So You Want to Lose 20KG? Here’s How I Would Do It. Part 1 of 4.
This article forms Part 1 of a four-part clinical nutrition series looking at how I would approach losing 20 kg of body weight while prioritising the reduction of body fat, preservation of skeletal muscle and improvement of long-term metabolic health.
Part 1: Introduction & Planning establishes the starting point—assessing current body weight, waist circumference, nutrition, energy requirements, activity and training before creating the initial deficit.
Part 2: Applying the Fundamentals moves into execution, covering calorie control, protein, carbohydrates, dietary fats, fibre, food quality, resistance training and physical activity.
Part 3: Regressive Analysis deals with what happens as body weight falls: analysing weight trends, identifying genuine plateaus, reassessing energy requirements, auditing adherence and deciding when—and when not—to adjust calories or activity.
Part 4: Putting It All Together combines everything into a complete fat-loss pathway, including 5 kg reassessment points, transitioning out of the calorie deficit and establishing a sustainable maintenance strategy.
The objective throughout isn't simply to become 20 kg lighter; it's to arrive there with less body fat, as much muscle and strength preserved as reasonably possible, better health and a strategy capable of keeping the weight off.
A Clinical Nutrition Guide to Fat Loss and Muscle Maintenance
Losing 20 kg is a significant goal. But if we're approaching it from a clinical nutrition and long-term health perspective, simply making the number on the scales smaller isn't really the objective. The goal is to reduce excess body fat while preserving as much skeletal muscle, strength and physical function as possible.
That distinction matters.Excess adiposity, particularly when accompanied by metabolic dysfunction, is associated with increased risk of type 2 diabetes, cardiovascular disease, several cancers and premature mortality. But muscle matters too. Skeletal muscle isn't simply something we train for appearance; it is metabolically active tissue, an important site of glucose disposal and a major contributor to strength, mobility and physical independence as we age.
So a successful 20 kg weight-loss phase shouldn't be judged solely by how quickly you can lose 20 kg.It should be judged by what you lose, what you keep and whether the process improves your health.
For the purposes of this series, we're also going to approach the problem without GLP-1-based weight-loss medication. That's not an argument against medications such as semaglutide or tirzepatide. These drugs have substantial clinical evidence behind them and can be appropriate treatments for obesity and its complications when prescribed to suitable patients. We're simply asking a different question: if I wanted to lose approximately 20 kg through nutrition, resistance training, physical activity and behavioural change, while doing everything reasonably possible to maintain muscle mass, how would I approach it?
I wouldn't start by removing carbohydrates. I wouldn't prescribe endless cardio. I wouldn't immediately slash calories to the lowest intake someone could tolerate, and I certainly wouldn't build the plan around detoxes, fat burners or whatever dietary trend happens to be dominating social media.
I'd start by understanding the person, establishing the starting point and building a plan.
Before You Diet, Understand the Starting Point
One of the biggest mistakes people make when deciding to lose a substantial amount of weight is starting with the intervention rather than the assessment.
They decide Monday is the day.
Calories are immediately slashed. Carbohydrates disappear. Alcohol is banned. The gym suddenly becomes a six-day-a-week commitment and the step counter gets set to 15,000.
For a couple of weeks, everything looks fantastic.
Body weight falls rapidly and the intervention appears to be working.
But we haven't necessarily learned anything about why the person gained the excess weight, what their actual energy requirements are, which parts of their existing diet need changing, what their current activity levels are or whether the intervention they're following is remotely sustainable.
Before changing someone's nutrition, I'd therefore establish a baseline.
I'd want to know body weight and waist circumference. Where appropriate, I'd consider body-composition measurements, while recognising the limitations of the method being used. I'd want to understand current food intake, protein consumption, fruit and vegetable intake, fibre, alcohol, snacks, drinks and the less obvious sources of energy such as oils, sauces and calorie-containing coffees.
I'd also want to know what happens at the weekend.
That's important because nutritional assessments based entirely on someone's Monday-to-Thursday behaviour can provide a very distorted picture of their average weekly intake.
I'd look at resistance training, daily movement, occupation, sleep and general lifestyle. If someone spends ten hours a day sitting at a desk, that's relevant. If they're already walking 15,000 steps every day, that's relevant too.
The objective isn't to collect data for the sake of collecting data.
It's to understand what we're actually working with before deciding what needs changing.
When most people say they want to lose weight, what they actually want is to lose body fat.
The distinction is important.
Your total body weight isn't simply stored fat. It includes skeletal muscle, bone, organs, glycogen, water and the contents of your gastrointestinal tract.
The scales combine all of this into one number.That's why someone can apparently “lose” several kilograms very quickly when beginning a restrictive diet. Some of that initial change may be body fat, but changes in glycogen, associated water and gastrointestinal contents can contribute substantially to short-term movements in scale weight.
This is also why an aggressive calorie deficit isn't automatically superior simply because the scales move faster.
During prolonged energy restriction, lean tissue can also be lost. If our goal includes preserving muscle and physical function, the composition of the weight being lost becomes important.
Our objective therefore isn't maximum weight loss at any cost.
It's a controlled reduction in body mass in which we attempt to maximise the contribution from adipose tissue while minimising unnecessary loss of lean tissue.
That principle will become particularly important when we discuss protein intake and resistance training in Part 2.
Twenty Kilograms Is the Destination, Not the Immediate Target
If someone tells me they need to lose 20 kg, I don't particularly want them waking up every morning thinking about the entire 20 kg.
Psychologically, it's a large target.
Physiologically, it's also unlikely that the exact intervention we establish at the beginning should simply continue unchanged until the final kilogram disappears.
Consider someone beginning at 110 kg and eventually reaching 90 kg.
The 90 kg version of that person is not identical to the 110 kg version.
They have less total mass to move around. Their energy expenditure may be lower. Their activity may have changed. Their training capacity may have changed. Their hunger and dietary behaviour may have changed.
The intervention therefore needs to evolve.
I prefer thinking about a substantial fat-loss goal in smaller phases, perhaps approximately 5 kg at a time.
Lose the first 5 kg.
Then reassess.
If everything is working, there may be no reason to change anything.
Continue.
At the next meaningful checkpoint, reassess again.
This transforms losing 20 kg from one enormous diet into a series of manageable interventions.
Calories Matter, but We Need to Establish the Right Starting Point
At the centre of intentional fat loss sits an unavoidable physiological requirement: a sustained energy deficit.
Over time, energy expenditure must exceed energy intake.
That doesn't mean all diets are nutritionally equivalent, nor does it mean appetite, protein, food quality, fibre, sleep and behaviour don't matter.
They absolutely do.
But none of them remove the requirement for an energy deficit.
The challenge is establishing how large that deficit should be.
We can use validated prediction equations to estimate resting energy expenditure and then estimate total daily energy expenditure based on activity. These calculations are useful, but they remain estimates.
The calorie number displayed by your smartwatch is an estimate.
The number produced by an online calculator is an estimate.
The calories supposedly burned during your treadmill session are an estimate.
So I'd use the calculation to establish a reasonable starting point rather than treating it as biological fact.
Suppose someone's estimated maintenance requirement is around 2,700 kcal per day.
I'm not immediately going to prescribe 1,500 kcal because a larger deficit should theoretically produce faster weight loss.
Perhaps we begin closer to 2,200–2,300 kcal.
We then collect data.
Is body weight trending down?
Is waist circumference reducing?
Is hunger manageable?
Can the person train effectively?
Can they actually adhere to the diet?
If the answers are broadly yes, then the intervention is doing its job.
The calculation starts the process. The results calibrate it.
One of the most common approaches to substantial weight loss is also one of the least intelligent.
Someone simultaneously introduces an enormous calorie deficit, daily cardio, huge step targets and multiple resistance-training sessions.
This can produce dramatic initial weight loss.
But it also leaves very little room to manoeuvre.
What happens when the person is 10 kg lighter and progress genuinely begins to slow?
Do we cut another 500 calories?
Add another hour of cardio?
Push steps to 20,000?Instead, I'd rather establish the minimum effective intervention required to produce meaningful progress.
Start with an appropriate calorie deficit.
Establish consistent resistance training.
Maintain or gradually increase daily activity where necessary.
Then monitor.
If the intervention is working, leave it alone.
If it eventually stops producing the expected result, we reassess before changing anything.
Fat loss doesn't need to feel progressively more miserable in order to be effective.
If our stated objective is fat loss with muscle maintenance, then muscle preservation can't become something we suddenly start worrying about halfway through the diet.
It needs to influence the plan from day one.
Two variables become particularly important here: protein and resistance training.
Protein provides the amino acids required to support muscle protein turnover, while resistance exercise provides the mechanical stimulus encouraging the body to maintain muscular tissue and function.
This doesn't mean everyone trying to lose weight needs to train like a competitive bodybuilder.
But it does mean that if preserving skeletal muscle matters, some form of appropriately programmed resistance training should form part of the intervention.
For many adults, somewhere around two to four resistance-training sessions per week may be appropriate depending on experience, ability, programme design, recovery and individual circumstances.
The objective isn't to see how much exercise someone can survive while dieting.
It's to provide sufficient training stimulus to preserve muscle and strength while allowing enough recovery to continue performing effectively.
Daily Movement Matters Too
Going to the gym doesn't automatically make someone physically active for the rest of the day.
You can complete a hard 60-minute workout and spend most of the remaining 23 hours sitting down.
This is why I'd establish baseline daily movement before dramatically increasing it.
Steps are particularly useful because they're easy to measure.
But there's nothing metabolically magical about 10,000 steps.
If someone currently averages 3,500 steps per day, increasing towards 5,000 or 6,000 may be a much more realistic initial intervention than immediately demanding 15,000.
Conversely, someone already averaging 12,000 steps probably doesn't need an arbitrary increase simply because they're dieting.
Again, we're establishing a baseline and making the smallest useful change.
Nutrition Has to Do More Than Produce Weight Loss
If weight loss were purely a mathematical exercise, we could simply calculate calories and finish the article here.
But clinical nutrition has a higher standard.
We need to consider protein, essential fatty acids, fibre, vitamins, minerals, fruit and vegetables, hydration, satiety, food quality and the ability to sustain the intervention.
A diet dominated by nutritionally poor foods could theoretically produce weight loss if total energy intake remained sufficiently low.
That doesn't make it the diet I'd prescribe.
When calories are restricted, nutritional quality arguably becomes even more important because we're trying to supply adequate nutrition from a smaller total energy budget.
This is where foods such as lean protein sources, fruit, vegetables, whole grains, legumes, dairy or appropriate alternatives, nuts, seeds and unsaturated fats become valuable.
Not because individual foods magically cause fat loss.
But because we're attempting to construct a calorie-controlled diet that simultaneously supports nutritional adequacy, satiety, muscle retention, training and long-term health.
Part 2 will explore exactly how I'd build that diet.
Your Diet Has to Survive Your Life
The perfect nutritional intervention on paper is useless if the person can't live with it.
People work long hours.
They have families.They go to restaurants.
They attend weddings.
They travel.
They have stressful weeks.
They occasionally sleep badly.
And many people drink alcohol.
A successful fat-loss intervention needs to accommodate the person's real life rather than requiring them to temporarily withdraw from it.
That doesn't mean every behaviour needs to remain unchanged.
If someone regularly consumes large amounts of alcohol, eats takeaways four evenings a week and barely moves during the day, then meaningful changes will probably be necessary.
But those changes need to be realistic enough to survive longer than the initial burst of motivation.
Because losing 20 kg isn't particularly useful if the intervention required to achieve it is so restrictive that the person immediately abandons everything when the diet ends.
We're not simply planning how to lose the weight.
We're beginning to establish how someone might eventually live at the lower body weight.
So, Before We Start…
If I were preparing to lose 20 kg, I wouldn't start tomorrow by eating as little as possible.
I'd spend some time understanding where I am now.
I'd record my body weight and waist circumference. I'd look at what I'm actually eating across an entire week. I'd establish my approximate energy requirements, protein intake, daily movement and current resistance training.
I'd then create a reasonable initial calorie deficit rather than the largest deficit I could tolerate.
I'd establish resistance training from the beginning.
I'd set an achievable activity target.
And then I'd stop obsessing about the full 20 kg.
I'd focus on the first meaningful checkpoint.
Get there.
Collect the data.
Then decide what happens next.
Practical Takeaways From Part 1
Define the objective correctly. The goal is predominantly fat loss while preserving as much skeletal muscle, strength and physical function as reasonably possible.
Establish your baseline. Record body weight, waist circumference, current food intake, activity and training before making major changes.
Estimate rather than assume your calorie requirements. Prediction equations provide a starting point; your subsequent weight trend helps refine the estimate.
Start with a moderate energy deficit. The most aggressive intervention isn't automatically the best intervention.
Think in months rather than weeks. Twenty kilograms represents a substantial change in body mass and should be treated accordingly.
Use smaller checkpoints. Think about the first 5 kg, reassess, and then continue rather than psychologically chasing all 20 kg from day one.
Resistance train from the beginning. If muscle retention matters, provide a reason for your body to retain muscle.
Establish daily activity. Don't blindly prescribe arbitrary step targets. Determine the baseline first.
Don't judge progress from one weigh-in. Look at trends alongside waist circumference, training performance and other relevant measurements.
Don't spend every intervention immediately. Leave yourself somewhere to go if genuine adaptation or a plateau occurs later.And perhaps most importantly:
Don't build a diet that you can't imagine maintaining some version of once the weight has gone.
Coming Next — Part 2: Applying the Fundamentals
In Part 2, we'll turn the plan into food.
We'll look at how I'd establish protein intake for muscle retention, where carbohydrates and dietary fats fit into the diet, fibre and micronutrient requirements, eating for volume, meal construction and how resistance training and daily activity integrate with the nutritional intervention.
Because understanding that you need a calorie deficit is relatively straightforward.
Building one that provides adequate nutrition, protects muscle, manages hunger and can be sustained long enough to lose 20 kg is where the real work begins.