Why Does My Weight Fluctuate Day to Day?

Find out how bowel movements, food, and menstruation affect your weight

If you weigh yourself every morning, you probably notice that the number on the scale can change significantly from one day to the next. Sometimes the reason for the daily weight fluctuation is obvious. Perhaps you indulged in a big meal before bedtime that resulted in weight gain or had a very sweaty workout that resulted in weight loss. But there are be other reasons that your weight fluctuates day to day.

If you are trying to lose weight or change your body composition, you might be tempted to believe that the daily weight change is due to fat loss or fat gain. And that is a possibility. But there are many other factors that affect your weight from day to day.

Daily Weight Fluctuation: What's Normal?

weight scale
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In a recent interview about how to lose water weight, Dr. Kathleen Wyne, a board-certified endocrinologist, said that a five-pound weight shift is typical for most people day-to-day, but that the number on the scale can change by as much as 20 pounds depending on your body size. So why the big swing? And what causes these frustrating weight fluctuations from day to day? These factors contribute to an increase or decrease on the scale.

Sodium

High salt foods ​can cause water retention. The extra water adds up to pounds on the scale. Some people are very sodium sensitive and may retain more water and others are less so.

So what if you don’t think you consume a lot of sodium? Many of us don’t overuse the salt shaker at mealtime. But sodium can hide in unexpected places. Cold cuts, frozen meals, and savory sauces are often high in sodium. Canned soup is another common culprit. Many varieties of low-calorie soup are very high in sodium. But even homemade soups can have a lot of added salt. If you’re a dieter who is trying to cut back on big meals, you might notice an increase in the scale when you replace a big meal with low-calorie soup—even though the weight is simply water retention.

Carbohydrates

If you love bread, pasta, rice and other starchy carbs, the weight gain you see on the scale may be related to your carb intake. For every gram of carbohydrate you consume, your body retains three grams of water in order to store the fuel source. For that reason, if you eat a very high carbohydrate meal, your body weight is likely to increase because of the water weight, not because of increased fat. In addition, many refined carbohydrate foods are also high in sodium. For example, a spaghetti and meatball meal with parmesan cheese may cause you to retain water due to the carbohydrate intake and due to the high salt content.

Food Weight

Food intake will, of course, cause your weight to increase slightly as it is processed by your body. The food you consume can weigh a few ounces per meal to a few pounds per day. The water in food can cause your weight to increase as well, and many times after you've been eating a lot this water weight is what causes the number on the scale to rise. Additionally, according to some experts, consuming two cups of water—from beverages or water in food—increases your weight by one pound.

So what happens to all of that weight? It doesn’t automatically stick to your thighs. The calories in food are either used to fuel your body’s natural processes or the energy is stored to be used later. Waste products are processed and excreted by your body in the form of urine and stool (bowel movements).

Bowel Movements

You’re likely to see some fluctuations on the scale due to bowel movements. Have you ever wondered how much your stools weigh? In a research study, investigators found that you might produce 125 to almost 170 grams of stool per day. That’s less than a half pound. However, other studies report average daily stool weight to be roughly 106 grams per day—less than a quarter pound.  Still, other sources report that you might poop up to one ounce per day for every 12 pounds of body weight. The bottom line? Normal bowel habits vary but you won’t see major weight fluctuations from bowel movements alone. In fact, even when you lose stool weight, there will still be digestible material in transit. Normal physiological fecal transit time is estimated to be vary between 40 and 60 hours with an optimal whole gut transit time of 24-48 hours. Transit time is improved if you consume more dietary fiber.

Exercise

Exercise can cause you to sweat and lose water weight. Exercise experts estimate that the average person loses approximately 25 to 45 ounces of fluid per hour during exercise, especially intense cardiovascular activity. But of course that number can vary greatly based on weather conditions and other factors. And fluids lost from sweat shouldn’t make a difference on the scale. Why? Because fluids lost during exercise should be replaced. So if you notice that you lose weight consistently after exercise, you may want to come up with an improved hydration plan.

But other forms of exercise can cause daily weight fluctuations as well. Lifting weights or any form of strength training can cause your muscles to retain water. Why does this happen? When you participate in strength training, you create tiny tears in the muscle. Your muscles store and use water to repair the damage. When you create and repair these micro tears your muscles become larger and stronger.

Medication

Certain medications may cause you to gain weight. Some increase your appetite, some may cause you to retain water, and according to the Obesity Action Coalition, “others may affect how your body absorbs and stores glucose, which can lead to fat deposits in the midsection of your body.” If you are on a medication to treat conditions including diabetes, high blood pressure, mood disorders, seizures or migraines you may notice an increase of up to several pounds per month, according to OACs medical experts. “Some people may gain a few pounds throughout the course of a year, while other people can gain 10, 20 or more pounds in just a few months.”

If you notice a sudden increase on the scale after you start a new prescription, don’t stop taking the medication. Instead, talk to your doctor or pharmacist. Sometimes the weight gain is normal and to be expected, but other times it may be an indication that something is wrong.

Menstrual Cycle

Most women notice some degree of bloating from fluid retention immediately before and during their menstrual period. Studies have shown that fluid retention peaks on the first day of menstrual flow. It is lowest during the mid-follicular period (the middle phase of your cycle) and then gradually increases over the eleven days surrounding ovulation.

The authors of a broad one-year study found that the fluid retention was not linked to ovarian hormone changes. But other studies have linked fluctuations in estradiol and progesterone (your ovarian hormones) to changes in binge eating and emotional eating. So while there may not be hormonal changes that cause weight gain it might be that those cravings you get before your period may be causing you to eat more or eat different foods than you would normally eat—causing increased fluid retention and possible an increase in weight from food and water intake.

If you are trying to lose weight during your period, it's important to be aware of and manage these menstrual eating patterns. A few days of high-calorie, high-fat eating can easily undo a few week’s worth of consistent dieting.

Alcohol Intake

Alcohol is a diuretic, so it is possible that you could notice an immediate weight decrease if you end up urinating more than usual while drinking. In fact, researchers have found that alcohol can produce urine flow within 20 minutes of consumption leading to urinary fluid losses and possible fluid imbalance. But the imbalance may also cause your body to retain fluids from the beverages you consume and from food that you eat. Many drinkers eat, or overeat salty foods that cause water retention. The end result is that it is very possible to see a weight increase on the scale after drinking.

When Will My Weight Go Back to Normal?

There are many reasons that daily weight fluctuations occur. Most of the changes can be linked to changes in water weight and normal bodily functions. So there really is no "normal weight." You probably don't need to worry if you see a small shift from day to day. You can even buy a body weight scale that measures your percent water to see how your fluid levels change throughout the month.

When should you be concerned about daily weight changes? If the number on the scale continues to increase or stays elevated for more than 5-7 days then it may be an indicator of a medical concern or simply increased body mass. But keep in mind that both muscle and fat increase your mass, so your weight gain isn’t necessarily bad.

Sources:

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Epstein M. Alcohol’s impact on kidney function. Alcohol health and research world. 1997;21(1):84–92. https://www.ncbi.nlm.nih.gov/pubmed/15706766

Hildebrandt B, Racine S, Keel P, et al. The effects of ovarian hormones and emotional eating on changes in weight preoccupation across the menstrual cycle. The International journal of eating disorders. 2014;48(5):477–86. https://www.ncbi.nlm.nih.gov/pubmed/24965609. 

Timm DA, Thomas W, Boileau TW, et al. Polydextrose and soluble corn fiber increase Five-Day fecal wet weight in healthy men and women. The Journal of Nutrition. 2013;143(4):473–478. doi:10.3945/jn.112.170118. http://jn.nutrition.org/content/143/4/473.short.

White CP, Hitchcock CL, Vigna YM, Prior JC. Fluid retention over the menstrual cycle: 1-Year data from the prospective ovulation cohort. 2011;2011. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3154522/

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