If you’re trying to lose weight, you might think counting calories and eating less is the way to go, but that’s not always the case, according to Terry Fairclough, a top personal trainer and co-founder of Your Body Programme.

As a personal trainer, I’ve heard a lot of different ideas and questions about the best diet for weight loss. Should we count calories? How many are we supposed to eat? Should we be on low-fat, low-carb, or high-protein diets? Is fasting a good idea? Are we supposed to eat small, regular meals three times a day?

While all these strategies might have their place depending on your body type, goals, and activity levels, one thing you should not do is undereat. We’ve all seen someone start cutting calories drastically to get that beach body—the weight drops off quickly, right? Sort of, but it might not be in the best way.

While creating a calorie deficit can lead to weight loss, it doesn’t necessarily mean you’re losing fat, which is what most people want. Our typical Western diet is generally larger than it should be, so a slight calorie deficit might be necessary because most people tend to overeat in the first place. However, many believe that eating way too little is the only way to shed pounds, but that’s not true.

When we eat, our body turns carbohydrates into glucose, a sugar that fuels our cells. If our body doesn’t need that glucose right away for energy or exercise, it’s stored as glycogen in the muscles and liver. Each glycogen molecule is linked with two to three water molecules. When we need energy or aren’t getting enough glucose from food, glycogen breaks down into glucose for fuel. So, when you cut calories, you’re mainly losing stored carbs and water—not fat.

Most of us want to lose fat, but a long-term calorie deficit can make your body hold onto fat while breaking down protein instead. Since protein is active biologically, having more of it helps burn more fat while at rest. This is why it’s crucial to eat enough calories that include fats, carbs, and proteins.

If you think cutting out fats helps lose fat, rethink that. Fat is our body’s most significant and long-lasting energy source, providing more than double the energy of carbs or protein. It’s stored in our muscle fibers and is readily available during exercise. Unlike limited glycogen stores, your body fat is a pretty endless energy source. During workouts, stored fat breaks down into fatty acids that fuel muscles.

Removing fats entirely could mean lacking energy to lose the fat you want. Additionally, severely cutting calories and nutrients can cause deficiencies affecting the body’s systems, like the immune, liver, and digestive systems, potentially leading to health problems and slowing your metabolism.

Issues from undereating might include fatigue, malnutrition, osteoporosis, anemia, polycystic ovary syndrome, depression, hormone-related conditions, and fertility issues. Extreme calorie cutting stresses the body. When stressed, our bodies release cortisol, a hormone that breaks down energy stores. In the short term, cortisol can cause weight loss, like when someone loses weight in tough times.

But long-term stress leads to big issues: proteins break down while the body clings to fat. Protein breakdown slows metabolism, heightened cortisol levels increase fat storage around the belly, and cortisol affects the thyroid hormone, impacting metabolism. Stress lowers digestive function because energy goes to muscles during ‘fight-or-flight’ situations. Undereating may mean missing essential nutrients needed for health and basic functions, affecting training and results in weight loss or fat loss quests.

Sleep quality can also suffer. When blood sugar drops, adrenaline steps in to balance it, often waking you up. Poor sleep affects liver detox, immunity, exercise, and productivity—factors linked to weight gain.

Bodybuilders often cut calories significantly to get lean before competitions and then eat more afterward, but doing it wrong can make them ill. Consistently cutting calories can lead your body to store any extra calories as fat due to a ‘famine mode.’

Ultimately, it’s crucial to eat the right amount of calories, carbs, fat, and protein for your body type, goals, activity level, height, weight, and age. My program helps people determine their calorie needs based on their own body types, proving repeatedly that increasing calorie intake can actually lead to fat loss.

Consume a variety of lean proteins, like lean beef, chicken, eggs, and fish. Vegans can choose pulses, legumes, tofu, and tempeh. Also, include healthy carbs from fruits, vegetables, sweet potatoes, quinoa, brown rice, and whole wheat pasta, alongside healthy fats like avocados, nuts, seeds, olives, and olive oil.

You have just one body, so take care of it by staying healthy, well-nourished, and keeping your metabolism active.