It's the biggest question in health and fitness, but what's the verdict? Writer Sam Rider calls on the experts to end the debate once and for all.

So, this is it. What it all comes down to. The question. And certainly, top of mind with New Year resolutions still in the air, races on the horizon and summer holidays in the distance. Simply: what is more important: diet or exercise?

To provide the definitive answer to this age-old question, we broke the topic down into four more manageable segments, asking:

• What is more important if you’re trying to lose weight?
• If you’re hoping to age like a fine wine?
• If your goal is to lift heavier or run faster?
• Or you’re struggling to sleep at night?

We then pored over the research and quizzed experts in each field to settle this score once and for all.

What's more important... for weight loss?

Surprisingly, this one is fairly clear cut. “The first thing you need to keep in mind, always, is that your diet, not your exercise regimen, will control your body composition results,” says Sean Murphy, Global Personal Training Director at Ultimate Performance, one of the world’s foremost authorities on physical transformations.

“The best way to lose weight is to reduce the number of calories you eat on a daily basis. It really is that simple,” he elaborates. “Exercise will increase your overall calorie burn. But, if you didn’t do any exercise whatsoever, yet made drastic reductions to your daily calorie allowance, you will lose weight. Period.”

It’s all thanks to the calorie deficit principle – in other words, consuming fewer calories than you expend on a daily basis. “Creating a calorie deficit through a balanced diet is generally deemed more effective [for weight loss] as, without dietary changes, exercise alone proves ineffective,” explains Jess Stansfield, Junior Nutrition Manager at Huel.

A 2017 review on the subject found that “most, but not all, study data indicate that exercise alone plays a very small role in weight loss”. The same paper quoted the American College of Sports Medicine and the American Diabetes Association who jointly declared “up to 60 minutes [of physical activity] per day may be required when relying on exercise alone for weight loss”.

Now, if you were to hit this exercise quota year-round, but chased every workout with a KFC bargain bucket, it’s unlikely you’d lose weight. “You cannot out-train a bad diet,” says Murphy. “That’s just a fact.”

“Rather than thinking one is better than the other, treat diet and exercise as teammates that work together to achieve your goal,” adds Stansfield, noting that regular exercise offers multiple health benefits far beyond weight loss, including lowering the risk of chronic health conditions and uplifting mood.

So diet is the clear winner for weight loss. But what about shedding body fat – rather than simply weight – while retaining lean muscle? For this, living in a perpetual kcal hole and foregoing exercise will only get you so far.

“You need to be a bit smarter,” says Murphy. “Yes, you need to remain in a calorie deficit. But you also need to signal to your body to tap into fat stores rather than muscle tissue.”

To do this you need to exercise. Specifically, you need to lift weights. “Regular resistance training improves your body’s ability to lose weight, manage blood sugar levels and build and retain muscle,” says Murphy, “which in of itself can help sustain successful weight loss.”

On this last point, one study showed regular weight training, for just 30 minutes three times per week, improved the trained muscle’s ability to use glucose in both healthy and diabetic subjects. In other words, the more muscle you can retain, the better your body will be at using glucose as fuel, rather than storing it as fat.

Verdict: Diet 80% - 20% Exercise [this is a guesstimate based on judgement of the research]

What's more important... for healthy ageing?

Time is a cruel mistress.

With every year that passes we get closer to conditions like osteoporosis (weak bones), sarcopenia (muscle wasting), anabolic resistance (reduced ability to convert protein to muscle) and menopause (which can lead to an increase in visceral fat, bone weakening and muscle mass decline in women).

Regular resistance training can help ward off this natural decline and lead to healthy ageing, says Murphy, pointing to one paper showing it can also reduce muscle inflammation – but to reduce your biological age (a key indicator of healthy ageing), a recent study suggests more time in the gym isn’t the solution.

The year-long research published at the start of 2023 investigated the effect of diet, exercise, or diet plus exercise on subjects aged over 65. The diet group reduced their daily caloric intake by 500-750 calories. The exercise group performed a mix of flexibility, cardio and resistance training for 90 minutes three times per week.

Intriguingly, exercise alone didn’t improve markers of biological age. Only diet or diet plus exercise proved effective interventions.

Murphy says avoiding ingredients that trigger inflammation, like highly processed junk food, and tapering off carbohydrate intake as you age to keep insulin sensitivity high, can also help you live longer. “The effects of inflammation are cumulative over a lifespan,” he says. “And lower IGF [insulin-like growth factor-1] levels are linked to a longer life expectancy.”

Nevertheless, the PT insists regular strength training is crucial for older populations too. “The highest risk correlation when it comes to people living longer isn't the amount of body fat they carry. It's not their overall weight. It's the amount of lean mass they retain,” he says, pointing to a 2012 study that linked a high lean mass with longer life expectancy and a low body fat with the opposite.

“As I’ve said, the best way to build lean mass is to resistance train,” he adds.

Stansfield agrees, noting that regular exercise should help maintain muscle mass and bone density as you age, while promoting mental well-being. “Increasing the intake of vital nutrients like protein, calcium and vitamin D – rather than merely restricting calories – will also further contribute to healthy ageing, while helping prevent chronic diseases,” she says.

“It's a simple yet powerful combination.”

Verdict: Diet 60% - 40% Exercise

What's more important... for performance?

This all depends on what you’re training for.

If you want to emulate Dwayne “The Rock” Johnson, diet probably comes first. “You’d need to eat thousands of calories a day and manage your diet very carefully,” Murphy points out. If, more likely, you’re trying to run a faster 10K or hit the target more often on the 5-a-side pitch, exercise – in the form of training and practice sessions – should be your priority.

When it comes to building lean muscle – a prerequisite for most sports and Murphy's special subject – exercise is all important. “There is no ‘superfood’, no pill, no magic food group, nothing that will allow you to build lean muscle without working out,” he says.

That said, if you’re chasing PBs but neglecting your diet, it would be like “trying to climb a mountain with one hand tied behind your back,” says Murphy.

“If you’re eternally in a calorie surplus, you will gain weight. If your diet is really poor, extra calories will be stored as fat, not muscle, no matter how hard you train – both unsightly subcutaneous fat and dangerous visceral fat wrapped around your internal organs.”

On the flipside, if you chronically undereat, your performance will suffer, leaving you susceptible to fatigue and injuries. Not consuming enough calories is one of the most common reasons for hitting the dreaded training plateau, for example, says Murphy.

Consuming high-quality protein, such as lean chicken, turkey or fish, Murphy adds, should be your number one priority. “Protein is arguably the number one macronutrient responsible for achieving positive body composition results,” he says. “It’s also the number one macronutrient most people undereat.”

The Reference Nutrient Intake (RNI) for adults in the UK is 0.75g of protein per kg of bodyweight. Murphy believes that if you’re serious about building muscle, and training appropriately, you should at least double this – so around 120-160g of protein per day for someone weighing 80kg.

Timing is similarly important. Spreading your protein intake throughout the day, instead of skewing it to dinner only, has been shown to increase daily protein synthesis rates by up to 30% over a seven-day period.

A 2018 meta-analysis on the subject backs this up. It found that coupling post-resistance exercise protein consumption of 20-30g (or 0.25-0.30g per kg of bodyweight) with regular protein intake spread out across the day of around 1.6g per kg of bodyweight to be the optimal formula for “favourable muscle adaptations to exercise training”.

Verdict: Diet 30% - 70% Exercise

What's more important... for sleep?

Sleep, diet and exercise are inextricably linked. What we eat and drink and how and when we move can all have a dramatic impact on what happens when we shut our eyes at night.

And yet, according to Dr Sophie Mort, Clinical Psychologist and Mental Health Expert at Headspace, “the jury is currently out” on which has the largest effect on sleep. “The link between sleep and the food we eat, and the physical activity we engage in, is so intricate,” she says, that it’s hard to pick a side.

A recent review of 61 studies exploring this subject underlines her point. In the review, published in Sleep Science and Practice, in October 2023, its authors noted that “nutrients such as tryptophan-rich foods [like cheese, chicken, eggs, fish and nuts], antioxidants, melatonin, micronutrients, and fruits positively affect sleep quality”.

The collective research also found the leading cause of sleep restriction to be “excessive energy intake”. Specifically, short sleep duration was mainly correlated with excessive energy intake of carbohydrates and fat. So a late night stuffed crust pizza is a definite no-no.

As for exercise, for the most part physical activity was found to improve sleep duration and sleep onset (the ability to fall asleep). “Wherever the exercise rate increased, the amount of sleep improved,” the research found, with the caveat that “some studies claimed that physical activity did not affect sleep enhancement”.

Daily walking, of the various forms of exercise measured, was the most effective intervention for improving sleep duration and latency for both active and inactive individuals.

So, as you might have guessed, diet and exercise both play a pivotal role. But if you’re among the 48% of UK adults which regularly struggle to get a good night’s sleep, or the eight million clocking under five hours of kip per night – according to a recent Headspace survey – what else can you do?

Well, to treat insomnia, which one third of people in the UK will experience at some point in their lives, the NHS recommends common sense habits like exercising regularly during the day but not within four hours before lights out, and to avoid alcohol, tea or coffee at least six hours before bed.

Dr Mort, however, says it’s important to consider possible changes outside of exercise and diet too. “Taking a moment for yourself to practise mindfulness meditation can help to lower the heart rate by igniting the parasympathetic nervous system and encouraging slower breathing, thereby increasing the chances of a quality night’s sleep,” she says.

“The Headspace app is a great place to start,” she adds.

Verdict: Diet 50% - 50% Exercise

Overall: Diet 55% - 45% Exercise

Words: Sam Rider