How Does Mild Dehydration Affect the Body?

By Chloe Bennett, B.Sc.Reviewed by Dr. Jennifer Logan, MD, MPH

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• What is Mild Dehydration?
• The Effects on The Brain
• Effects on Mood
• Effects on Energy Levels
• Effects on Gastrointestinal Function

Water is essential for human life. It is a vital component for extracellular fluids and all living cells. In the human body, body weight is comprised of approximately 55% and 75% of water in infants and the elderly, respectively. Therefore, it’s imperative to ensure that the body has adequate stores of water for processes such as homeostasis to occur effectively.

Despite the well-researched importance of staying hydrated, previous studies suggest that 40% of women, 60% of men, and 20% of children across 12 countries did not meet the recommended water intake values outlined by the European Food Safety Authority. So what are the effects of mild dehydration on the body?

What is Mild Dehydration?

Water plays an essential role in metabolic processing, and the balancing electrolytes alongside regulating body temperature and acting as lubricant for internal organs and joints. Therefore, it’s important for humans to maintain balance between water output and input in order to remain constantly hydrated.

Water balance is influenced by water input versus water output and several factors can influence it such as, environmental conditions, level of physical activity, dietary intake, and age. If the balance of water input and output isn’t controlled, then it can result in deficits or excesses.

Mild dehydration can occur when fluid loss or diminished fluid intake leads to a decrease in total body water content.

Research suggests that individuals can become dehydrated if they lose just 2% of their total body weight due to water depletion. This can cause a range of adverse effects and symptoms, including:

• Inability to focus on tasks
• Light-headedness
• Dizzy spells
• Headaches
• Thirst
• Dry tongue or mouth
• Muscle weakness
• Tiredness
• Feelings of lethargy
• Dry skin

Dry lips and dark color lips caused by dehydration. Image Credit: FlyingFlokerr / Shutterstock

It is recommended that individuals consume between 1.2 -1.5 liters the equivalent to six to eight glasses of water per day for those living in average climates. This water intake can be in the form of liquid water as well as through food consumption, so diet composition affects how much drinking water must be consumed each day. Also, water present in other beverages (such as milk) contributes toward the total water intake needed for a day. However, beverages containing alcohol should not be counted toward this total, as alcohol can actually increase water loss.

For those living in hotter climates, or a quite active, water intake may need to be increased to account for water lost as sweat. If an adequate amount of water isn’t consumed, even mild dehydration can cause a range of health consequences.

The Effects on The Brain

Thirst sensation can be triggered if the body loses between 1-2% of its total water content. As a result, this can lead to deficits in cognitive function which may be of particular concern for those in hot climates as well as the elderly and the very young.

Specifically, mild dehydration has been found to affect short-term memory, alertness and concentration, and subsequently impair performance on tasks such as visual-motor tracking, perceptual discrimination, arithmetic ability and psychomotor skills. However, these effects are thought to be short-lived and are thought to be able to be reversed by the consumption of fluids.

Further studies suggest that cases of mild dehydration can alter the visual vigilance and visual working memory response latency of men. However, when comparing performance of men and women on the same tasks, key aspects of cognitive performance in women are not substantially altered. This disparity in results, may been that sex differences may need further research.

Effects on Mood

Alongside experiencing cognitive deficits, research has highlighted that those with mild dehydration are also more susceptible to experience mood alterations.

A particular study has shown that adults under conditions of mild dehydration reported significantly higher self-reported scores of fatigue, anger and confusion.

Effects on Energy Levels

As well as deficits in cognitive performance, mild dehydration has also been found to severely impair physical performance. Individuals with mild dehydrating undergoing demanding physical activity, have been found to experience a decrease in performance due to increased fatigue, reduced motivation and endurance.

Furthermore, mild cases of dehydration are thought to adversely affect physical performance due to an increase in body temperature, which causes an increase in the perceived effort need to complete the task and can ultimately make it harder to performance in general.

Additionally, as a consequence of dehydration decrease stroke volume and increase heart rate can occur, resulting in an increase in the heart’s work to maintain homeostasis.

Effects on Gastrointestinal Function

Fluids are typically absorbed in the proximal small intestine. Research has found that fluid intake is closely linked to the likelihood of developing bouts of constipation.

One study found that fluid intake restriction resulted in reduced frequency and weight of stools  and an increased tendency to develop constipation. However, like most symptoms, gastrointestinal function seemed to improve following an increase in water consumption.

Mild dehydration can have a range of adverse health effects that can impair both cognitive and physical function. Most importantly, most of the effects can be reversed by increasing water intake.

Further Reading

• All Dehydration Content
• Dehydration - What is Dehydration?
• Dehydration Treatment
• Causes of Dehydration
• Dehydration Symptoms