Fasting & the Rebound Effect: Do We Eat More When We Don't Fast?

The Environmental Problem With Compensation

Imagine someone who fasts three days every week.

It sounds like they must be eating considerably less.

But what if they compensate?

Suppose fasting produces intense hunger, followed by very large meals on eating days.

The person may consume nearly as many calories across the week as before.

This is sometimes called a rebound or compensation effect.

Why It Matters Environmentally

If the environmental benefit of fasting depends partly upon consuming less food, compensation can undermine that benefit.

The relevant measurement is not:

How many days did I fast?

It is:

How much food did I consume over the whole period?

An Interesting Real-World Parallel

Research on environmental interventions repeatedly demonstrates that behavioral responses can reduce the expected benefit of an intervention.

A 2025 study of meat-free days in university cafeterias, for example, found substantial reductions in food-related emissions on intervention days but also observed some rebound in animal-based meal consumption on subsequent days.

The principle is highly relevant to fasting.

Fasting Day vs. Annual Consumption

One fasting day may have little environmental significance if it leads to a large compensatory feast.

But repeated fasting that genuinely lowers annual food consumption could have a different outcome.

What Should You Track?

If environmental impact is part of your fasting experiment, track:

• Calories on fasting days
• Calories on eating days
• Weekly average
• Food categories
• Animal products
• Food waste

This allows you to distinguish fasting frequency from actual reduction in consumption.

The Key Question

Does fasting cause me to eat less — or simply to eat differently?

The answer could determine much of its environmental significance.