Why Does Weight Fluctuate From Morning to Night?

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It's typical for a person's weight to change throughout the course of the day.
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If you are watching your weight, it can be tempting to step on the scale several times a day to check your progress. Resist the urge! These frequent weight checks can be frustrating and misleading, since it's normal for weight to fluctuate at least a few pounds within the course of a day.

In addition, it's normal for your morning weight to be a bit lower compared to your afternoon or evening number.

What Causes Weight Fluctuation?

Daily changes in weight are normal, although shifts in body water — not body fat — explain most of the daily fluctuations, according to an article published in the July 2017 issue of Physiological Reports. Since an average of 60 percent of an adult's body weight comes from water, per the Academy of Nutrition and Dietetics, even small losses of water can show up as weight loss on the scale.

Here are some of the key factors that could cause the number on the scale to shift from AM to PM.

Fluctuating Fluids

The water in your body shifts from morning to night. Body water is lost through sweat, respiration, urination and with bowel movements, according to Harvard Health Publishing.

Exercising and being in hot temperatures cause an even greater loss of body water. Illness, including symptoms of diarrhea, fever and vomiting, can also cause water loss — and a temporary drop in weight, according to the Merck Manual.

Body water is replenished by drinking beverages and eating water-containing foods.

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The Glycogen Factor

The loss or gain of glycogen — the storage form of glucose found in the liver and muscles — is another cause of short-term weight fluctuations. Glycogen is stored with up to 3 or 4 parts water, so when this energy source is broken down, the body also loses extra water — and weight, according to the article in Physiological Reports.

Glycogen can be quickly depleted — and weight can be lost — from extended exercise, or as a result of very low-carbohydrate or fasting diets, per an April 2018 article in Nutrition Reviews. On the other hand, someone who has depleted glycogen stores from dieting or exercise can regain weight when carbohydrate foods are consumed and glycogen stores are built back up.

Intestinal Contents

This factor makes intuitive sense: Your weight will temporarily change when you eat or drink, simply because your body now contains what you consumed. Just drinking 16 ounces of water can spike your weight by about a pound. This temporary weight of food and liquids stays with you as your body digests and absorbs water and nutrients, until it sends waste out of the body via urine and stool.

Even as stool is in your intestines, it adds to your weight. The average daily weight of stool in humans is 128 grams — or 0.28 pounds — according to research published in the September 2015 issue of Critical Reviews in Environmental Science and Technology. But rest assured, these weight changes are temporary and when you urinate or have a bowel movement, your weight will go down a bit.

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Other Factors

Other factors can explain short-term weight fluctuations. For example, if you are dehydrated, your weight will be lower. If you tend to retain water, a condition known as edema, you can gain weight.

Edema can occur in pregnancy, in premenstrual syndrome, or due to other medical conditions such as liver, kidney or heart disease, per the Mayo Clinic. Consuming too much sodium, a mineral important for water balance, can worsen edema, according to Harvard Health Publishing.

Certain medications can also lead to water retention, per the Mayo Clinic. Steroids and certain blood pressure and diabetes medications are common types of drugs that can cause puffiness or swelling. Alternatively, diuretics or "water pills" can lead to weight loss since these medications remove extra body fluid. Taking any of these drugs can cause weight fluctuations between morning and night.

How Much Does Weight Fluctuate?

On average, daily weight can fluctuate from 1 to 2 kg (2.2 to 4.4 pounds), per a July 2017 article in the journal Physiological Reports. Body weight is a bit lower in the morning, because there's less food and fluids consumed overnight, and also due to water lost through sweat and breathing, per the American Physiological Society.

But not everyone is the same, and above-average weight fluctuations can occur in people with more muscle mass, or related to changes in diet and exercise.

Water makes up around 75 percent of muscle mass, according to an August 2019 review in Nutrients. So people with higher amounts of muscle mass could experience weight fluctuations of several pounds per day as they experience changes in muscle mass.

Runners can lose several pounds of water weight after a several-mile run, and fluid replacement is essential for safety and performance, according to the Academy of Nutrition and Dietetics.

Drastic changes to diet can also cause weight loss or gain that exceeds the daily average of 2.2 to 4.4 pounds, according to Mayo Clinic. For example, if carbohydrate foods are nearly eliminated in the diet, the body uses glycogen stores for energy — which can lead to weight loss of 10 or more pounds in a week. Resuming a diet that includes carbohydrate foods after a brief fast or low-carb diet can also lead to several pounds of weight gain, as the glycogen stores are restored.

When to Weigh Yourself

If you are tracking your weight, you may be more interested in changes to muscle and fat weight instead of the temporary fluctuations from fluids, foods and intestinal contents. Changes to fat and muscle mass occur more gradually as these are influenced by the calories and nutrients in your diet along with your physical activity.

To assess these weight changes, check your weight once daily at about the same time each day.

Since weight tends to be lower in the morning, most people weigh early in the day, after urinating or having a bowel movement — and before eating breakfast. As you keep track of your daily weights, hopefully you will observe positive trends over time.

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When to Talk to Your Doctor

Weight changes that are lower or higher than the average of 2.2 to 4.4 pounds a day could be related to medications or health conditions. Let your doctor know if you are experiencing weight fluctuations outside of the average range or outside of the range recommended by your doctor, particularly if you are being treated for any medical conditions or if you are taking any prescription medication.

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