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  • Writer's pictureJess Elliott, MA, CSCS, TSAC-F, XPS

10 Possible Causes of Sudden Weight Gain - Part 1

Updated: Oct 29, 2023


One question which never seems to get old is “How the f*%$! did I gain ___ pounds in ___ day(s)?!?! ← Notice the excessive punctuation to convey the amount of stress this experience creates. Learn what could be causing sudden weight gain.

Now, you’ll notice a couple of blanks in the above section. That’s because everyone's experience is unique. Our weight fluctuations could range from 1 to 10 pounds, and it could vary over 1 to 7 days. But I can tell you this…I’ve NEVER been asked, “why did I gain 20 lbs over 180 days (that's ~6 months, by the way)?” Not once.

The slow crawl never surprises anyone. We know whether our actions caused weight gain over a prolonged period of time. Rather, it’s the seemingly overnight (and sometimes, it actually IS overnight) increase in the number on our bathroom scale - ever-judging us with its stupid numbers. Little, square f*cker.

Before we banish the scale to the confines of a dark, unused closet, throw the damn thing in the trash, or refuse to step on it until we’re 100% confident it will display a number within a tolerable range, let’s take a deep breath and dive a bit deeper. Unless you ate an extra ~3500 calories in a single day (without any change in energy expenditure), you didn’t gain a pound of fat overnight (unless there’s some unknown medical condition at work). This is where you breathe a sigh of relief. I know this is what you really came here for - validation that you didn't gain 3 pounds overnight. That being said, if you DID eat an extra ~3500 calories without increasing energy expenditure, then it’s likely you gained weight from consuming an excess of fuel (remember food is fuel...delicious, life nurturing fuel).

So, is weight gain/loss as simple as calories in, calories out? Yes and no. I actually want to throw a book at people who make this overly reductionist statement - just me? This conversation is about the no - the times when our weight on the scale is fluctuating but not because we’re eating more or exercising less (as “calories in, calories out” would suggest).

In this post, I address the first 5 of 10 possible causes of acute weight gain (that have nothing to do with gaining fat or muscle). Why the first 5 only? Because you have a life, and listing all 10 in one article would WAY too long to read in a single sitting. Again, this article addresses normal weight fluctuations that may be sudden, but that are temporary. As a long-time coach and data-lover, I meticulously track daily metrics from bodyweight and hours of sleep to macronutrient ratios. This list summarizes my observations over several years of formal tracking. See if you can figure out what could be contributing to your sudden “weight gain.” And for the record, it might not be only one of these things at a time.



1. Less Total Sleep than Normal

In my experience, this is instant. But, let’s back up a bit. To ensure an “accurate” reading on the scale, it’s recommended to weigh yourself first thing in the morning and in the same outfit (or lack thereof) for consistency. I always take my initial bodyweight reading (I track changes throughout the day to check hydration levels, especially following long runs or long hikes) upon waking and after using the restroom. This allows me to get a consistent read on my body’s status at the start of each day.

Now, for anyone interested in understanding their body’s patterns and responses, I recommend using a tracker like Whoop (what I use) or an Oura ring. When I get out of bed, I process my recovery through my device, and this begins my sleep analysis. Within 2-10 minutes, I get my sleep and recovery metrics. My sleep metrics show: total hours of sleep, time in various sleep stages, total sleep needed, total time in bed, and sleep performance. When I say “lack of sleep,” I’m referring to total sleep time as opposed to total time in bed. Total sleep eliminates “awake time” or “wake events.”

On days when my total sleep is in the 5 hour range or lower, my weight on the scale is often 1-2 pounds higher than normal. As I mentioned above, it’s an instant change in the scale.


2. Caffeine Intake

For all of you coffee and pre-workout lovers, I get it! I’m not here to bash either of those. So don’t cancel me just yet.

Personally, I love and miss drinking coffee on a regular basis, but alas, my body doesn’t perform well with regular caffeine intake. When I drink coffee for more than a few days in a row, I face nasty headaches and significant fatigue. Additionally, there are only a few formulations of pre-workout that I tolerate well for as long as a month at a time. But after tracking my performance and recovery scores on pre-workout (and even the effects of coffee), I decided the juice isn’t worth the squeeze for me (notice I said “for me” not “for everyone”).

How does this impact my weight on the scale? This one isn’t so instantaneous. If I drink coffee for a few days in a row, that’s when I start to see the effects on the scale. Even adjusting my hydration levels to compensate for the diuretic effects of caffeine doesn’t make a difference. Instead, rather than feeling like my body receives an added boost from the coffee, it feels like it’s fast-forwarding my energy. In other words, any perceived boost in energy feels like it’s at the cost of my later stores of energy. Rather than having consistent energy throughout the day, coffee will enhance my energy in the immediate hours around consumption, but I’m left feeling drained and fatigued in the back half of the day. Then, I need more sleep each day, and even with the extra sleep, I feel exhausted. It’s a horrible downward spiral. By this time (after 3-ish days of coffee intake), my weight on the scale can increase anywhere between 2-4 pounds.

Unfortunately, it takes a few days of zero caffeine just to get my energy levels sorted. In terms of scale weight, it might take a full week (upon eliminating coffee) to get back to normal. Keep in mind, this is simply my experience based on how my body responds. As a result, I decided the exhaustion and prolonged recovery period isn’t worth a very minor boost in energy. Fortunately, I’ve found other strategies to keep my energy levels high throughout the day, but that is a post for another time.

What about pre-workout? Ah, the days of my pre-workout rituals... Sadly, I don't have data from that long ago. However, I’ve recently experimented with pre-workout (120-150mg of caffeine each morning) on two separate occasions, both for a full month at a time. And guess what? Not a single, consistent difference in daily energy expenditure or weight on the scale.

I'm sure some of you are thinking, "isn’t caffeine a performance enhancer?" Absolutely - it’s one of the oldest. So, how do I use it? Very sparingly. I’ll use caffeine on race days, and that’s it. This keeps my caffeine sensitivity high, allows me to perform at my best on race day, and it minimizes any undesirable side effects.

3. Low Hydration Levels

I know it seems counterintuitive, but dehydration causes water retention. I’ve found that my sweet spot for hydration is 132-145 fluid ounces of water per day. On the rare occasion my water levels fall short (under 110 fluid ounces of water), my weight goes up the next morning - usually about 1 pound. Once I increase my fluid levels, I’m back to normal within a day or two.

If you aren’t properly hydrating and you start increasing your water intake, two things will happen immediately: 1. You’ll start peeing constantly, and 2. Your weight on the scale will increase. Your body will require a couple of weeks to adjust to the new hydration levels. So, expect that a bump in hydration (especially if it’s a big one) will cause the weight on the scale to go up temporarily. Once your body adjusts, it’ll go back to normal - possibly a bit below normal because it won’t be retaining as much water weight when properly hydrated.

Additional note: Remember, caffeine is a diuretic so any fluid intake with caffeine should be subtracted from your daily water intake NOT added. Watch your caffeine intake because this could be digging you into a hydration hole without you realizing it. Liquid doesn’t automatically equal hydration.

4. Higher Salt Intake

When I was working as an EMT, we had a saying… “where salt goes, water follows.” This came from IV Therapy class, which is all about fluid balance. Salt is going to pull water into the cells creating a temporary increase in weight.

In my day-to-day life, I don’t add too much salt to my food. When cooking, I use it for prepping steak and roasting certain veggies and that’s about it. As a result, it doesn’t impact my weight 99% of the time. Salt only becomes a scale factor for me when I eat out somewhere. This often looks like a 1-2 pound increase the very next morning. Keep in mind, even “clean” or “healthier” options on a menu can be heavy with salt (or at least heavier than what you normally eat).

So, don’t freak out when you see that number on the scale go up. Eating out while on vacation? Enjoy it!!! Unless you’re substantially overeating that higher scale weight is likely caused by salt and higher carbohydrate intake (which we’ll cover next). It might take a few days to a week to get back to “normal,” but it will go back (unless you actually gained weight from over-consuming). Hydrate well, and it will speed up the process.


5. Higher Carb Days

I love a high carb day. Now, this is tricky because it’s pretty impossible to standardize what is considered “high.” Depending on your metabolic flexibility, training status, physical activity levels, and individual needs, you might thrive on a higher carb lifestyle, a moderate carb lifestyle, or a lower carb lifestyle. Notice, I’m using the word “lifestyle,” because “diet” is associated with a goal of weight loss. And that’s not what I’m referring to here.

Personally, I opt for carb-cycling (and calorie-cycling for that matter) in daily life. I enjoy food, AND I also look at food as fuel for my body. To that end, I fuel my body based on the day’s demands. On long hiking days, I’ll boost my carbs a bit. On long run days, I’ll boost my carbs a lot as it's a higher intensity than hiking (in other words, more anaerobic for me personally) and higher intensity activity relies predominantly on carbohydrates. This sustains my energy levels during activity AND helps me to replenish muscle glycogen post-activity.

What’s the outcome? I weigh more the next day. How much? It depends on how much I increased my calories and carbohydrate intake. It could be a 1-3 pound increase. It’s important to note that every gram of glycogen stores 3-4 grams of water with it. So it’s not just the glycogen (aka stored and converted carbohydrates) that causes the scale to go up, it’s the additional water that comes with it.

In the personal example I listed, I talked about how my high-carb days are my longer activity days. This leads us into possible cause #6, which we’ll pick up in Part 2! Stay tuned!


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