How does dehydration hurt our metabolism and make us tired?
TL;DR: Staying hydrated is an important part of keeping a balanced metabolism and reducing stress (stress = metabolic imbalance). Dehydration causes the liver to put more glucose into the bloodstream, slows down the time it takes for glucose to move through the body, and causes our brains to swell. This leaves us tired and causes headaches. (about 6 min read)
Metabolic tips for staying hydrated to reduce metabolic stress:
Stay hydrated! Your pee shouldn’t be darker than the inside of a lemon.
Drinking liquids that increase cortisol (like too much caffeine) or BGLs (like sugary drinks) will dehydrate you. You may temporarily feel better due to the increased glucose, but this will quickly wear off.
Drink water that’s supplemented with whatever salts you lost in your sweat - but not more than that. If you aren’t sweating, skip the “sports” drinks and go for actual water.
Proper hydration is an important part of your “nutrition” plan. How, you might be asking yourself, can water with literally zero nutrients contribute to balanced nutrition? That’s a good question. Water makes up the majority of our blood volume (and about 55-60% of our total body weight). Being dehydrated reduces the amount of water in our blood and organs, which leads to changes in our metabolism and increased physiological stress.
Dehydration reduces blood volume. Your blood carries all of the nutrition your organs will use. This includes glucose (obviously) as well as fats you’ll use for energy, vitamins and minerals your cells need to function properly, and other metabolites like ketone bodies and other sugars. When you’re dehydrated, your blood is thicker - so even though you have the same amount of all the things in your blood, you have less blood in total. This makes it harder for your heart to pump blood around your body and subsequently slows down the rate of nutrition delivery to your different organs.
Slower blood flow = less glucose getting to your brain and muscles. Since the thicker blood is harder to pump and the reduced volume is messing up the pressurized system of your blood vessels, glucose is getting to your brain slower. However, the reduced blood volume is also increasing the concentration of glucose in your blood - your BGLs increase even if your liver doesn’t put more glucose into the system (which it does, so a double whammy).
This can be hard to visualize, but think about it this way: let’s say your blood is like a carousel with 100 little water animals (100% hydration) that glucose can ride on. Your brain can take a glucose molecule every time a little water animal with a glucose rider passes by. If there are enough water animals on the carousel, then there’s going to be a new glucose rider passing by the brain every time the carousel turns by 1% (1/100). Now, when we get dehydrated, all the glucose riders stay on the carousel but there are fewer water animals to ride. If our blood volume is reduced by 10%*, then we now have 100 glucose riders on just 90 water animals. The carousel has to turn 10% more before a glucose rider will pass by the brain (1.1%, 1/90).
So even though there is the same amount of glucose in the blood, it takes longer and uses more energy to get that glucose to the brain. And we feel tired or have “brain fog”. It also takes longer for enough glucose to get to our muscles when we’re exercising, which leads to decreased performance, feeling more tired, and “heavy limbs”. If you’ve ever felt like you just can’t pick up your legs as much after a long run/hike/bike ride (or the day after a long night out), dehydration is part of the reason why.
Dehydration increases blood glucose concentration AND total glucose in the blood. One way that your body tries to keep enough glucose getting to the brain is for the liver to release more glucose into the bloodstream. So the glucose is not only more concentrated from reduced blood volume, but now it is extra concentrated since there is more total glucose as well.
Dehydration causes your brain to swell. Another adjustment your body makes to help get more glucose to the brain is to “open up” (dilate) the blood vessels in the brain so they can accept more glucose, once the glucose finally flows by. Dilated blood vessels increase pressure inside your head - your brain is literally swelling. The increased pressure leads to headaches, in addition to brain fog. This is one reason why dehydration can be a trigger for migraines.
Salt balance is an important part of hydration. If you are sweating, then most of the problems from dehydration will come from reduced blood volume, losing ions (which you need for your neurons and muscles to fire), and your muscles using glucose faster than you can replenish your BGLs. Staying properly hydrated while exercising (even in the cold!) requires replacing those lost salts and electrolyte supplements may be helpful.
If you aren’t sweating, dehydration is going to increase the concentration of salt in your blood. Increased salt and BGLs are stressful on your blood vessels (more about that later). This stress messes up your blood pressure, can decrease insulin sensitivity, and increase inflammation if it goes on for too long. It’s better to save the sports drinks for hydrating to replace sweat, and just drink water for hydrating when we aren’t sweating.
Metabolic tips for staying hydrated to reduce metabolic stress:
Drink water throughout the day! Your pee shouldn’t be darker than the inside of a lemon.
Drinking liquids that increase cortisol (like too much caffeine) or BGLs (like sugary drinks) will dehydrate you. You may temporarily feel better due to the increased glucose, but this will quickly wear off.
Drink water that’s supplemented with whatever salts you lost in your sweat - but not more than that. If you aren’t sweating, skip the “sports” drinks and go for actual water.
*some studies have shown that blood volume was reduced by 6-13% in runners after marathons.
Well et al., (1982) Hematological changes following a marathon race in male and female runners. Eur J Appl Physiol Occup Physiol.
Maughan et al., (1985) Estimation of plasma volume changes during marathon running. Br J Sports Med.
