Hydration is the most under-tracked variable in macros
Water and electrolytes affect scale weight, lift performance, energy, and appetite, yet almost no one tracks them. Here's a clinical framework for getting it right.
A patient came in last month convinced her cut had stalled. Five weeks in, scale flat for two of them, energy crashing by 3pm, lifts deteriorating, and the moodiness her partner had started noticing. She’d done everything right by the numbers: 1,650 calories, 140g protein, lifting 4x/week. She came in expecting me to tell her to drop another 100 calories.
I asked about water. About 60 ounces a day. Electrolytes? She’d cut out the LaCroix because of the sodium, and she’d been avoiding salt at meals because she’d read it caused water retention. She’d also recently cut her carbs from about 180g to 110g.
That last part was the giveaway. The first week or two on lower carbs flushes a lot of water weight (every gram of glycogen binds roughly 3 grams of water). She’d lost the water and then never replaced the sodium and potassium that left with it. The flat scale wasn’t a stalled cut. It was a dehydrated body adapting badly to an intake she could otherwise have sustained.
We didn’t change her calories. We added 800 mL of water and a half-teaspoon of salt per day, plus an electrolyte packet on training days. Two weeks later her energy was back, her lifts were back, and the scale had dropped 1.8 pounds. Nothing about the diet was wrong. Hydration and sodium were doing all the work.
What hydration actually affects
If you ask a typical macro tracker user what hydration does, you’ll get “it’s good for you” or “fights bloat.” Both true, both incomplete. Here’s the actual mechanism layer, which is more useful than the slogan layer:
Plasma volume. Total body water sits in two main compartments: intracellular (inside cells) and extracellular (between cells and in the bloodstream). When extracellular volume drops, plasma volume drops with it. Plasma is what your heart pumps. Lower plasma volume means lower stroke volume, higher heart rate at the same workload, and faster perceived effort. This is why your sets feel heavier when you’re dehydrated.
Glycogen storage. Each gram of stored muscle glycogen binds about 3 grams of water. If you’re chronically under-hydrated, you store less glycogen, which means lifts feel weaker and recovery between sets gets longer.
Cognition. Studies in moderate dehydration (around 2% bodyweight loss in fluid) show consistent decreases in reaction time, working memory, and mood. The effect size is small in individual studies but the direction is replicated. You’re not imagining the afternoon brain fog when you’ve had one glass of water by 2pm.
Appetite. This one is messier in the literature, but the clinical pattern is real: people who are mildly dehydrated tend to mis-attribute the signal as hunger. I see this in patient food logs constantly, where a 3pm “snack” was actually a thirst response. Drink the water first, wait fifteen minutes, then decide if you’re still hungry.
Scale weight. The most visible and least useful effect. A 1.5 to 3 pound morning fluctuation from hydration shifts is normal and tells you almost nothing about fat balance.
The baseline numbers
The clinical baseline I use is about 35 mL of water per kg of bodyweight per day for a sedentary adult. For a 70-kg woman that’s 2,450 mL, roughly 83 ounces, or about ten 8-oz glasses. The old “eight glasses a day” advice was approximately right for a small, sedentary person and approximately wrong for everyone else.
For training, add 500 to 750 mL per hour of moderate exercise, more in heat. If you’re sweating visibly, you need to replace it. A weigh-in before and after a hard training session, with no fluid in between, gives you a personalized number: every pound lost on the scale during exercise is about 16 ounces of fluid you owe yourself back.
If you’re on a GLP-1, hydration matters more than the baseline suggests. Reduced thirst is a common side effect, and the slowed gastric emptying means you may feel “full of water” before you’re actually hydrated. I tell patients on these medications to set hydration on a schedule, not a thirst cue, for at least the first three months.
If you’re breastfeeding, add 500 to 1000 mL on top of baseline, distributed across the day. The body will prioritize milk production over your hydration status, and you’ll feel it.
Electrolytes, briefly
Water alone isn’t hydration. Hydration is water plus the electrolytes that hold it where it belongs. The three that matter for most people:
Sodium. The most consequential and the most demonized. Reference range for healthy adults is roughly 1,500 to 2,300 mg/day at rest, and meaningfully higher (3,000 to 5,000 mg) for people training hard, sweating heavily, eating low-carb, or on a diuretic-adjacent diet. I see far more sodium-deficient cuts in my practice than sodium-excess ones. If you have diagnosed hypertension or kidney disease, your numbers are different and you should work with your physician, not a blog post.
Potassium. Reference intake around 2,600 mg/day for women, 3,400 mg for men. Most people don’t hit this and don’t notice unless something else goes wrong. Bananas get the marketing but potatoes, beans, leafy greens, yogurt, and salmon are all higher per serving.
Magnesium. Roughly 310 to 420 mg/day depending on sex and age. Deficiency is common and tied to muscle cramps, poor sleep, and elevated stress markers. Most multivitamins under-dose it. A 200 to 400 mg supplement is reasonable for most adults; check with your doctor if you have kidney issues or take certain blood pressure medications.
I don’t generally recommend electrolyte powder brands to patients because the field is full of marketing and the formulations change. The principle is what matters: if you’re cutting carbs, training hard, sweating, or just under-eating, your electrolyte needs go up and food alone may not cover the gap.
Does coffee count
Mostly yes, with caveats. The “coffee dehydrates you” claim came from a 1928 study with eleven subjects and has not held up in modern research. Habitual coffee drinkers show no net diuretic effect at typical intakes; the fluid in the coffee outweighs the modest diuretic effect of caffeine. I count it as roughly 80% of a water equivalent for tracking purposes.
Tea: yes, count it.
Diet soda: yes, count the fluid, though the carbonation can mess with satiety signals in some people.
Alcohol: no, treat as a fluid debit, not a credit. Each standard drink costs you roughly 100 mL beyond the volume of the drink itself.
How to actually track it
Most macro apps treat water as a check-box. That’s better than nothing but loses the precision that matters. What I have patients do:
Track ounces, not glasses. “Glasses” is undefined. A glass at home and a Starbucks “tall” and the cup in your office kitchen are three different volumes.
Front-load. Aim for 30 to 40% of your day’s water by 10am. The cognitive and appetite benefits are bigger when hydration is steady through the morning, not when you’re chugging a liter at 9pm.
Pair training and water on the same view. If your tracker shows yesterday’s water alongside yesterday’s session, you’ll catch the days you under-drank around a hard workout. This is what I’d like Macroline to do natively, and I’ve passed the suggestion along.
Notice the indicators. Urine pale yellow most of the day. Not waking up with dry mouth. Not feeling parched after lifts. Lift quality consistent across the week. Skin not crepe-y in your 30s. These are the downstream signals that tell you the hydration math is working.
The number on the bottle isn’t the point. The point is having enough water and enough sodium in your system to support the training, the cognition, the mood, and the recovery that are all sitting on top of it. You can hit perfect macros and miss this and feel terrible, which is what my patient was doing.
Most of my “stalled cut” cases are not actually stalled cuts. They’re hydration and electrolyte cases dressed up as macro problems. Check the water before you check the calories.
Macroline is not medical advice. If you have kidney disease, heart failure, hypertension, or any condition affecting fluid balance, your numbers are individualized and should come from your physician.