The apparel industry didn’t suddenly become unstable because of GLP-1 drugs.

What these medications have done is accelerate change, exposing planning assumptions that were already under strain. Fit models built on slow-moving size curves. Forecasts that assume bodies change gradually. Inventory strategies designed for predictability rather than volatility.

GLP-1 didn’t introduce new problems, it removed the buffer that once hid them. But it has compressed timelines, intensified variability, and made existing weaknesses harder to ignore.

For merchandise planners, product leaders, and forecasting teams, GLP-1 matters less as a trend and more as a signal that highlights how apparel planning must evolve in 2026 and beyond.

1. GLP-1 is accelerating body change, not redefining it

GLP-1 medications such as Ozempic, Wegovy, and Mounjaro are contributing to faster, more frequent changes in consumer body size. For some consumers, this means moving through multiple sizes in a single year, driving short-term wardrobe replacement rather than stable demand growth.

This acceleration has commercial consequences. Smaller sizes are selling through faster in certain categories, while demand softens elsewhere. Analysts estimate that if planning models are not updated, retailers could face billions in misaligned inventory over the next few years.

Why this matters

Bodies are shifting faster than traditional planning cycles were designed to accommodate, leaving systems built for gradual, predictable evolution out of sync with reality. The risk is not missing demand, but misreading temporary replacement spend as long-term growth.

2. Size curves were already fragile, but GLP-1 made that more visible.

Even before GLP-1 entered the conversation, size planning accuracy across the industry was struggling. In many organisations, fewer than half of planned size distributions align with actual sales.

GLP-1 hasn’t broken size curves, but it is stress-testing them.

As demand shifts mid-season, planners are being forced to revisit size allocations that were previously locked months in advance. Some brands are adjusting curves more frequently, while others are discovering that their systems simply don’t support rapid recalibration.

Why this matters

Static size curves no longer work - planning teams need tools and processes that allow for ongoing correction, not annual course-setting. Without that flexibility, volatility translates directly into markdown risk.

3. Fit has become both a design concern and a planning risk.

Fit has always been a return driver, what’s changed is its strategic importance.

As bodies change more quickly, outdated grading and size assumptions increase fit mismatches, particularly online. Poor fit doesn’t just lead to returns, it creates dead stock and waste and garments that don’t fit well often have no viable resale path.

This is where GLP-1 acts as a multiplier - faster body change amplifies existing fit inconsistencies and exposes gaps between planning, product, and customer feedback loops.

Why this matters

Fit can no longer sit downstream of planning. Brands that fail to integrate fit intelligence, including returns data, customer feedback, and size performance, directly into forecasting and assortment decisions will see margin erosion and rising waste.

4. Demand may rise but confidence in forecasts will fall.

GLP-1 users often increase apparel spend during periods of rapid body change, replacing clothing out of necessity and, for some, rediscovering confidence in what they wear. These demand spikes can be meaningful, but also misleading.

For planners, the challenge is distinguishing short-term replacement and confidence-led spend from structural growth. Without that distinction, teams risk overcommitting inventory based on signals that fade as bodies and behaviours stabilise.

Beyond volume, shifting body confidence is influencing how consumers shop. Some are gravitating towards more form-fitting silhouettes whilst others double down on comfort and function. This adds complexity not just to size planning, but to silhouette, fabric, and category mix.

Why this matters

Planning in 2026 rewards resilience more than precision. You win by adapting faster, not by guessing better. Scenario planning, shorter commitment windows, and faster read-and-react cycles are becoming even more essential, regardless of GLP-1 adoption rates.

5. Inclusivity still matters as distributions shift.

While attention has focused on smaller sizes selling faster, a significant proportion of consumers still fall into extended size ranges. Height, body proportions, and fit needs do not disappear with weight loss.

The real risk is overcorrecting - planning for today’s demand without losing tomorrow’s customer. Reducing extended size offerings too aggressively may temporarily ease inventory pressure but undermines customer trust and long-term loyalty.

The most effective brands refine size curves using data while maintaining inclusivity, supported by demand sensing and flexible allocation to serve multiple body realities simultaneously.

Why this matters

GLP-1 does not negate the need for inclusive sizing. Precision and inclusivity are not opposites, they are planning competencies.

Signals, Not Symptoms

GLP-1 did not create volatility, but it has further stripped away the illusion of stability.

The assumptions that once underpinned apparel planning, namely slow body change, predictable size curves, and fixed commitment windows, were already under pressure. GLP-1 simply accelerated the moment when those assumptions stopped holding.

These issues sit at the heart of modern merchandise planning and extend far beyond health trends. Fit, size, and forecasting are no longer fixed inputs, but moving variables that require ongoing adjustment.

The GLP-1 effect is not a trend to react to, but a signal to adapt. Brands that treat this moment as a wake-up call, investing in agile planning, fit-driven insight, and flexible size strategies, will be better positioned for whatever comes next. Those that do not will continue planning for a stability that no longer exists.

Many of the planning challenges explored here are being unpacked in more depth across upcoming PI discussions, including April’s 'Sizing & Replenishment That Actually Works' Spotlight and May’s RE:PLAN event in New York, where planners focus on practical execution rather than theory.

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