Yes, lifting weights can increase your Body Mass Index (BMI) because muscle is denser and heavier than fat, but this doesn't necessarily mean you're unhealthy; BMI doesn't distinguish between muscle and fat, so very muscular individuals (like athletes or bodybuilders) can have a high BMI that indicates "overweight" or "obese" when they actually have low body fat. A higher BMI from muscle mass reflects increased fitness, not excess body fat.
Impact statement. Body mass index (BMI) is a common measure of body fatness but overestimates body fat in subjects with high muscle mass.
Bodybuilders or other very muscular people can have a high BMI because of their muscle mass, even though they're not necessarily in the overweight range for BMI. BMI can also underestimate body fat in people who have lost muscle mass, such as some older people.
For example, compared to 71% of adults who didn't do resistance training, those who did about two 30- to 60-minute resistance training workouts a week were 30%, 41%, and 31% less likely to become obese, based on BMI, waist size, and body fat.
Gaining weight while working out can be due to several factors, including muscle growth, water retention, changes in dietary habits, hormonal fluctuations, and digestion adjustments. These changes are often temporary and indicate that your body adapts to increased physical activity.
It's totally normal to actually gain a few pounds when you begin a serious exercise program. At first, when your muscles aren't used to strenuous activity, your body will hold on to extra water weight to help it heal and to fuel your muscles, and then you might gain a little weight as you build more lean muscle.
When you exercise, your body loses fluids through sweat. As you rehydrate, you may retain water, leading to a temporary increase in weight. Additionally, as you gain muscle mass through strength training, you might see an increase on the scale, even if your body fat decreases.
BMI does not differentiate between body fat and muscle mass. Therefore, body builders and people who have a lot of muscle bulk will have a high BMI but are not overweight. Fat predominantly distributed around the waist is also a risk factor for health complications such as cardiovascular disease and Type II diabetes.
'The 3-3-3 split is simply three strength sessions, three cardio days and three active recovery days across the week,' says personal trainer Aimee Victoria Long. 'I think it's trending because it feels clear, achievable and balanced – women are craving structure that supports their energy, not drains it. '
The Scale is Going Up
Muscle growth is often slow and gradual, even if your training is on point. You might not see your biceps pop right away. The first sign that you're building muscle may be an increase in body weight. The number on the scale goes up because muscle tissue is denser and heavier than fat.
However, BMI cannot differentiate between body fat and muscle mass. This means there are some exceptions to the BMI guidelines: Muscles – body builders and people who have a lot of muscle bulk will have a high BMI, but are not overweight.
When Arnold Schwarzenegger was Mr. Universe, at 6'2” and 235 pounds, he had a body mass index (BMI) of 30.2—a number that would classify him as “obese.” But with a waist circumference of less than 34 inches, the bodybuilder had a body roundness index (BRI) of just 2.6, which classified him as “very lean.”
Similarly, you may notice increased muscle definition, less softness, and an overall leaner physique. Checking in on how your clothes fit can be another useful rule of thumb. Since muscle is denser than fat, if you've gained weight but your clothes fit a little looser, that's a good sign you may have gained muscle.
For example, someone who lifts weights regularly, and has a high amount of muscle mass, will have falsely high BMI because BMI uses their overall weight, failing to distinguish between weight that is from muscle or weight from fat.
A BMI of 25 is the start of the overweight category (25.0-29.9), moving just above the healthy weight range (18.5-24.9) for most adults, indicating a higher level of body fat compared to a healthy weight, though it's not yet considered obese (30+).
What kills muscle gains most are poor recovery (lack of sleep, overtraining, high stress/cortisol), insufficient or poor-quality nutrition (not enough protein/carbs, excessive processed foods/sugar/alcohol), and inefficient training (too much cardio, bad form, focusing on isolation over compound lifts). Chronic stress elevates cortisol, which breaks down muscle, while inadequate protein, calories, and sleep directly impede repair and growth, making recovery paramount.
Stage 1 overtraining symptoms include:
You don't need to spend hours a day lifting weights to benefit from strength training. You can see significant improvement in your strength with just two or three 20- or 30-minute strength training sessions a week.
BMI has been criticized as being an inaccurate tool. One reason is that a person with lots of muscle and minimal body fat can have the same BMI as a person with obesity who has much less muscle.
People who train for strength tend to do heavier weights, lower reps, which builds more dense muscle. To get as strong as possible, strongmen will eat enough to ensure maximum strength gains, which also will build fat. Bodybuilders also eat to ensure maximum muscle gain, and put on some fat.
Men between 14-24% and women between 21-31% (varying by age) fall within normal parameters for good health. This range balances metabolic function with disease risk factors. Obese: Body fat percentages above 25% for men and 32% for women (in the 20-29 age group) are classified as obese.
People naturally lose muscle after 40, especially women after menopause. Because muscle burns more calories than fat, this can slow down your metabolism and make it harder to shake those stubborn pounds.
If the weight fluctuation is very rapid (for example, a weight increase of 1-3 kg from one day to the next), it's definitely water, because fat doesn't accumulate that quickly! If it's water retention, swelling is often localized, manifesting as puffiness in the legs, ankles, hands, and abdomen.
Weight gain despite eating little can result from underestimating calorie intake, consuming high-energy or processed foods, lack of physical activity, dehydration, poor sleep, or hormonal imbalances like PCOS or perimenopause.