Can Dehydration Cause Fatigue?

Author : Ari Whitten
Medical Reviewer: Evan Hirsch, M.D.

Can dehydration cause fatigue

Overview of Fatigue and Dehydration

Fatigue is one of the most common complaints brought to doctors and it is commonly linked back to dehydration or not drinking enough water. Dehydration occurs when your body loses more fluids than it takes in making it difficult to carry out normal day to day functions. 

Even mild dehydration can cause moodiness, problems concentrating, headaches and fatigue. There is a range of water intakes or consuming enough fluids that are optimal – you don’t want to drink too little water but you also don’t want to over hydrate. 


Even though it is less common, over hydration is also important to be aware of. Most commonly found among fitness enthusiasts, it causes major stress on the body by constantly trying to flush out the large excess over water. This often leads to the release of vital minerals and nutrients found in the water leaving you feeling fatigued and weak.

Reasons You are Dehydrated

There are several potential dehydration causes leading to chronic fatigue; being in a very hot climate, low phosphorus intake, spending a lot of time outdoors in a dry climate or altitude, high vitamin D levels, exercising without rehydrating, higher pantothenic acid (B5) and pyridoxine (B6) intake, not hydrating adequately while working during the day, low adrenal output (due to adrenal insufficiency), low sodium chloride (table salt) intake, which prevents the body from storing water, very high protein intakes, high calcium, magnesium, zinc and chromium intakes, extremely high sodium or potassium intakes, taking laxatives, excessive urination, diuretics or other medications and also taking certain herbal supplements, which act as diuretics.

What Happens to Your Mentality When You Are Dehydrated?

Hydration enhances mood and energy levels by increasing vasodilation in the arteries of the brain which increases blood flow to the brain and enhances oxygen and glucose delivery to the brain cells, thus improving mood, energy and cognitive performance. The same thing happens for muscular performance. There is a direct correlation between cognitive and physical performance and overall energy levels.1

In one study that tested the hydration levels of adolescents in a hot climate they found that, “The prevalence of the dehydration of the adolescents during school time appeared to be extremely high, and thirst was not driving those adolescents to drink enough.” Hydration was correlated with alertness and the ability to concentrate and inversely correlated with fatigue.2

Research has also found that women who were not sufficiently hydrated after exercise reported difficulty in getting work done and scored lower on questionnaires assessing mood. They also reported higher incidences of headaches and fatigue. This was with only a mild  dehydration and could occur in normal life.3

A similar study done in men showed; “In conclusion, mild dehydration without hyperthermia in men induced adverse changes in vigilance and working memory, and increased tension/anxiety and fatigue.”4

A recent study on Athletes also found that almost all felt fatigued after limiting fluids and water-rich foods for 15 hours. They also had lapses in memory and difficulty concentrating.

Other symptoms of dehydration include:

  1. Low Mood
  2. Dizziness
  3. Confusion
  4. Irritable
  5. Sleepy
  6. Low Body Temperature
  7. Heart Rate Variability 

More severe dehydration can also cause shrinkage in brain volume and other brain changes associated with poor mood and cognitive performance.5

How to Fight Fatigue By Staying Hydrated

For most people allowing thirst to guide how much water you drink is a good idea. However, if you live at high altitude, in a very hot climate or perform lots of exercise, you should make a conscious effort to consume enough water.

If you have trouble drinking enough water throughout the day you can;

  1. Try to drink at least two glasses of water before any planned physical activity
  2. Drink 2-3 glasses of water before each meal
  3. Bring a reusable water bottle with you on the go
  4. Make water more appealing by adding lemon, lime, orange wedges, etc. 
  5. Incorporate high-water content foods such as soup, fresh fruits and vegetables


The best way to gauge how much water you should drink is to check the color of your urine. It should be slightly yellow, but not dark orange or brown color. Contrary to popular belief your urine should not run clear. 

Water is en essential part of health and well-being and plays a prominent role in stabilizing energy levels throughout the day. When you are feeling dizzy, fatigued, or weak, it may be your first thought to grab caffeine. Unfortunately, this spikes your adrenaline and blood pressure, which ultimately leads to an energy “caffeine” crash. 

Instead, you may want to take your water consumption into consideration and track how much water you are drinking on a daily basis.


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Medically Reviewed ByEvan Hirsch, M.D.

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