Hydration: How much fluid do we need?

How much fluid do we need?

Depending on what you eat, you can get about 4 cups of water from the foods you are eating on a daily basis.  

  • Raw fruits and vegetables contain mostly water

  • Cooked grains and legumes contain some water when they absorb water through the cooking process.

  • High fat foods like nuts, seeds, oils and butter have very little water (which is why hydration is so important on a Ketogenic diet).

While we get some water from food, most of our fluid intake comes from what we drink.  The body will regulate thirst to take in more water when we need it.

You are likely to feel more thirsty when:

  • The temperature is warmer

  • The humidity is lower

  • You’ve eaten something salty

  • You’ve consumed alcohol

  • You’ve been sweating

You are likely to feel less thirsty when:

  • It’s cooler

  • It’s humid

  • You haven’t sweated much

There’s generally a lag time between losing fluid and the body triggering thirst.  Staying ahead of dehydration becomes important for people exercising or working in extreme conditions as even slight dehydration can worsen thinking, focusing, and performance.

How much should you drink?

While the numbers will be dependent on a lot of variables, generally:

  • Most adults need 3L (12 cups or 96 ounces) of fluid daily as a baseline.  If you are consuming a reasonable amount of fruits and vegetables, you can assume 1L (4 cups or 12 ounces) is coming from your food and assume 2L (8 cups or 64 ounces) from drinking.

  • Bigger people need more fluid than smaller people as you have more that needs water.  

  • People who are sick and are losing water through diarrhea and / or vomiting will need additional fluid as well as electrolytes for replenishment.

  • If it’s warmer and / or drier, you might need ½ L (2 cups or 8 ounces) more.

  • If you’re exercising hard, you might need up to 6L ((24 cups or 192 ounces) per day.

If you want a more specific number, you can estimate your fluid needs by bodyweight.  For every kilogram of bodyweight, ingest 30-40mL of water.

Feel free to use this calculator.

The effects of dehydration:

Dehydration can simply mean that we’re not drinking enough water to go about our daily life activities.  Often it means means that we are losing more water than we are taking in.

As the percent of body water loss increases, the consequences become more severe:

0.5%: Increased strain on heart

1%: Reduced aerobic endurance

3%: Reduced muscular endurance

4%: Reduced muscle strength, reduced motor skills, heat cramps

5%; Heat exhaustion, cramping, fatigue, reduced mental capacity

6%: Physical exhaustion, heatstroke, coma

10-20%: Death

But why? Because your blood plasma is mostly water, when you sweat you lower your blood volume.  The lower the blood volume, the harder the heart has to work to move blood through the body. As a result, heart rate increases to meet the demand.

Symptoms of dehydration:

Usually you don’t notice your thirst until you’ve lost about 1-2% of your body water.  Unfortunately as the numbers above indicate, you will have already started to suffer from a performance standpoint.  Beyond 2% you will start to notice more serious symptoms like:

  • Headache

  • Fatigue

  • Low Blood Pressure

  • Dizziness / Fainting

  • Nausea

  • Flushing

  • Rapid heart rate

You may recognize a lot of these symptoms if you’ve ever suffered from a hangover!

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