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Our Science and Methodology

Nutrition Built on Evidence. Not Guesswork

At Fettle, every recommendation is grounded in established nutrition science and refined through real-world use. We don’t believe in rigid plans, extreme diets, or one-size-fits-all formulas. We believe nutrition should adapt as your body, habits, and goals change — because real people don’t stay static.

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Foundational Principles

Our Guiding Principles

Before we explain how Fettle works, it's important to understand what guides every decision we make.

Nutrition should adapt over time

Your body isn’t static. As you lose fat, gain muscle, or change activity levels, your calorie and macro needs change too. Any system that doesn’t adapt will eventually fall out of sync.

Trends matter more than single data points

Sustainability beats extremes

Nutrition should adapt over time

Your body isn’t static. As you lose fat, gain muscle, or change activity levels, your calorie and macro needs change too. Any system that doesn’t adapt will eventually fall out of sync.

Trends matter more than single data points

Sustainability beats extremes

Nutrition should adapt over time

Your body isn’t static. As you lose fat, gain muscle, or change activity levels, your calorie and macro needs change too. Any system that doesn’t adapt will eventually fall out of sync.

Trends matter more than single data points

Sustainability beats extremes

Macro Calculation

How Fettle calculates your nutrition

Fettle’s engine combines established formulas with adaptive logic to create personalised, evolving targets.

Step 1: Baseline Energy Needs

We start by estimating your baseline daily energy needs using clinically validated equations based on height, weight, age, and sex.

This provides an initial estimate of how much energy your body requires at rest and during daily life.

Step 1: Baseline Energy Needs

We start by estimating your baseline daily energy needs using clinically validated equations based on height, weight, age, and sex.

This provides an initial estimate of how much energy your body requires at rest and during daily life.

Step 1: Baseline Energy Needs

We start by estimating your baseline daily energy needs using clinically validated equations based on height, weight, age, and sex.

This provides an initial estimate of how much energy your body requires at rest and during daily life.

Step 2: Activity & Lifestyle Adjustment

Your baseline needs are then adjusted based on your reported activity levels and lifestyle.

This helps estimate your Total Daily Energy Expenditure (TDEE) — the number of calories your body burns on an average day.

Step 2: Activity & Lifestyle Adjustment

Step 2: Activity & Lifestyle Adjustment

Step 3: Goal-specific Adjustments

Based on your goal — fat loss, maintenance, or muscle gain — we apply controlled adjustments to your calorie target.

These adjustments are designed to:

Support progress without unnecessary fatigue

Reduce muscle loss during fat loss

Avoid overly aggressive targets that are hard to sustain

Step 3: Goal-specific Adjustments

Based on your goal — fat loss, maintenance, or muscle gain — we apply controlled adjustments to your calorie target.

These adjustments are designed to:

Support progress without unnecessary fatigue

Reduce muscle loss during fat loss

Avoid overly aggressive targets that are hard to sustain

Step 3: Goal-specific Adjustments

Based on your goal — fat loss, maintenance, or muscle gain — we apply controlled adjustments to your calorie target.

These adjustments are designed to:

Support progress without unnecessary fatigue

Reduce muscle loss during fat loss

Avoid overly aggressive targets that are hard to sustain

Macro vs Calories

Protein, Fat, & Macro Allocation

Calories alone don’t tell the full story.
Fettle allocates macronutrients with specific safeguards in place.

Protein

Protein targets are prioritised to:

Support progress without unnecessary fatigue

Reduce muscle loss during fat loss

Avoid overly aggressive targets that are hard to sustain

Protein is especially prioritised during fat-loss phases.

Protein

Protein targets are prioritised to:

Support progress without unnecessary fatigue

Reduce muscle loss during fat loss

Avoid overly aggressive targets that are hard to sustain

Protein is especially prioritised during fat-loss phases.

Fats

Dietary fat is never allowed to drop below a minimum safe level. Adequate fat intake is essential for:

Hormonal health

Nutrient absorption

Overall wellbeing

Fats

Dietary fat is never allowed to drop below a minimum safe level. Adequate fat intake is essential for:

Hormonal health

Nutrient absorption

Overall wellbeing

Carbohydrates

Carbohydrate targets are set after protein and fat needs are met. This allows flexibility while ensuring:

Energy levels are supported

Training performance doesn’t suffer

Diets remain practical and enjoyable

Carbohydrates

Carbohydrate targets are set after protein and fat needs are met. This allows flexibility while ensuring:

Energy levels are supported

Training performance doesn’t suffer

Diets remain practical and enjoyable

Adaptive Technology

Why Fettle recalculates every week

Most nutrition apps stop here.
Fettle doesn't.
Every week, Fettle reviews your progress and adapts your targets accordingly.

Why weekly updates matter

Weekly recalculation keeps nutrition targets aligned with real physiological change — not daily fluctuations.

By adjusting at a consistent interval, Fettle responds to meaningful trends while avoiding overcorrection.

What Fettle looks at.

Weekly updates consider trends such as:

Body weight change over time

Body measurements (where available)

Activity patterns

Goal adherence

Adjustments are made cautiously and incrementally to support steady progress.

Why weekly updates matter

Weekly recalculation keeps nutrition targets aligned with real physiological change — not daily fluctuations.

By adjusting at a consistent interval, Fettle responds to meaningful trends while avoiding overcorrection.

Fettle's foundation

Evidence-led, not trend driven

Fettle Is Built Using:

Clinically validated nutrition formulas

Evidence based macronutrient guidelines

Established principles used by healthcare & sports nutrition professionals

We Deliberately Avoid

Fad diets

Extreme restrictions

One-size-fits-all-rules

How Fettle uses BMI

BMI: Useful Context, Not the Full Picture

BMI is often misunderstood.

Fettle uses BMI as a screening and context tool, not a diagnosis.

BMI can:

Distinguish between fat and muscle trends at a population level

Account for fat distribution

Provide an initial reference point

But BMI alone cannot reflect individual body composition — which is why Fettle never relies on BMI in isolation.

Fettle's Purpose

What this means for you

To improve accuracy and understand broader nutrition patterns, Fettle may analyse insights processed in an anonymised, aggregated form.

Fettle handles the complexity

So you can focus on eating well and staying consistent - without overthinking it.

Eating better

Clear guidance that fits your goals, preferences, and real-world habits.

Staying consistent

No daily perfection required - Fettle looks at trends, not single days

Making progress without stress

Progress is measured over time, with adjustments made carefully and gradually.

Start eating Smarter Today

Fettle plans your nutrition before problems appear. We calculate how much to eat, show you what fits your targets, and turn meals into a ready-to-shop plan - updating everything weekly based on real progress, not guesswork

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