Longevity Isn’t Just About Eating Less: What Fasting Studies in Monkeys Really Tell Us

One of the most important insights from the science of ageing might be that fasting is not inherently beneficial. It’s context-dependent and without understanding that, we risk misapplying a tool that can just as easily harm as heal.

A now well-known study comparing two rhesus monkey cohorts explored calorie restriction and longevity. Both groups had their caloric intake reduced but only one group saw extended lifespan. The difference?

 • One group was fed a standardised chow (nutritionally poor, high in sugars and processed ingredients)

 • The other was fed a natural, whole food-based diet (more in line with an ancestral template)

The group on the processed diet saw clear lifespan extension with fasting. The other group, already eating in alignment with biological norms, saw no significant benefit. 

In other words:

Fasting from poor nutrition improves outcomes.

Fasting from a nutrient-rich, metabolically appropriate diet may not.

From a functional nutrition perspective, this is critical.

The metabolic benefit of fasting often hinges on what it’s replacing. If the baseline diet is inflammatory, dysglycaemic, and lacking in key co-factors, calorie restriction is a net improvement. But when the body is well-nourished, metabolically balanced, and not in a chronic stress state, induced fasting may offer diminishing returns or even trigger adaptive stress responses that accelerate ageing.

This matters greatly in real-world application:

 • In already nutrient-depleted, inflamed, or hormonally dysregulated individuals

 • In midlife women or chronically stressed clients with poor sleep or HPA dysfunction

 • In clients with low metabolic flexibility or poor bile flow and mitochondrial output

Fasting is often touted as “anti-ageing,” but it’s the restoration of metabolic signalling and cellular repair that drives longevity and that doesn’t require extreme caloric deprivation. 

In fact, prolonged fasting under the wrong metabolic conditions can:

 • Suppress thyroid output

 • Impair detoxification and bile clearance

 • Elevate cortisol and blood glucose

 • Accelerate lean tissue loss in older individuals

Calorie restriction is not a universally beneficial longevity tool. We must assess metabolic context first: nutrient status, stress load, hormonal balance, and dietary quality.

In functional nutrition, the question is not “Should I fast?” but:

“Is my body in a position to benefit from fasting or will it interpret it as another stressor?”

When you optimise mitochondrial health, circadian rhythm, bile flow, protein intake, and micronutrient sufficiency you may not need to fast at all. Nourishment is longevity.

⸻

Lauren Dyer, BSc, MSc, IFMCA, Dip.CLN

Nutritional Therapist

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