How to lose fat while maintaining muscle, according to the personal trainers of celebrities and business execs

• When we say we want to lose weight, we typically mean shedding fat but not muscle.
• Muscle helps us to look "toned" and supports our metabolism.
• To lose fat without losing muscle, eat enough protein and strength train regularly, two top trainers said.

If you want to lose fat without losing muscle, there are three things you need to know.

That's according to Magnus Lygdback, a Hollywood personal trainer who helped Alexander Skarsgård prepare to play a Viking warrior in "The Northman," and Harry Cox, a personal trainer to busy C-suite execs based in London.

To lose weight, you need to eat fewer calories than you use, but if that number drops too low, you'll start losing muscle along with fat.

"Because your body is in an energy deficit, it's going to try and find energy wherever it can, and sometimes that ends up being muscle," Cox told Business Insider.

You may look slimmer, but it could backfire in the long run, because muscle mass is essential for overall health, he said. It plays a key role in regulating metabolism, including glucose levels, and increases the number of calories your body uses at rest. All this impacts body composition. 

We also naturally start to lose muscle with age, and building and maintaining it allows us to move around freely, lift things, and prevent injury longterm. 

Both trainers endorsed establishing long-term healthy habits like eating a balanced diet and getting enough sleep over quick fixes, which can be structured around the following three tips. 

1) Strength train with progressive overload at least twice a week 

Strength training regularly, particularly if you're trying to lose fat, will help you preserve muscle mass, Cox said. It signals to the body that we need it and must keep it, he said.  

The most effective and evidence-backed way to build muscle is to train with progressive overload. This is where you work your muscles to the point of failure — where you could barely do even one more rep— and continually add more weight or reps as your body adapts over time.

Cox said the same principle applies to maintaining muscle. "If you're just going to the gym and lifting some of the lightest dumbbells and hope that's going to help you preserve muscle, that's not the case. The training needs to be somewhat intense," he said.

He said to aim for a minimum of two strength training sessions a week, but that you'll likely get better results from three or four.

Each workout should include at least one pulling exercise — such as a chin-up, pull-up, or lat pull-down — a push exercise — such as a push-up, bench press, or shoulder press — and some kind of leg exercise — such as a lunge, squat, or deadlift.

"If you start putting in place those healthier habits of eating more protein, exercising more frequently, a lot of the time, the fat loss happens as a byproduct without even focusing on it," Cox said.

2) Eat enough protein

Protein is the building block of muscle, so eating enough is crucial for maintaining muscle while losing fat, Lygdback said.

Whether or not you're trying to lose weight, you should aim to eat the same quantity of protein  to maintain your muscles , he said.

"You don't really want to adjust that too much if you ask me. And I never do," he said.

The scientific literature recommends eating between 1.6 and 2.2 grams of protein per kilogram of body weight a day to support recovery and maintain or build muscle mass, Cox said.

A person who weighs 60 kilograms would aim to eat 120 grams of protein each day, for example.

If you're struggling to eat enough protein, the trainers recommended educating yourself on which foods are high in protein, using protein powder if necessary, and not being afraid to order double protein in a restaurant.

3) Stay in a gentle calorie deficit

To lose weight, you need to burn more calories than you eat, known as a calorie deficit. But building muscle requires being in a calorie surplus. That's why it can be hard to lose weight without shedding muscle instead of fat.

You can achieve it though by tracking macronutrients (protein, fats, and carbohydrates), Lygdback said . You don't have to weigh out your food at every meal, but it might be helpful to track your meals for a few days to get a sense of what 30 grams or protein, or 500 calories of rice looks like, for example, he said.

Chicken, rice, and broccoli is the staple gym bro diet. But you don't need to eat that to reach your goals, he said. Just make sure you have enough protein, while eating in an overall calorie deficit. "You want to make sure to keep the protein high enough, and the carbs and fat low enough," he said.

Lygdback recommended cutting your daily calories by 10 to 20%. "You don't want to go too extreme," he said.

Business Insider previously reported on how to calculate your macros depending on your weight and activity levels.