It’s Very Hard to Out-Train a Bad Diet but It’s Equally Difficult to Out-Diet a Sedentary Lifestyle

You’ve probably heard the saying that fat loss is 80% diet and 20% training. While I get the sentiment––it’s hard to out-train a bad diet––I’m not a fan of these arbitrary ratios because, for one, people are just pulling these numbers out of their asses, and two, both are important, just for different reasons. 

That’s why I say, “Eat for fat loss, train for muscle, strength, and health.” It highlights the role diet and training play in fat loss instead of making one seem more important. 

And today, I want to explain why physical activity shouldn’t be an afterthought if the goal is fat loss. Let’s start with the most fundamental reason…

It’s good for you, duh

Excuse me while I point out the obvious and risk losing you before we’ve even really begun, but regular physical activity is great for your health and it can help you live longer. 

Research suggests that active people reduce their risk of dying by about 30-35%, which translates to living about 2-4 years longer (and possibly more). Importantly, active people tend to enjoy more years of disability-free life. 1 Does Physical Activity Increase Life Expectancy? A Review of the Literature, Reimers CD et al. 2012

Exercise can increase energy expenditure

Out of all the components of your total daily energy expenditure, like the number of calories you burn at rest (resting metabolic rate; RMR), the energy required to digest and store the food you eat (thermic effect of food; TEF), and the calories expended during physical activity (PAEE), physical activity is the most variable, ranging from ~5-40% of total daily energy expenditure. 2 Homeostatic and non-homeostatic appetite control along the spectrum of physical activity levels: An updated perspective, Beaulieu K et al. 2018

As such, if we took two people of the same age, sex, weight, height, etc., but one was active while the other was sedentary, the active individual would expend more energy. 

To help illustrate, I used the Mifflin-St Jeor Equation to calculate the resting metabolic rate (RMR) for a hypothetical male and female and then multiplied their RMR by the standard activity level multipliers. In the image below, you can see their total daily energy expenditure (TDEE) at different activity levels.

From a fat loss perspective, a sedentary individual would need to diet on significantly fewer calories than someone active, which can make sticking to the diet harder.

Sure, most people aren’t going to reach the very high end of the activity rung. But even moving from sedentary to lightly active can net you an additional 200-300 calories, which can make a fairly sizeable difference to how you feel and how well you adhere to the deficit.

Caveat time

Okay, it’s time for a quick segue to clarify that the relationship between exercise and energy expenditure isn’t linear. Beyond a certain point, more exercise doesn’t directly translate to an equal increase in energy expenditure.

For instance, if you burn 500 calories per day through exercise, your total daily energy expenditure (TDEE) won’t necessarily increase by exactly 500 calories. This is due to the compensatory effects like increased fatigue and reduced spontaneous activity. 3 Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans, Pontzer H et al. 2017

That said, this compensation doesn’t eliminate the entire increase in energy expenditure. On average, about 30% of the extra calories burned are compensated for, meaning you still get a net increase in TDEE (it’s just not a perfect 1:1 relationship), and the extent of this compensation will be larger if you’re in a calorie deficit (versus maintenance or a surplus) and have higher body fat levels. 4 Energy compensation and adiposity in humans, Careau V et al. 2021 5 Physical Activity and Total Daily Energy Expenditure in Older US Adults: Constrained versus Additive Models, Willis EA et al. 2022

So, while increasing activity won’t have as big an effect as people think––no, your HIIT class didn’t burn 1000 calories despite what your smartwatch says––it will have an effect. 

Additionally, even after accounting for energy compensation, exercise can still help reduce the impact of metabolic adaptation––i.e., the normal reductions in metabolic rate as a consequence of weight loss. If you’re losing weight and your metabolism slows by 100 calories, this drop will feel more significant if you’re sedentary because your baseline TDEE is lower.

For example, a sedentary female eating 1,200 calories per day who experiences a 100-calorie metabolic slowdown may have to drop to 1,000–1,100 calories just to continue losing weight. On the other hand, if she were active and eating 1,600 calories per day, she would have room to drop to 1400-1500 calories without the diet feeling unsustainable.

Cool? Cool. Let’s get back to our regular scheduled programming.

Muscle matters

Fat-free mass (FFM)––everything in your body that isn’t fat––is the strongest predictor of how many calories you burn at rest. In other words, the more fat-free mass you have, the higher your metabolic rate. 6 Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine

By extension, losing muscle will have a measurable impact on RMR. For example, individuals in a calorie deficit who aren’t resistance training tend to lose about 25% of the total weight lost as muscle. 7 Proportion of caloric restriction-induced weight loss as skeletal muscle, Heymsfield S et al. 2024

Skeletal muscle, like your biceps, burns around 13 kcal/kg (6 kcal/lb), while body fat burns about 4.5 kcal/kg (2 kcal/lb). Therefore, if someone loses 10kg (22 lbs) of total body weight, their resting metabolic rate will decrease by around 66 calories. But if they were resistance training and maintained all of their muscle, the decrease in their metabolism would only be 45 calories.

Note: I’m intentionally keeping the example simple. In reality, losses in organ mass will have a greater impact on RMR reductions since organs require more energy than skeletal muscle. For example, your organs account for 70-80% of resting energy expenditure despite making up less than 10% of total body weight. 8 Contribution of individual organ mass loss to weight loss-associated decline in resting energy expenditure, Bosy-Westphal A et al. 2009 9 Normalizing resting energy expenditure across the life course in humans: challenges and hopes, Müller MJ et al. 2018

And this isn’t just hypothetical. Pedro and colleagues meta-analysed the effects of resistance-based exercise programs on body composition in overweight and obese individuals. When resistance training was combined with a calorie deficit, fat loss averaged 5.1kg (11 lb) with a non-significant lean mass loss of -0.2 kg (-0.4 lb). Conversely, dieting without resistance training increases the percentage of weight lost as muscle. 10 Resistance training effectiveness on body composition and body weight outcomes in individuals with overweight and obesity across the lifespan: A systematic review and meta-analysis, Pedro L et al. 2022

But maintaining muscle isn’t the only thing that will impact your metabolism––gaining muscle will also have an impact.

If a pound of muscle burns 6 kcal/lb (13 kcal/kg), gaining 10 lb of muscle would increase your metabolic rate by 60 calories. 

However, as Greg Nuckols pointed out, that 6 kcal figure only applies if you lie around all day. So, assuming you’re not in a coma, the actual figure is closer to 10 kcal per pound (22 kcal/kg) when factoring in daily activity and exercise. In other words, gaining 10 lb of muscle would increase your metabolism by 100 calories. Sure, it’s not exactly massive, but it’s still something. 

Furthermore, the research generally shows that physical activity increases metabolic rate. A meta-analysis by Mackenzie-Shalders and colleagues found that different exercise interventions––resistance training, cardio, or a combination of the two––increased resting metabolic rate by an average of 80 calories per day. 11 The effect of exercise interventions on resting metabolic rate: A systematic review and meta-analysis, Mackenzie-Shalders K et al. 2020

Another study examined the effect of regular physical activity on resting metabolic rate and body composition in women aged 35-50, finding significantly higher metabolic rates and lower body fat levels in active middle-aged women compared to their sedentary counterparts. 12 Effects of habitual physical activity on the resting metabolic rates and body compositions of women aged 35 to 50 years, Gilliat-Wimberly M et al. 2001

But there’s more to muscle than a metabolic boost. 

From a health perspective, resistance training improves glucose metabolism, making it easier to regulate blood sugar levels. And if you’re into that whole living longer thing, muscle mass has a protective effect on mortality risk. 13 Skeletal muscle hypertrophy rewires glucose metabolism: An experimental investigation and systematic review, Baumert P et al. 2024

Secondly, research shows that people who stay more physically active throughout life tend to retain more muscle as they age. While ageing inevitably takes its toll, building more muscle mass in your younger years acts as a buffer against age-related muscle loss. To illustrate, absolute muscle mass remains relatively stable until the late 40s, after which it starts to decline at an average rate of 1.9 kg (4.2 lb) per decade in men and 1.1 kg (2.4 lb) per decade in women. 14 Physical activity and fat-free mass during growth and in later life, Westerterp KR et al. 2021 15 Skeletal muscle mass and distribution in 468 men and women aged 18–88 yr, Janssen I et al. 2000

SO:

• A man entering his 50s with 40 kg of muscle will still have around 38 kg at 60 and 34 kg by 80. Meanwhile, a man who starts with 25 kg at 50 will drop to 23 kg at 60 and 19 kg by 80.
  
  
• Similarly, a woman who enters her 50s with 30 kg of muscle will have 28 kg at 60 and 27 kg by 80, while someone starting with 18 kg at 50 will decline to 16.9 kg at 60 and 14.7 kg at 80.

But maybe you don’t care about a modest increase in metabolism or the health and longevity benefits of retaining (and gaining) muscle. Do you care about avoiding fat regain? Well, evidence suggests losing more muscle during a diet can increase the chances of weight regain through increased appetite. 16 Associations between the proportion of fat-free mass loss during weight loss, changes in appetite, and subsequent weight change: results from a randomized 2-stage dietary intervention trial, Turicchi J et al. 2020

Dulloo and colleagues noted that during the Minnesota Starvation Experiment, the men’s hunger remained elevated despite regaining all of the body fat they had lost during the experiment. It wasn’t until their fat-free mass had been fully restored that hunger levels returned to normal. 17 Collateral fattening in body composition autoregulation: its determinants and significance for obesity predisposition, Dulloo AG et al. 2018

Speaking of appetite…

Better appetite regulation

Nearly 70 years ago, Mayer et al. studied factory workers in West Bengal, India, uncovering a fascinating relationship between physical activity, food intake, and body weight. 

Sedentary workers overate relative to their needs, moderately active workers had better appetite control, and highly active labourers ate more but maintained a healthy weight due to higher energy expenditure. 18 Relation between caloric intake, body weight, and physical work: studies in an industrial male population in West Bengal, Mayer J et al. 1956

Their findings suggested that appetite regulation improves within a “zone of regulation”—a range of activity where food intake naturally aligns with energy needs. For sedentary individuals, appetite is often dysregulated, leading to overeating. Adding exercise brings them back into the regulated zone, where their intake better matches their energy needs.

Decades later, modern studies are confirming these findings. Shook et al. examined the relationship between energy intake, physical activity, appetite, and weight gain during a 12-month follow-up period in a sample of 421 individuals. They found that Individuals with the lowest physical activity had the highest caloric intake and gained the most body fat. Interestingly, the same group also underreported their food intake more than individuals with higher physical activity levels. 19 Low levels of physical activity are associated with dysregulation of energy intake and fat mass gain over 1 year, Shook RP et al. 2015

There are a few potential mechanisms for how exercise might influence appetite regulation. Acute exercise suppresses hunger by lowering ghrelin (the ‘hunger hormone’) and increasing PYY and GLP-1 (satiety hormones), though these effects are short-lived. Regular physical activity can also enhance appetite sensitivity over time, making the body more responsive to satiety signals and reducing the risk of overeating. Finally, some evidence suggests that exercise alters brain reward pathways, reducing the drive for high-calorie foods and improving food choice regulation. 20 The complex pattern of the effects of prolonged frequent exercise on appetite control, and implications for obesity, Blundell JE, Beaulieu K. 2023 21 The Impact of Physical Activity on Food Reward: Review and Conceptual Synthesis of Evidence from Observational, Acute, and Chronic Exercise Training Studies, Beaulieu K et al. 2020

Supporting this, Hägele et al. found that higher energy turnover led to a sustained decrease in ghrelin and an increase in GLP-1, creating a hormonal environment that naturally reduces hunger and prevents compensatory overeating. 22 Appetite Control Is Improved by Acute Increases in Energy Turnover at Different Levels of Energy Balance, Hägele FA et al. 2019

Now, to address something that might be on your mind—if exercise improves appetite regulation, why can’t I just do more of it while in a deficit?

The answer lies in understanding two types of appetite regulation: homeostatic and non-homeostatic appetite.

• Homeostatic appetite regulation is driven by the body’s physiological need for energy (aka ‘true hunger’)—hormones like ghrelin (which stimulates hunger) and PYY/GLP-1 (which promote satiety) help balance intake with expenditure.
  
• Non-homeostatic appetite regulation is influenced by external and psychological factors—like food availability, stress, habits, and reward-driven eating (e.g., eating dessert when you’re already full).

So:

• If you’re sedentary and eating more, you’re in an “eat more, move less” state where appetite is dysregulated—homeostatic signals don’t properly align with energy needs, often leading to overeating. Exercise can help nudge you back into the regulated zone, where energy intake better matches expenditure.
  
  
• If you’re highly active and eating more, you’re in a “move more, eat more” state where appetite and energy balance are well-coupled and your body self-regulates intake in response to increased expenditure.
  
  
• But if you’re in a large calorie deficit because you’re exercising a lot while eating fewer calories, you enter a “move more, eat less” state that disrupts homeostatic regulation. As a result, the body ramps up hunger to push you to eat more while increasing fatigue to make you move less. At the same time, non-homeostatic factors—like cravings for high-calorie foods—become stronger. All of this is your body’s way of telling you to calm the fuck down.

Weight maintenance

I’m going to go out on a limb here and assume that you don’t just want to lose fat––you want to be able to maintain that fat loss, yes? Well, physical activity can help here, too. According to one systematic review, increased physical activity is “the most consistent positive correlate of weight loss maintenance.” 23 Successful weight loss maintenance: A systematic review of weight control registries, Paixão C et al. 2020

Similarly, Ostendorf and colleagues found that weight maintainers took more steps and had higher physical activity levels than normal-weight and obese controls. 24 Physical Activity Energy Expenditure and Total Daily Energy Expenditure in Successful Weight Loss Maintainers, Ostendorf DM et al. 2020

One reason for this could be due to something called “energy flux.” At the end of a diet, you can approach weight maintenance in one of two ways:

• Low energy flux: Continue eating fewer calories relative to what you were eating at the start of a diet.
  
  
• High energy flux: Increase your physical activity and eat a bit more food. This could reduce hunger (see previous point) and increase your chances of maintaining your weight.

While both states lead to energy balance, a high-flux state could help mitigate weight regain by closing the ‘energy gap’ – the difference in energy requirements before and after weight loss. 25 Increasing Energy Flux to Maintain Diet-Induced Weight Loss, Melby CL et al. 2019

At first blush, the last paragraph might sound contradictory because what about the constrained energy model? 

The answer to that question lies in a 2022 study by Willis et al. The researchers tested both the additive and constrained energy expenditure models in a large Western population and found that when participants were at maintenance or a surplus, the additive model held true (to a point), whereas when participants were in a deficit, the constrained model held true. 26 Physical Activity and Total Daily Energy Expenditure in Older US Adults: Constrained versus Additive Models, Willis EA et al. 2022

Specifically, here’s how total energy expenditure changed across the different energy balance groups:

• Positive energy balance (i.e. calorie surplus):
  • Bottom decile: 2175 kcal/day
  • Top decile: 2880 kcal/day
    
• Neutral energy balance (i.e. maintenance)
  • Bottom decile: 2346 kcal/day
  • Top decile: 2776 kcal/day
    
• Negative energy balance (i.e. calorie deficit):
  • Bottom decile: 2428 kcal/day
  • Top decile: 2372 kcal/day

Note that in the ‘negative energy balance’ group, the most active individuals (top decile) were burning fewer calories than those in the bottom decile, indicating a constraint on energy expenditure. This explains why some people are stuck dieting on very low calories while exercising excessively without losing weight—their bodies adapted to conserve energy. Conversely, weight maintainers in neutral or positive energy balance can perform more activity without experiencing the same metabolic constraints, allowing them to maintain their weight while burning more calories overall.

When it comes to weight loss, a lot of people tend to fall on one end of the extreme––they either focus wholly on intense exercise with no regard for their diet, or they put off physical activity and only focus on restricting their calories. To paraphrase Goldilocks, “The sweet spot is in the middle, fuckface.” 

In reality, the goal isn’t just weight loss––it’s to approach your diet and training in a way that maximises fat loss and minimises muscle loss. But more importantly, it should you set up for success long after your diet ends. 

Out of all the benefits of physical activity, “burning calories” wouldn’t even crack the top ten. Yes, exercise increases energy expenditure, but it also improves health and gives you the best chance at living a longer, higher quality life while maintaining physical function long into your twilight years. It also regulates appetite, preserves muscle, and prevents metabolic slowdown. And when it comes to keeping weight off long-term, regular movement is one of your best bets.

No matter your goal, regular physical activity isn’t optional—it’s essential.

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