The science

An engine that reasons about your body — out loud.

Fettle's Adaptive Engine reads your weekly trend, chooses an explicit state, and shows you its reasoning. No black box. No silent stalls.

The science

An engine that reasons about your body — out loud.

Fettle's Adaptive Engine reads your weekly trend, chooses an explicit state, and shows you its reasoning. No black box. No silent stalls.

The science

An engine that reasons about your body — out loud.

Fettle's Adaptive Engine reads your weekly trend, chooses an explicit state, and shows you its reasoning. No black box. No silent stalls.

Principles

Three rules the Engine never breaks.

Trend over noise

Daily scale weight is mostly water, glycogen and timing. The Engine works on your smoothed weekly trend, so one salty dinner never rewrites your plan.

Trend over noise

Daily scale weight is mostly water, glycogen and timing. The Engine works on your smoothed weekly trend, so one salty dinner never rewrites your plan.

Decisions, not just numbers

Every week ends in an explicit state — hold, adjust, protect, or protocol. A number tells you what; a state tells you what happens next.

Decisions, not just numbers

Every week ends in an explicit state — hold, adjust, protect, or protocol. A number tells you what; a state tells you what happens next.

Show the working

Every change comes with its reasoning on screen. If the Engine holds your targets, you'll know why. If it moves them, you'll know by how much and what it saw.

Show the working

Every change comes with its reasoning on screen. If the Engine holds your targets, you'll know why. If it moves them, you'll know by how much and what it saw.

The trend line the Engine actually reads

Weekly metabolic analysis

Your check-in is a data point. Your trend is the truth.

Each check-in feeds a weekly metabolic analysis: rate of change against target, adherence context, and confidence in the data itself. Only when the trend is readable does the Engine act — and when the data is too messy to trust, it says so and protects you instead of guessing.

The trend line the Engine actually reads

Weekly metabolic analysis

Your check-in is a data point. Your trend is the truth.

The trend line the Engine actually reads

Weekly metabolic analysis

Your check-in is a data point. Your trend is the truth.

The Adaptive Engine

Every week ends in one of these.

These aren’t marketing names — they’re the actual decision states in the app.

Engine
HOLDING
Trend aligned.
Targets maintained.
ON TRACK
Engine
ADJUSTING
Trend usable.
Targets adjusted carefully.
SMALL UPDATE
Engine
STALLED
Plateau confirmed.
Plan escalated.
BREAKTHROUGH
Engine
DEFENSIVE
Cycle protected.
Targets held steady.
PROTECTIVE HOLD
Engine
DEFENSIVE
Diet break started. Targets eased for recovery.
RECOVERY PHASE
Engine
DEPLETING
Mini-cut started.
Short deficit active.
CUT PHASE

Engine
HOLDING
Trend aligned.
Targets maintained.
ON TRACK
Engine
ADJUSTING
Trend usable.
Targets adjusted carefully.
SMALL UPDATE
Engine
STALLED
Plateau confirmed.
Plan escalated.
BREAKTHROUGH
Engine
DEFENSIVE
Cycle protected.
Targets held steady.
PROTECTIVE HOLD
Engine
DEFENSIVE
Diet break started. Targets eased for recovery.
RECOVERY PHASE
Engine
DEPLETING
Mini-cut started.
Short deficit active.
CUT PHASE

Holds & adjustments

On-target weeks hold. Off-target trends get measured, deliberate calorie adjustments — small when the drift is small.

Protocols

Structured multi-week strategies: diet breaks to restore adherence and hormones, mini-cuts, and reverse phases into maintenance or gain.

Data-integrity locks

Insufficient data, statistical lockout, quarantine, behavioural freeze — when the Engine can’t trust the data, it refuses to guess and tells you what it needs.

Protective states

Period Sentinel and Step Sentinel hold your targets when hormonal or activity signals would corrupt the reading. Protection, not punishment.

The decision screen — reasoning included

Transparency

You see the decision, not just the outcome.

The Engine’s decision logic is written by a registered dietitian and reviewed by our pharmacist founder — clinical judgement, encoded, not a black-box model. Every weekly analysis ends in a decision screen: what the Engine saw in your trend, which state it chose, and what changes as a result. This is the difference between being coached and being told a number — you learn how your own body responds while the Engine does the maths.

The decision screen — reasoning included

Transparency

You see the decision, not just the outcome.

The decision screen — reasoning included

Transparency

You see the decision, not just the outcome.

A different approach

Not expenditure-guessing. State-based decisions.

Apps like MacroFactor estimate your energy expenditure by comparing logged calories against weight change — which means the estimate is only as good as your logging. Fettle takes a different route: the Engine works from your weekly trend and adherence context, places you in an explicit state, and applies evidence-based adjustment rules for that state.

The practical difference: Fettle stays decisive even when logging is imperfect, protects you when the data can’t be trusted, and always shows its reasoning — then finishes the job by turning the updated plan into your shopping basket.

fettle

Built in the UK. Meal plan in. Groceries out.

Always in fettle.

Terms

Privacy

Refunds

Cookies Settings

fettle

Built in the UK. Meal plan in. Groceries out.

Always in fettle.

Terms

Privacy

Refunds

Cookies Settings

fettle

Built in the UK. Meal plan in. Groceries out.

Always in fettle.

Terms

Privacy

Refunds

Cookies Settings