If you’re trying to lose weight, you might think the key is to count calories and eat less, but that’s not necessarily the case, says Terry Fairclough, a personal trainer and co-founder of Your Body Programme.

As a personal trainer, I’ve heard all sorts of opinions on the best diet for weight loss. Should we count calories? Go low fat, low carb, or high protein? Should we be fasting or eating small, regular meals? While each of these approaches might have its place based on body type, goals, and activity levels, one thing’s for sure: no one should be undereating.

We’ve all seen someone start counting calories and dramatically cut their intake to get that beach body. While a calorie deficit can lead to weight loss, it doesn’t necessarily lead to fat loss, which is what most people aim for. In reality, many of us need a slight calorie deficit just because we were overeating before. But under eating is not a sustainable solution for weight loss.

When we eat, our body converts carbohydrates into glucose, a sugar that fuels our cells. If the glucose isn’t used for energy right away, it’s stored as glycogen in the muscles and liver, along with water. When we cut calories, the initial weight loss often comes from losing this stored glycogen and water, not fat. Prolonged calorie deficits can cause the body to hold onto fat and instead break down protein, which is not what we want.

Protein is crucial because it helps build muscle, which in turn burns more fat. That’s why it’s important to consume a balanced mix of all three macronutrients: fats, carbs, and protein. And contrary to popular belief, fat is a vital and enduring energy source. It provides more than twice the energy of carbs or protein and is stored in muscle fibers, making it accessible during exercise. Cutting fat from your diet can leave you without the energy needed to burn unwanted fat.

Additionally, cutting calories and restricting essential nutrients can lead to deficiencies that impact our overall health, including immune, liver, and digestive systems. Under eating can cause fatigue, malnutrition, osteoporosis, anemia, hormone problems, and even fertility issues. Extreme calorie deficits stress the body, leading to increased cortisol, the stress hormone, which might cause short-term weight loss but long-term fat gain.

When the body is stressed, it breaks down protein for energy, slows metabolism, and stores more fat, especially around the belly. High cortisol levels also interfere with thyroid function, further slowing metabolism. Moreover, stress reduces digestive efficiency, making it harder to absorb necessary nutrients, which can affect our training and results.

Under eating affects sleep too. When blood sugar levels drop, adrenalin is released to balance it out, waking you up. Poor sleep impacts liver detoxification, immunity, productivity, and can lead to weight gain. Bodybuilders often cycle their calorie intake to get lean, but doing it wrong can make them ill.

Continually cutting calories can eventually break down basic body functions and alter metabolism, making weight loss feel impossible. The body goes into famine mode, storing any excess calories as fat.

The bottom line is to consume the right amount of calories, carbs, fats, and proteins suitable for your body type, goals, and lifestyle. My program, Your Body Programme, helps people figure out what they need to stay healthy and energized. Restricting calories is not necessary; in fact, increasing them can aid in fat loss.

Make sure to eat plenty of lean proteins like beef, chicken, eggs, and fish, or plant-based options like legumes, tofu, and tempeh. Include healthy carbs from fruits, vegetables, sweet potatoes, quinoa, brown rice, and whole-wheat pasta. Don’t forget healthy fats from avocados, nuts, seeds, olives, and olive oil.

Focus on nourishing your body and maintaining a balanced diet to keep your metabolism active and healthy.