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Exercising for weight loss? These are the mistakes a personal trainer sees all the time

Wooden dumbbells and kettlebells, boxing gloves spaced symmetrically on a gym rack against a bamboo fence

Exercising for weight loss? These are the mistakes a personal trainer sees all the time

Lucy Gornall

Published: August 26, 2026 at 4:29 PM

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Experts explain where you might be going wrong in your workouts

When you’re on a mission to lose weight, it’s natural to increase your exercise. You might schedule in endlessly long runs or cram in HIIT classes at the gym. After all, the more you sweat, the better your results, right? Well, not necessarily.

Sean Murphy, chief personal training officer at Ultimate Performance, has been a PT for over 20 years, and he’s seen it all.

“Most people assume the problem is effort. They think they're not training hard enough or not being disciplined enough to lose weight. But in my experience, it's almost never that. The people who come to us are usually trying very hard indeed, and the effort is simply going into the wrong places, or being undone somewhere they haven't thought to look.”

Here he shares the most common mistakes that cause people to go wrong. If you’re exercising to lose weight, make sure you’re not making any of these errors.

For more exercise advice, check out our guide to the exercises you should be doing at every age, from your 20s to your 90s...

Eating too little

If you want to lose weight, then eating as little as possible feels like the obvious answer. But according to Sean, the problem is what severe restriction does to your behaviour over time.

“When you cut calories hard and cut out everything you enjoy, you can usually hold it together for a few days, sometimes a week, maybe two. Then something gives, such as a bad day or a social event, and the diet falls apart. Often, because you've been so restrictive, you may fall off in a big way.” This can mean you end up eating more and more, regaining any weight you’ve lost.

Don’t forget, you also need food to fuel your workouts. Will Duru, PT and founder of fitness app 12Reps explains: “If you’re running on empty, you won’t have the energy to exercise properly, and you also won’t have the fuel needed to recover properly. This can leave you exhausted and more likely to skip workouts altogether.”

tiny meal on a plate consisting of one shrimp, a tiny potato, and a single short asparagus stalk

Fail to prepare, prepare to fail

If you’re going into a weight-loss regime without any sort of plan, then exercise doesn't always become a priority. You may end up just squeezing it in when you have the time.

“I often see clients who want to lose weight and change their body shape, but their exercise routine is an afterthought,” says Will.

Instead, having a structured plan can help you to hit your weight loss targets quicker.

“Before the week begins, work out what you’re going to do and when. Book it into your calendar like an appointment so it’s less likely that you’ll cancel,” adds Will.

“Schedule a rest day or two as well, but remember you don’t need to sit still on a rest day. Walking, yoga, pilates, stretching – these are all low-impact and calm ways to move.”

To do list for managing expectations

Don’t assume all calories are the same

Calories are the driver behind weight change: more calories consumed than you burn can lead to weight gain, but the other way round can lead to weight loss.

“Two diets containing the same calories can feel completely different to live with, and can leave you somewhere different at the end,” says Sean. “Protein is the first reason why. It's the most satiating of the three macronutrients – alongside carbohydrates and fat – so it does more to keep you full for the calories it costs. It's what protects your muscles while you're eating less too.”

Aim for a portion of protein in each meal. Think chicken, eggs, greek yogurt, fish, tofu, turkey, cheese and even beans and lentils. Sean suggests eating 80% whole foods too, to keep you full. Check out our favourite high-protein recipes for inspiration.

That's the practical argument for building most of your diet from whole foods.

“Processed or preserved food isn't inherently bad for you but whole foods just tend to fill you up more for the calories they contain, and they carry the vitamins and minerals your body needs more than usual when you're trying to lose weight and eating less overall.”

Plate of crab egg fried rice

Doing all the cardio and none of the resistance training

Ultimately, weight loss comes down to calories in vs calories out. So of course you might want to do whatever workout burns the most. Typically, a cardio session such as a long run or a big session on the bike. But it’s important to balance the cardio out with weight training.

“When you're in a calorie deficit, your body is looking for ways to cut its energy costs, and muscle is an easy solution. Your body will use it for energy unless you give it a reason to keep it, and that reason is resistance training,” says Sean.

Will explains that muscle is metabolically active tissue, which means it uses energy even when you’re resting. So ideally you want to keep as much of it as possible.

“Aim for two to three, 30- to 45-minute resistance sessions each week, making sure the last rep in any set is nearly impossible, so you know you’re really challenging your muscles,” says Will.

Eco-friendly outdoor gym ideas and equipment made of natural materials

Ignoring all other movement

A 45-minute workout followed by an entire day sitting down could make weight loss very slow.

“Ideally you want to keep moving throughout the day,” says Will. “Walking, taking the stairs, housework, fidgeting, carrying the shopping – these all burn more calories than you might think and can help with your weight-loss goal.”

A family walking

Eating as a reward for your workout

So you’ve just completed a gruelling workout and now you’re ravenous. But you’ve earned a huge feed, right? Turns out, you may not have burned as much as you think.

“An hour of hard resistance training burns somewhere between 200-300 calories above what you'd have burned sitting still. Cardio can push that higher, but even a genuinely strenuous hour is unlikely to clear 500 calories,” says Sean.

While these numbers are good, your effort could be undone very quickly by choosing the wrong post-workout foods or eating a much greater number of calories. Take a Saturday morning for example. “A croissant is around 250 calories; an almond one is closer to 500. So an hour of real effort is undone in about 90 seconds,” warns Sean.

Research backs this up. In a study published in the journal Plos One, people tended to eat bigger portions after exercise, which could end up eating into the calorie deficit they’d worked so hard for.

Sean also warns that, often, fitness trackers and gym cardio machines can overestimate calorie burn, so you may not have burned as much as you thought anyway. This doesn’t mean you can't eat after training. Sean recommends a post-workout protein shake with a banana and peanut butter.

“Your muscles are primed for protein to kickstart the recovery process at that point, so it's a sensible time to have it.” Or try greek yogurt with berries and honey, or cottage cheese on toast.

Greek yoghurt, nuts and blueberries

Thinking change happens overnight

Weight loss takes time – if it was that easy, we’d all be able to do it.

Sean explains that sustainable fat loss runs at roughly ½-1% of your bodyweight a week.

“For most people that's one to two pounds. It doesn't sound like much and it isn't meant to. Anything much faster is usually costing you muscle. Give it months and judge it on the trend across a few weeks instead of whatever the scale says on a Tuesday morning.”

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Lucy Gornall

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