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slug: foundations
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# Foundations

What aging is, what measurement means, and which evidence rules should govern a longevity claim. This section gives the vocabulary that makes the rest of the field readable.

Start with [Healthspan vs. Lifespan](healthspan-lifespan.md), then move into the mechanism, measurement, and evidence concepts that keep protocols from becoming guesswork. [Hallmarks of Aging](aging-hallmarks.md) names the mechanism map; [Biological Age](biological-age.md), [Pace of Aging](aging-pace.md), and [Frailty Index](frailty-index.md) name measurement frames; [Inflammaging](inflammaging.md) names the integrative chronic-inflammatory state most other hallmarks converge on; [Cardiovascular-Kidney-Metabolic (CKM) Syndrome](ckm-syndrome.md) names the cardiometabolic risk cluster that later diagnostics, drugs, and lifestyle entries keep returning to; [The Longevity Pyramid](longevity-pyramid.md) gives interventions an order-of-operations frame; [Hormesis](hormesis.md) explains why dose matters; [Metabolic Flexibility](metabolic-flexibility.md) explains fuel switching across fasting, feeding, rest, and exercise; [Evidence Tiers](evidence-tiers.md) keeps claims tied to what human data can actually show. [Lifestyle Theater](lifestyle-theater.md) names the trap of performing visible longevity signals while neglecting the base behaviors that do the work.

Read straight through, or land on a specific entry and follow its outgoing links into later sections.

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- [Next: Healthspan vs. Lifespan](healthspan-lifespan.md)
- [Previous: Article Map](article-map.md)