Weight seems fairly straightforward. After all, it’s just a measure of your body mass, right? But a lot goes into what your specific number is — energy, metabolism, hormones, and more. And if you’re trying to lower your weight, the process is also somewhat complex, going way beyond calories in and calories out. Ready to learn how it all goes down? Weight loss 101 starts now.
What is weight exactly?
When you step on a scale, the number that flashes back at you — your weight — is a measure of your total body mass. That includes a combo of bones, skin, muscles, fat, and water. Some components of your body weight don’t really change over most of your lifetime — like bones (unless you have osteoporosis, which is when your bones thin) — while other parts fluctuate constantly. “If you are expecting your period, it’s entirely possible to gain three pounds of water weight overnight,” says Dr. Lawrence Cheskin, M.D., professor of nutrition and food studies at George Mason University, adjunct professor at Johns Hopkins School of Medicine, and author of Weight Loss for Life: The Proven Plan for Success. “It’s not because you ate a pound of butter before you went to bed. It’s because hormonal changes are causing you to retain fluid.”
How the body uses energy
The body requires energy to function, and we get that energy in the form of calories from food. Technically, calories are a measure of heat required to raise the temperature of a liter of water one degree Celsius. The process of turning that heat into energy is what the metabolism does.
Calories are burned by the body in three ways:
- Basic metabolic functions like breathing, sleeping, thinking, and circulating bodily fluids
- Digesting food
- Fueling physical activity
A measure known as the basal metabolic rate, or BMR, is the number of calories your body uses to keep you alive. “The bigger you are, the more calories you need,” explains Amy Hess-Fischl, RDN, a certified diabetes care and education specialist at the University of Chicago. The exact number you need to consume is partly dictated by your height, weight, age, biological sex, and level of activity.
When it comes to fuel (as in, where your body gets that energy from), glucose is the body’s main source. Glucose comes mostly from carbohydrates in food. Once you eat carbs, the glucose either hangs out in the bloodstream or is sent to the liver. “Liver glucose is always used first and lasts about 24 hours,” Hess-Fischl says. “Think of it as a conveyor belt constantly giving us what we need. Every time we eat, we fill it back up again.”
Glucose that isn’t used during activity is converted to glycogen, which is stored for use later. On average, there are 500 grams of glucose stored in skeletal muscle (the main muscles in your body like your abs and quads) and 100 grams stored in the liver. “When those storage areas are full, excess calories are stored as fat,” says Hess-Fischl. “The more calories that are not needed, the more that get sent to fat storage — it’s as simple as that.”
Weight loss: How it happens
Research links fat loss to a reduced risk of diabetes, improved blood pressure, better blood sugar, increased mobility, and less joint pain. You’ll also have a brighter mood and feel more energetic. So, how does it happen?
Cutting back on how many calories you consume (and burning some through physical activity) forces your body to tap into your energy reserves and causes weight loss. But while you probably wish that your body would draw solely from fat stores first, that’s not the case. Glycogen stores are hit first, and because it takes a good amount of water to store the glucose, you lose a lot of water weight as you burn through it.
This quick drop in water weight is why early-stage weight loss feels so exciting — it typically happens fast, showing up on the scale right away. The next phase is where things can get tougher. This is due to certain biological drivers triggered in your body when you lose weight, such as an increase in hunger.
Consistent and healthy lifestyle habits, like eating nutrient-rich food and adding activity to your day, are what’s needed for steady fat loss. Focus on nourishing your body with more filling foods, like lean protein and high-fiber fruits and veggies, without restricting yourself so much that you can’t stick with it.
Metabolism: What it is and why it matters
Metabolism is the body’s workhorse, the process that mixes calories with oxygen to create and release energy. And it’s involved in so many processes, like muscle contraction, thinking, breathing, digesting food, circulating blood, and waste removal.
Science tells us that the calories needed to keep living and breathing account for 60% to 70% of all the calories we metabolize in a day. It’s the other 30% to 40% of calories burned that you have some control over. Some people burn calories more quickly because they’ve inherited the ability to build muscle mass from their parents (more energy is needed to build and maintain muscle mass). But don’t be discouraged if that isn’t your case. Cheskin says there is something you can do to improve your metabolism: exercise.
“Resistant training and lifting weights are what’s needed to build lean muscle mass,” he says. “Having more muscle mass is the thing that helps you burn more calories — even when you aren’t using them.” And there’s another good reason to work out regularly with weights. Research indicates that people who exercise consistently are more likely to sustain their weight loss.
Speaking of weight loss, it does impact metabolism, but probably not in the way you’d expect. “It may seem counterintuitive, but as you lose weight, your metabolism slows down, which means you’ll need fewer calories to maintain the weight loss,” Cheskin explains. “Here’s the hard truth: A smaller body has lower energy needs.”
That said, there’s something that doesn’t slow metabolism down as much as you may think: your age. It’s long been thought that your metabolism will start to tank in your 30s, but that’s starting to be debunked. “Newer evidence shows that isn’t the case,” says Hess-Fischl. “Metabolism remains quite constant between the ages of 2 and 60, for both men and women. After 60, it drops off only slightly.”
How hormones impact weight loss
Created in the glands, hormones are the body’s chemical messengers that turn a variety of processes like hunger on or off. There are over 50 different hormones, and at least seven are directly linked to metabolism, hunger, and weight gain: estrogen, ghrelin (makes you feel hungry), leptin (tells your brain you’re full), glucagon-like peptide 1, insulin, testosterone, and thyroid hormone. (Several more play supporting roles in fat storage and weight loss.)
The tricky thing is that if even just one hormone is out of balance, it can throw off everything off. For example: If you aren’t producing enough thyroid hormone, which regulates metabolism, the body won’t burn calories as quickly and that can make weight loss more difficult. During perimenopause (that transitional period before menopause starts), women experience hormonal changes — most famously drops in estrogen and progesterone. These changes can affect where fat is stored in the body.
Lack of sleep and a hectic lifestyle can also disrupt weight-loss-related hormones. Take the stress hormone cortisol. It gets pumped out by the adrenal glands when you’re experiencing physical or mental stress, and a surplus has been linked to increased appetite and weight gain.
Hormone-related chronic conditions including polycystic ovarian syndrome (PCOS), Hashimoto’s disease, and Cushing’s syndrome have been found to make weight loss more difficult as well. If you’re having an especially hard time losing weight, speak to your healthcare provider to rule out a hormonal issue. In most cases, though, slight hormone level fluctuations alone aren’t enough to interfere with weight loss.
How appetite makes weight loss harder and what you can do
If you’re hungry all the time, losing weight can be even more difficult. Appetite is tricky, Cheskin says. “It varies from person to person due to things like learned exposures and habits.” It’s important to recognize that we can eat for many reasons. Sometimes, your body signals you to eat because it needs more fuel — for example, when you haven’t eaten for a while. This can sometimes be accompanied by physical sensations like lightheadedness, a growling stomach, or a headache. Other times, you may eat because something in front of you looks delicious or routine (like how you always watch TV with a bowl of chips). Habits, emotions, cravings, and your environment can cue you to eat when your body doesn’t actually require those calories for fuel.
What to do? Research suggests that eating regularly throughout the day can help you manage hunger. What you eat and how much you consume can dictate how long you’ll stay satisfied. Meals and snacks rich in fiber and protein (think beans, avocado, nuts, and low-fat cheese) can help you feel fuller longer.
Is exercise necessary for weight loss?
Let’s be clear, exercise alone — without eating fewer calories — likely won’t lead to the expected amount of weight loss, says Hess-Fischl. “The best way to lose weight is by reducing calories, but adding exercise helps burn some of the calories we’ve consumed and is linked to all kinds of other health benefits beyond weight,” she says. Those benefits of exercise, like reducing your risk for certain diseases, making you feel great, and improving your overall well-being, is key to achieving true weight health.
Regular exercise — at least 2.5 to 5 hours of moderate activity a week — can actually lower mortality, or risk of dying. Moderate activity is any exercise that gets the heart rate up — breathing quickly but not becoming out of breath — like walking briskly. Research also shows that exercise, specifically resistance training, can reduce the loss of lean muscle that occurs in the early stages of weight loss and helps maintain weight loss.
The bottom line
The number on the scale is impacted by some factors that you can’t do much about, including your hormones and genetics. And losing weight is a complex process, dependent on things like the type of food you eat and your body weight. Weight and its impact on your health are also very individual — you can consume the same number of calories as your friend and have very different outcomes. But there’s science to support the lifestyle changes that, in time, will make a difference: eating less, moving more, reducing stress, and prioritizing sleep.