Your weight can fluctuate throughout your lifetime, and it goes up and down daily (just step on the scale and you'll see). For the sake of this article, we spoke to Holly Lofton, MD, director of the Medical Weight Management Program at NYU Langone Health, and Eduardo Grunvald, MD, FACP, medical director of the UC San Diego Weight Management Program, about fluctuations in weight mainly due to fluids as well as fat loss and gain. It's harder to put on muscle than it is to put on fat - we've written about this before - and Dr. Grunvald noted initially that if we're talking about longer-term change in weight, "we're talking about fat weight, although muscle weight can also fluctuate." There are instances, too, when medical disorders will affect your weight, which we discuss ahead.
Why Does My Weight Fluctuate Daily?
Dr. Lofton said that weight tends to go up during the day - as much as five pounds - because we eat and drink and don't eliminate all of it. This, she said, is the obvious reason. It also has to do with sodium in certain food, which draws in water. Things that are rich in sodium (potato chips, for example) keep water in the bloodstream, which makes our weight on the scale increase. "We urinate less with the salty food and thus hold onto more water," she explained. Other things that may affect weight include physical activity. "Every time we move, our muscles contract. They use carbohydrates and water to contract, and that can lead to some temporary swelling, which can manifest as weight gain on the scale," Dr. Lofton said.
Seeing three, maybe four, extra pounds on the scale by the end of the night, Dr. Lofton said, is normal. Anything greater than five pounds she considers to be abnormal. Dr. Grunvald agreed. Daily weight fluctuation due to water retention in a healthy person usually only registers as a few pounds, he said. On the other hand, patients who have kidney, liver, or heart disease may hold onto more water. "Even some pituitary issues may affect water retention or the ability of the kidneys to produce urine, and thus, they hold onto more fluid," Dr. Lofton added. She also pointed to pregnancy as another instance where retention of fluids would increase.
In addition, sometimes people can have leg swelling throughout the day if they tend to be on their feet a lot. "Because they're in an upright position, they can get what's called dependent edema, and if their blood vessels aren't as readily able to push fluid back up from the lower extremities, that can cause it to pool and that causes visible swelling, which can look like weight gain on the scale." This is referred to as vascular or venous insufficiency. If it happens to you, resting and elevating your legs helps, Dr. Lofton said.
Why Does My Weight Fluctuate Over Weeks and Months?
When your weight goes up or down over longer periods of time, this is weight primarily associated with body fat, Dr. Grunvald noted. This isn't the same as daily fluid retention fluctuations (Your body fat won't change in a day!). Losing weight (aka fat) over time can happen when you're in a caloric deficit, as we've frequently mentioned before in previous articles. "If you're having a consistent calorie deficit, over time you'll see a steady weight loss. Eventually you'll plateau," Dr. Grunvald explained. "If you've lost weight and now your lifestyle changes in the wrong direction, then you're going to see very rapid weight regain."
Dr. Grunvald noted that the scale isn't necessarily the best indicator of fat loss or gain, given that it can't tell you how much of the weight is water, muscle, or fat. Better indications, he said, are pant size, your waist circumference, your overall health, and your labs. You can read more about how to tell if you're gaining or losing muscle or fat specifically here (there are special body composition tests you can take).
Another aspect of weight fluctuations in terms of fat is what Dr. Grunvald referred to as "weight cycling" (some people call this "yo-yo dieting" if they're intentionally switching between periods of dieting and not dieting). "People go on a calorie restriction, whatever that may be, and they lose weight, and then they slip and revert back to their old habits," he explained. "They regain the weight and they have this pattern." This, he said, is very common and can happen several times over the course of someone's life, whether they're losing weight or regaining it.
Everyone's bodies are different - genetics are a huge factor in this - and the amount of weight you lose and gain can vary due to your family history and the vigilance of your eating, exercise, and lifestyle changes overall. "As soon as you stop the vigilance, your weight will usually come right back up," Dr. Grunvald said. "So it's the durability of the changes you make that are the big challenge."
Note: Trying to lose weight in a short amount of time is not effective because it's not easily sustainable. Dr. Lofton said that with any diet that's restrictive, "What happens when you reach that very low point, before it starts to come back up, is your body effectively puts the brakes on your weight loss to avoid you losing too much. It goes into, essentially, a mode of preservation by slowing your metabolism and increasing your hunger hormone."
You can get over a weight-loss plateau, or even a period of time where you keep losing or regaining weight - Dr. Lofton suggests upping your physical activity and really tracking your caloric intake - but the most important thing is finding what you can maintain over time, Dr. Grunvald said. "Doing something that's going to result in weight loss but is not going to be sustainable for you as an individual, is what leads to failure," he told POPSUGAR. "It's really about what is going to be durable for you, not just what's going to work for three months."
Again, there are some conditions that can cause a shift in weight as well. Doctors will want to rule out any type of malignancy, Dr. Lofton said, "because malignancies are just overactive tumors that use up a lot of energy, so they're essentially eating our body's resources." Gastrointestinal disorders that cause malabsorption such as celiac disease can also lead to weight loss. Pancreatic insufficiency, she said, is another one, which is a situation where the pancreas, which is responsible for "producing digestive enzymes amylase and lipase that help us absorb fat and, to some degree, carbohydrates, isn't producing those hormones. We then get a deficiency of fat that leads to weight loss unintentionally." Hyperthyroidism, or an overactive thyroid, can cause weight loss, too.
As for weight gain, disorders such as hypothyroidism, or an underactive thyroid, can be to blame. Changes in hormones might influence your weight as well. Dr. Lofton noted that unintentional weight gain can be a side effect of some medications such as corticosteroids and antidepressants, and insulin very commonly has that side effect. If you're gaining or losing weight and you're not sure why, don't hesitate to contact your doctor. It's better safe than sorry to reach out to a medical professional "when things just don't make sense," Dr. Lofton advised.