through an 8-hour workday or an intense workout, or to simply pick up and tuck your kids into bed at night.

Every activity – from folding laundry to running sprints – requires adenosine triphosphate (ATP). Your body doesn’t store much ATP, so you constantly need to remake it. Starting a half marathon or getting up quickly to answer the doorbell? The speed and quantity for which ATP is needed for a movement, and the presence or absence of oxygen, dictates which energy system your body uses.

The Phosphagen-Creatine (PCr) system

You need energy, whether it’s picking up a child or sprinting a few steps to make a crosswalk light. Or maybe you are regularly lifting weights or splitting firewood with an ax. During this kind of short, intense activity, a large amount of power needs to be produced by the muscles, which creates a high demand for ATP. Creatine phosphate, which is creatine and a phosphate molecule, is stored in the muscles and required for this process; whereas carbohydrates, fat, and oxygen are not necessary. The PCr system is the fastest way to make and remake the energy that is used for an all-out movement that lasts up to about 10 seconds. But since there is a limited amount of stored creatine and ATP in our skeletal muscles, fatigue occurs rapidly thereafter.


For all-out exercise lasting 30 seconds to two minutes, glycolysis is the predominant energy system used. Glyco- (the prefix alludes to glycogen stored in muscles or carbohydrates from blood sugar) and -lysis (the suffix means to break down). Usually, purposeful exercise occurs in this timeframe – high-intensity exercises or interval exercises, like sprints, pull-ups, or weightlifting sets of 10 or more reps.

Although energy is created relatively quickly during glycolysis, it is not replenished as fast as the PCr system; but it is the second fastest way to make ATP. Typically, oxygen is not very plentiful when doing fast and intense movements for 30+ seconds, so lactate or lactic acid can build up in the muscles. When this happens, muscles can lose their ability to contract effectively, exercise intensity ultimately decreases, and fatigue commences.

Aerobic System

This energy system uses oxygen and supports lower intensity purposeful exercises and movements, which constitute the majority of demands all day long. The aerobic system uses a variety of reactions in pathways that support you when giving a presentation at work, walking around a fair with your kids, or completing household chores. It is also the energy-producing system in play when you are exercising at an intensity between that which allows you to easily hold a conversation and the point at which you are struggling to speak.

Although the aerobic system can provide longer-lasting energy, it is not used for quick bursts of energy. It uses blood sugar, muscle glycogen (stored sugar), and stored fat to create ATP via many chemical reactions. Fat provides exponentially more energy than sugar sources, thus sustaining this sub-maximal energy production for hours.

Many complex reactions are taking place in different parts of your cells to produce the energy you are using at this very moment. During these reactions, your body requires molecules like nicotinamide riboside, B vitamins, minerals, positively charged ions, and more, to support almost every step of these energy cycles – or energy cannot be made. Similarly, your body demands the macronutrients being used – calories from fats, proteins, or carbohydrates – or the body needs to make the substrates to support your activity.

What is causing my fatigue?

If you lack the spark in your day or you don’t have the pep in your step, then there are many reasons you might not be making enough energy or are feeling fatigued.

Illnesses of many kinds can be one reason. A blood test that looks at your red and white blood cells and a complete metabolic panel can help paint that picture. Examination of your red blood cells can indicate if you have enough iron, if your cells are of appropriate size and shape, and if you are carrying enough oxygen to your working muscles and brain. If not, then you might have anemia (which can be associated with low iron or low folate/B12). Examination of the numbers and proportions of the different types of white blood cells can indicate whether you have a low grade infection.

Food or environmental allergies can also cause fatigue. An allergic reaction results in histamine production in your body, which can alter pathways involved in cognition, respiration, circulation, and digestion, ultimately affecting the way you make and use energy.

Hormonal imbalances can be another cause of low energy. Stress is a factor that can adversely impact sleep quality and quantity, plus it raises adrenal hormones like cortisol, which can leave you feeling tired but wired – and eventually, just tired when your cortisol levels become depleted. Other hormone imbalances, like sex hormones, can influence how your liver plays a role in energy production. Thyroid hormone or insulin/glucose imbalances can also contribute to fatigue.

Prescription medications, like those prescribed for allergies, asthma, blood pressure, anxiety, or depression can also be a cause of low energy levels. Opioid pain medications and sleep prescriptions can cause long-term effects and disrupt your ability to sleep when you need to and to wake up when you want to.

Dehydration can result in fatigue. You will know you are dehydrated if your urine is apple juice color or darker; it should look more like light lemonade.

Dietary excesses or deficiencies can contribute to fatigue. Excess sugar in your diet causes your brain to crave more of that quick energy and makes you feel tired without it.

Magnesium is used in 600 chemical reactions in the body, so when it is deficient, your energy levels take a hit. Dietary antioxidants can also play a large role in cellular energy; for example, vitamins C and E, selenium, and CoQ10 support energy production in the mitochondria – the portion of the cell where most energy is produced.

Too much exercise or too little exercise can affect energy. Too much exercise can cause physical distress, disrupt sleep, and perhaps affect recovery; although this can be alleviated with a well-tailored exercise program. More often, sedentary behaviors can lead to serious illness and affect nutrient utilization, energy, and cardiovascular disease risk.

Sedentary individuals can decrease their fatigue by 65 percent by engaging in regular, low-intensity exercise.1

Lack of sleep from pain, feeling mentally stressed, poor GI health, or any combination is more common than you think. Considering the direct connection between your brain and gut, and that connection’s effect on almost everything in your body, it is no wonder these factors can play a significant role in sleep hygiene.

How can I increase my energy?

First, figure out the cause of your fatigue. Diet? Medication? Stress? If these factors are relatively unchanged, then maybe you need to add specific nutrients.

Work with your health-care professional to check your blood chemistry. A comprehensive blood panel, including blood cell counts, thyroid screening, etc., will indicate whether your red blood cells are transporting sufficient oxygen, if your body is fighting an infection, or if you have a hormone imbalance.

If you become physically exhausted from a low level of exercise, then you probably aren’t getting enough. If you exercise less than 150 minutes a week, then you might feel more energized by moving more. If you have not been exercising, then gradually increase your exercise over time.

  1. Puetz T, Flowers S, O’Connor P. A randomized controlled trial of the effect of aerobic exercise training on feelings of energy and fatigue in sedentary young adults with persistent fatigue. Psychother Psychosom 2008;77(3):167-174.

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