Introduction

If you’ve ever attempted to lose weight or are currently trying to do so, you’ve probably started tracking your body weight regularly. To your surprise, you’ve likely noticed that your body weight seems to do funny things even if you’re as perfect as possible with your diet and exercise.

One day it’s down, the next day it’s up, and it seems unpredictable. It might feel difficult to even know whether or not you’re losing weight because your body weight seems to do whatever it wants.

The truth is that this is the nature of your body weight. It’s completely normal for your body weight to fluctuate day to day and it’s not an indication that you’re doing anything wrong.

In fact, everyone’s body weight fluctuates daily for a number of different reasons.

Our goal with this article is to help you understand why your body weight fluctuates so much. We will discuss specific variables that you probably haven’t even thought about in relation to body weight, and we will give you some actionable steps to learn how to properly track your weight to ensure that you’re making progress in the right direction.

Why causes body weight fluctuations?

When we think about our body weight, most of us think about how much body fat we have. We think that any time we’ve lost weight we’ve “lost fat,” and any time we’ve gained weight we’ve “gained fat.” However, the two terms, weight and fat, are not synonymous. Your weight can change from day to day without there being any changes in your body fat. This is because there are other things that impact our weight.

First, our bodies are made of more than just fat. We have bones, muscles, and other organs as well. That being said, the weight of these components doesn’t really fluctuate from day to day. However, there is another variable that can fluctuate tremendously on a daily basis, which will impact our weight.

And that variable is... WATER!

Approximately 60% of our bodies are made of water, that’s a lot! And the amount of water that our bodies are holding on to can fluctuate from day to day which impacts your body weight.

For example, let’s say someone weighs 200 lbs. At 60%, they contain approximately 120 lbs. of water.

Imagine this person is dehydrated from spending all day in the sun and doesn’t drink sufficient water to rehydrate afterward so their water content drops from 60% to 59%. Now they only have 118 lbs. of water.

If nothing else changes, their body weight just dropped 2 lbs. without any changes in body fat.

In addition to water, food is another important variable to consider. The amount of food that we have in our digestive tract at any given time will also influence our body weight acutely. If you have more food in your digestive tract, you’re going to weigh more compared to having less food.

So far, this is all stuff you probably already knew. “If I drink a ton of water and eat a ton of food, I’m going to weigh more, duh.”

However, it’s not just about eating food and drinking water. In fact, there are a ton of additional variables that will influence how much water we retain or how much food is in our digestive system independent of simply how much we are consuming.

Variables that influence water and food retention

Salt (Sodium) Intake: Sodium intake can have an acute impact on body weight due to its role in regulating fluid balance. When we consume higher amounts of sodium, the body retains more water to maintain a balance with the higher sodium concentration. This temporary water retention can lead to a sudden increase in body weight, making the scale go up. However, it's essential to note that this weight gain is primarily due to retained fluids and not actual fat gain. This is one of the reasons why if you go “off your diet” for a day or two and have some salty foods, your weight might go up even if you track your calories properly.

Fiber Intake: If you ever eat substantially more fiber on one day compared to your normal fiber intake, you might see that your weight increases the next day. This is because fiber intake can influence body weight acutely through two primary mechanisms: water retention and the slowing down of gastric emptying. Soluble fiber, found in foods like oats, sweet potatoes, and beans, has the capacity to absorb water and form a gel-like substance in the digestive tract, which can temporarily increase body weight due to water retention. Additionally, fiber-rich foods take longer to digest, leading to a delayed gastric emptying process, which can increase the amount of food in your intestines at any given time, which again, can lead to acute weight gain.

Hormonal Changes (Menstrual Cycle): In the days leading up to and during menstruation, some women may retain more water due to hormonal fluctuations, particularly changes in estrogen and progesterone levels. This water retention can cause a temporary increase in body weight and lead to feelings of bloating and discomfort. It's important to note that this weight gain is usually transient and related to fluid balance, not actual fat gain. After the menstrual period, as hormonal levels stabilize, the excess water weight typically diminishes. These acute fluctuations in weight are a natural part of the menstrual cycle and should not be confused with long-term changes in body composition.

Dehydration (alcohol, weather, diuretics): Different factors such as alcohol consumption, hot weather, and the use of diuretics can influence hydration status. If you’re out in the sun all day or consumed a large amount of alcohol, you’re likely going to be dehydrated the following day, which will result in a drop in body weight. Excessive heat exposure leads to dehydration through increased sweat, and alcohol and other diuretics increase urine production, thus, contributing to dehydration as well. This is why if you go out for drinks with friends, you might wake up lighter the next day even though you indulged.

Intense Exercise: A very intense week of training, where the volume and intensity of your workouts surpass your norm, can lead to acute increases in body weight. This heightened level of exercise can cause significant muscle damage, inflammation, and swelling. As a result, the body often responds by retaining more water in the affected muscles, contributing to a temporary increase in body weight. This water retention is primarily due to the body's natural response to repair and recover from exercise-induced stress. While it might result in a short-term weight gain, it's crucial to understand that this is not indicative of fat gain but rather a response to the physical demands of intense training.

Stress Levels: Elevated stress levels can have an acute influence on body weight, primarily through their impact on water retention. Stress triggers the release of cortisol, a hormone that can lead to increased water retention. Cortisol promotes the reabsorption of sodium in the kidneys, resulting in sodium and water retention as part of the body's natural stress response. If you've had a stressful week at work or home, you might notice a slight increase in weight, even if your dietary habits have been on point. This temporary weight gain is largely attributed to stress-induced water retention and not fat gain. Managing stress through relaxation techniques and self-care can help mitigate these fluctuations in body weight.

Timing of Last Meal: The time between your last meal and when you weigh yourself can significantly influence your body weight acutely. If you typically weigh yourself first thing in the morning but had dinner later than usual and woke up earlier than usual, you might notice your weight is higher. This can be attributed to the effects of gastric emptying and the presence of undigested food in the intestines. Food in the digestive system can add weight temporarily due to water content and the physical mass of undigested food. As the body processes and eliminates this matter, your weight is likely to return to its baseline.

Caloric Density: The caloric density of your diet is another variable that can influence body weight acutely. A "healthier diet" that primarily consists of whole foods and fewer ultra-processed items tends to be less calorically dense, meaning you can consume more food volume for fewer calories. This shift in diet composition results in a greater physical weight of the food itself, which can temporarily raise your scale weight due to the sheer mass of the food consumed. Additionally, healthier diets rich in whole foods are typically higher in fiber, which can lead to increased water retention as fiber absorbs water in the digestive tract. While this may cause a short-term increase in body weight, it is unrelated to fat gain and reflects the changes in dietary composition and water balance.

How do you sort through the noise?

Admittedly, it can be stressful to see your weight fluctuate up and down every other day, especially if you’re trying to lose weight. So, what can you do to cut through the noise and determine whether or not you’re making progress toward your goal?

It’s important to understand that these fluctuations are “acute” changes in body weight, meaning short term. As we’ve mentioned multiple times, it’s not indicative of whether or not you’re losing body fat. For this reason, it’s important not to hyper fixate on what your weight is doing on any given day, but instead, focus on trends over time.

Rather than looking at your body weight each and every day to determine whether you’ve lost weight, take an average of your body weight for the week and only compare weekly averages.

For example, if you look at the data set below, it’s impossible to tell whether or not the person lost weight from week 1 to week 2. The numbers are all over the place! It might even seem to some people as if they gained weight on week 2 because they hit a “high” of 202 lbs, which is higher than any individual weigh-in on week 1. However, if we look at the average for these two weeks, week 2 (199.75 lbs average) is about half a pound lighter than week 1 (200.3 lbs average).

This is why it’s so important to look at weekly averages instead of daily fluctuations. It removes a ton of the noise and doesn’t leave you wondering “what the hell is going on with my weight.”

In addition, it’s important to weigh yourself under similar conditions each day to minimize noise in your weight measurement. We recommend weighing yourself first thing in the morning, with minimal clothing, before eating or drinking, and ideally, and after using the restroom. Doing this will help minimize fluctuations as much as possible, but not completely eliminate them.

Summary

• Body weight naturally fluctuates from day to day, and this fluctuation is often unrelated to fat loss or gain.
• Various factors influence acute changes in body weight, including water retention, food in the digestive tract, and other variables.
• Water, food, fiber intake, hormonal changes, dehydration, intense exercise, stress levels, and the timing of your last meal can all cause temporary weight fluctuations.
• To make sense of these fluctuations, it's crucial to focus on long-term trends rather than daily changes in body weight.
• Calculating weekly averages provides a more accurate representation of your progress, eliminating the noise from daily ups and downs.
• Consistency in weighing conditions, such as weighing yourself in the morning, with minimal clothing, before eating or drinking, and after using the restroom, can help minimize measurement variations.

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