The Real Cost of Diet-Only Weight Loss:
According to the Wales Activity Tracker Survey (2023), 46% of Welsh adults engage in physical activity 2–4 days a week, 19% do so 5+ days, and 22% report no weekly exercise at all. Of those who exercise, 59% cite managing their physical health as the primary reason. When we look at activity types, 57% walk for leisure, 23% walk for work purposes, and only 15% attend fitness classes or go to the gym.
Meanwhile, the National Survey for Wales (2021–2022) shows 62% of individuals are classified as overweight or obese, with 25% falling into the obese category, the results of which are similar UK-wide (64% classified as overweight/obese, 24% considered obese). Taken together, these statistics suggest that although a majority of the population is at least moderately active—primarily to manage physical health—only a small fraction (15%) frequents a fitness class or gym. From this, we can infer an even smaller segment is actively engaging in any type of deliberate resistance training as part of their exercise regime.
In our previous article, Beyond BMI, we highlighted how BMI (weight/height²) can often be misleading as a health metric because it doesn’t distinguish muscle from fat, or account for fat distribution. Some fat deposits, like visceral fat around the abdomen and organs, are considered far more harmful than subcutaneous fat, found just below the skin surface. A key takeaway was the importance of resistance training to preserve muscle mass and improve insulin sensitivity—particularly crucial for individuals at risk of metabolic disorders.
For many people seeking weight loss, the focus tends to land squarely on dietary changes - often ignoring the role of exercise entirely. However when we consider the downsides of losing weight without the preservation of muscle mass, or ensuring robust metabolic health, a question begins to emerge: is weight loss without resistance exercise doing more harm than good?
Exercise Matters - Even if Weight Loss Isn’t the Goal
Many individuals looking for weight loss gravitate towards cardiovascular exercise - walking, running, cycling - likely because it shows a clear and immediate spike in calorie burn. We see and sense the elevation in heart rate, and a single session does have the potential to yield a substantial daily energy deficit. By contrast, resistance training often has a more modest impact on heart rate and metabolic expenditure in the moment; you’re unlikely to “lift you way” into a large calorie shortfall the same way that running for two hours might.
However, the real value of resistance training lies in preserving or adding muscle mass, which not only supports long-term metabolic rate but also prevents the drastic muscle losses seen with diet-only weight loss interventions. The short-term calorie burn may seem minimal, and likely why we don’t naturally tend to gravitate towards it as an exercise choice to accompany active weight loss attempts, but it’s role in safeguarding muscle and maintaining overall metabolic health is crucial.
The Cost of Losing Weight Without Exercise
A caloric deficit is undoubtedly central to weight loss. However, for long-term weight maintenance and overall health, exercise—especially resistance training—is essential. Here’s why:
Muscle Loss
Without exercise, weight loss skews towards a greater loss of muscle alongside fat mass, with research showing this rate of loss can range from 22% to an alarming 46% of weight loss being muscle mass loss specifically.
In scenarios in which both poor sleep or high stress are present, muscle loss can spike to 60-80% of total weight loss. With significant calorie deficits for an extended period of time known to disrupt sleep patterns and induce stress, that’s a potentially terrifying combination from a long-term health perspective.
Less muscle mass also likely means a lower resting metabolic rate (RMR), making the potential for future weight gain more likely.
2. Fat Overshoot
After diet-induced muscle loss, the body may regain more body fat for the same increase in body weight, than it had before - known as fat overshoot.
For example, imagine someone starting at 85 kg who diets down to 70 kg, losing 7–8 kg of muscle in the process. Then, upon regaining 15 kg, they only recover 4–5 kg of muscle mass. Although they return to 85 kg on the scale, they’re now carrying 2–3 kg less muscle than before—leaving them in a worse position both metabolically and aesthetically.
3. Loss of Strength and Function
Muscle is not just for aesthetic purposes; it’s critical for mobility, balance, and day-to-day functional strength.
Reduced muscle mass, especially as we age (termed sarcopenia), correlates with higher rates of hospitalisation, disability, and mortality.
4. Blunted Muscle Protein Synthesis
A calorie deficit increases the rate of muscle protein breakdown. The body needs energy to meet it’s basic metabolic demands. To do this it can utilise carbohydrate sources readily available, as well as body stored body fat, and muscle protein.
Without resistance training to simulate muscle protein synthesis, muscle loss becomes a near certainty.
Preserving Lean Body Mass
In a calorie deficit, resistance training is the most reliable way to maintain (or yes, even gain) muscle. While High-Intensity Interval Training (HIIT) and aerobic exercise have their own benefits (especially for cardiovascular health), they’re far less consistent in preserving muscle during dieting phases, especially in individuals who aren’t new to exercise. Research on untrained individuals can often be noisy - fluid shifts and measurement variations can distort muscle data as to how much muscle an untrained individual can put on in a calorie deficit - however the consensus remains the same, lifting weights or resistance exercises are key.
So what does that look like specifically?
Minimal Effort, Major Returns: Unlike the time-consuming requirement to make cardiovascular changes, even 2-3 sets per week, per muscle group, can help to maintain or increase muscle mass when paired with adequate protein. In reality, this could mean a return on investment for just 2-3 sessions of ~30 minutes in length, per week.
Optimal Set Range: More sets in total typically lead to better outcomes with regard to increasing muscle mass, but the margin diminishes if you’re already close to your body’s capacity for recovery.
Various Activities: It doesn’t have to be free weights in a gym to reach maintenance levels of muscle stimulus, it can be bodyweight exercises, exercises with bands, or a simple machine circuits assuming the degree of intensity is high enough to stimulate muscle growth. Research has even suggested that modalities such as climbing or certain contact-based sports might maintain muscle mass through a calorie deficit, though the research is very limited.
Recommended Benchmarks
As we touched on within the Beyond BMI article, one way in which to measure muscle status is through the Fat-Free Mass Index (FFMI), which accounts for lean mass relative to height. For more information on this measure, make sure to check out the previous article, however as a general guideline:
Men: Aim to keep FFMI above 19.
Women: Aim to keep FFMI above 16.
Why Movement is Non-Negotiable for Long-Term Weight Maintenance
Whilst we’ve devoted most of the past two articles to the topic, it’s not just about muscle retention. Most successful body composition changes through weight-loss is sustained through substantial daily physical activity, such as:
8-12,000 steps per day, or
60-90 minutes of moderate activity, or
30-40 minutes of vigorous exercise most days of the week.
Without this ongoing activity, metabolic rate can decline with time, and the likelihood of weight regain increases. Overemphasising the dietary arm of weight loss alone might yield quick results, but it leaves a gap in the foundations that are needed for long-term success.
Putting It All Together
Calorie Deficit: A fundamental component of losing weight.
Resistance Training: Critical to protect muscle mass, maintain bone density, and uphold functional strength levels.
Protein Intake: Adequate dietary protein further helps stave off muscle loss during a calorie deficit.
Physical Activity: A strong precursor of success in maintaining long-term weight loss, tying everything together.
Ultimately, losing weight without exercise, and resistance training specifically, might not be unethical in the strictest of senses, but it can shortchange you in terms of health, functionality, and sustainability. From the sheer volume of muscle lost, to the metabolic rebound that often follows extreme crash dieting, the evidence points to exercise - particularly resistance training - as a must-have in any responsible, healthy approach to long-term weight management.
Final Thoughts
If you goal is genuine, lasting health - not just a shrinking number on the scale - exercise is non-negotiable. You’ll safeguard your muscle mass, maintain a higher overall metabolic rate, and foster better long-term weight maintenance. More than that, you’ll likely gain energy, resilience, and a far more robust defence against chronic disease.
In other words, pursing weight loss in isolation may be simpler, choosing to restrict calorie intake without choosing to exercise is an easier choice, but it comes with real risks. By combining a calorie deficit with a smart exercise programme - combining resistance training, aerobic conditioning, and sufficient daily movement - you can create a powerful synergy that fosters a stronger, healthier you.