Gaining Weight Despite Dieting? 7 Surprising Reasons Why

Maybe you’ve heard this one: “Which weighs more, a pound of muscle or a pound of fat?”

Short answer: They weigh the same. A pound is a pound!

But they sure don’t look the same.

What do I mean by that?

Muscles are much denser than fat, meaning they have a smaller volume for weight. So, 15 pounds of fat will take up a lot more space in your body—giving you a larger and softer appearance. Meanwhile, 15 more pounds of muscle would make you appear firmer and more toned despite your weight being the same.

While you might see the scale go up, it might be a good thing. Gaining more muscle can not only give you a more toned appearance, but it can actually increase your metabolic rate, which can help you burn more calories even when you're not working out (1).

Water retention is a surprisingly common reason for seeing the scale jump, even when you’re doing everything right. Since water makes up a big part of your body—about 60%—it’s not unusual for shifts in water weight to move the scale by ten pounds or more from one day to the next (2, 3).

But how does this relate to working out?

Well, when you start a new workout routine, your muscles are put under stress, leading to tiny tears as they adapt. This is a normal part of muscle recovery, but it can cause your body to hold onto extra water as it heals. This temporary water weight gain is usually minor and should go away once your muscles recover and your body finds balance again.

It might seem counterintuitive, but drinking plenty of water can actually help reduce water retention. By staying well-hydrated, you help your body flush out excess fluids, keeping your weight more consistent and avoiding those sudden jumps on the scale.

Your total daily energy expenditure (TDEE) is the total number of calories your body burns in a day, and it's made up of four parts (4):

• Basal Metabolic Rate (BMR): the energy your body needs just to stay alive
• Exercise Activity Thermogenesis (EAT): covers the calories burned during exercise
• Non-Exercise Activity Thermogenesis (NEAT): includes everyday movements like walking or fidgeting
• Thermic Effect of Food (TEF): the energy used to digest and process food

Sometimes, we can get into the mindset that if we exercise, we don't have to do anything else for the rest of the day. But here’s the thing—exercise only accounts for about 5% of your TDEE (3). NEAT, however, can account for up to 30% of your total daily calories (4).

According to researchers at the University of Missouri–Columbia, when you sit for hours at a time without any movement, your body stops producing lipase—a fat-inhibiting enzyme that can be very helpful in achieving your weight-loss goals (5).

You probably know that getting enough sleep is crucial for your overall health and well-being. But here's the thing—if your sleep quality isn't great, it could lead to weight gain (6).

A 2022 review found that poor sleep is associated with eating more frequently and eating foods higher in carbohydrates and unhealthy fats—all of which may lead to weight gain (7).

Another review found the following sleep factors affected weight loss (8):

• Short sleep duration
• Frequent awakenings during the night
• Poor sleep quality
• Longer sleep onset

It also found that staying in bed and waking up later reduced the likelihood of getting enough exercise each week, which is important for avoiding weight gain.

Chronic stress is a common problem that can affect your weight (9).

High levels of the stress hormone cortisol have been shown to increase hunger and your desire for highly palatable, calorie-dense foods, which can cause weight gain (10).

Interestingly, stress management may promote weight loss.

In an 8-week study of 45 obese adults, those who engaged in relaxation techniques like deep breathing lost significantly more weight than those who only received standard dietary advice (11).

This is a surprisingly easy—and often overlooked—reason for that unexplained weight gain.

Weighing yourself at different times of the day can lead to fluctuations that might make it seem like you've gained weight when, in fact, you haven't. This happens because food and drinks don’t just contain calories; they also have physical weight.

When you eat or drink, the weight of that food or liquid temporarily adds to your body weight until it's fully digested and eliminated. This is why it's crucial to be consistent with the timing of your weigh-ins to get an accurate picture of your progress.

Personally, I hop on the scale every morning right after waking up and using the bathroom. That way, my stomach and bladder are empty, giving you a more accurate weight reading without any external factors throwing it off.

While this is a less likely explanation than the others, it is possible that your unexpected or sudden weight gain is being caused by underlying health issues or medical conditions.

• Hormone-Related Conditions: Hormone conditions can lead to unwanted weight gain, even if you're engaging in healthy habits. For example, polycystic ovary syndrome (PCOS) affects about 5 million women of reproductive age annually and can cause insulin resistance, chronic inflammation, and weight gain (12).
• Insulin Resistance: Working out requires fuel, and while you might be focused on how much protein you’re eating, it’s actually glucose that immediately fuels your muscles for a workout. When your body doesn’t respond well to insulin, it’s known as insulin resistance (13). A variety of factors can cause insulin resistance, including thyroid gland issues.
• Thyroid Issues: Thyroid issues are a common cause of unexplained weight gain, and an estimated 20 million people in the U.S. have some form of thyroid disease (14). Your thyroid gland (located at the base of your neck) releases hormones that control metabolism—aka, the way your body uses energy (15). An underactive thyroid can lead to sudden weight gain because if your body isn’t receiving enough of those regulating thyroid hormones, it stores more weight (16).

If you suspect any of these conditions, it’s best to discuss it with your doctor or healthcare provider.