Caloric restriction: revision 1 to 2

Revision 1

Revision 2

categories: ["longevity"]

tags: ["dietary restriction", "aging", "fasting", "calerie", "nutrient sensing", "clinical trials"]

summary: "Sustained reduction of energy intake without malnutrition, the oldest and most reproducible lifespan-extending intervention in animals and the least proven in humans."

summary: "Sustained reduction of energy intake without malnutrition, the oldest and most reproducible lifespan-extending intervention in animals and the least proven in humans."

updated: "2026-07-27"

updated: "2026-10-29"

humanEvidence: "CALERIE, the only randomized trial in healthy non-obese adults, ran two years at roughly half the intended restriction and moved cardiometabolic and immune markers; no human lifespan data exist, and the two rhesus studies disagree."

humanEvidence: "CALERIE, the only randomized trial in healthy non-obese adults, ran two years at roughly half the intended restriction and moved cardiometabolic and immune markers; no human lifespan data exist, and the two rhesus studies disagree."

access: "Requires no prescription and nothing to buy, since it is a change in eating; commercial fasting-mimicking formulations are sold direct to consumers."

access: "Requires no prescription and nothing to buy, since it is a change in eating; commercial fasting-mimicking formulations are sold direct to consumers."

reversibility: "reversible"

"caption": "Dietary intervention",

"rows": [

{ "label": "First reported", "value": "1935 (McCay et al., rats)" },

{ "label": "Typical protocol", "value": "20–40% below ad libitum intake" },

{ "label": "Typical protocol", "value": "20-40% below ad libitum intake" },

{ "label": "Proven in", "value": "Yeast, worms, flies, rodents" },

{ "label": "Primate evidence", "value": "Conflicting" },

{ "label": "Human trial", "value": "CALERIE, 2 years, biomarker endpoints" },

```

**Caloric restriction** is a sustained reduction in energy intake, usually 20 to 40 per cent below

what an animal would eat freely, while maintaining adequate vitamins, minerals, and protein. In

what an animal would eat freely, while maintaining adequate vitamins, minerals and protein. In

laboratory rodents it remains the most reproducible intervention in biogerontology, extending median

laboratory rodents it remains the most reproducible intervention in biogerontology: it extends

and maximum lifespan and delaying most age-related pathologies. In primates the picture is

median and maximum lifespan and delays most age-related pathologies. In primates the evidence

genuinely conflicted, and in humans the evidence stops at intermediate biomarkers measured over two

genuinely conflicts, and in humans it stops at intermediate biomarkers measured over two years.

years.

## Origins

Underfeeding and longevity were linked before anyone had a mechanism. Clive McCay's 1935 experiments

Underfeeding was linked to longevity before anyone had a mechanism. Clive McCay's 1935 experiments

at Cornell showed that rats fed a restricted but nutritionally complete diet from weaning grew more

slowly, matured later, and lived substantially longer than freely fed controls.[^mccay1935] For

slowly, matured later and lived substantially longer than freely fed controls.[^mccay1935] For

decades the effect was attributed to slowed growth, then to reduced metabolic rate, then to reduced

decades the effect was attributed first to slowed growth, then to reduced metabolic rate, then to

oxidative damage. None of those explanations survived scrutiny; restricted animals do not, in

reduced oxidative damage. None of those explanations survived scrutiny. Restricted animals do not,

general, have lower mass-specific metabolic rates, and the free-radical account has weakened alongside

in general, have lower mass-specific metabolic rates, and the free-radical account has weakened

the wider retreat of that theory in [[mitochondrial-dysfunction|mitochondrial biology]].

along with the wider retreat of that theory in [[mitochondrial-dysfunction|mitochondrial biology]].

The current framing is nutrient sensing, which appears in most catalogues of the

The current framing is nutrient sensing, which most catalogues of the [[hallmarks-of-aging]] list as

[[hallmarks-of-aging]] as a distinct axis. That framing owes much to invertebrate genetics:

a distinct axis. That framing owes much to invertebrate genetics.

[[cynthia-kenyon|Cynthia Kenyon's]] laboratory showed that partial loss of a single

insulin/IGF-1-family receptor doubles nematode lifespan, which gave nutrient sensing a demonstrated

control point rather than a correlation. Restriction reduces signalling through insulin and IGF-1,

control point instead of a correlation. Restriction reduces signalling through insulin and IGF-1,

lowers mTORC1 output, activates AMPK, and raises [[autophagy]], engaging the same targets addressed

lowers mTORC1 output, activates AMPK and raises [[autophagy]], engaging the same targets that

pharmacologically by [[rapamycin]] and [[metformin|biguanides]]. It also lowers markers of

[[rapamycin]] and [[metformin|biguanides]] address pharmacologically. It also lowers markers of

[[inflammaging]] and, in rodents, reduces the burden of [[cellular-senescence|senescent cells]] that

[[senolytics|senolytic drugs]] are designed to remove. Work in flies and mice complicates even this: restricting protein, or the

[[senolytics|senolytic drugs]] are designed to remove. Work in flies and mice complicates even this

single amino acid methionine, reproduces much of the effect without reducing calories at all, which

account. Restricting protein, or the single amino acid methionine, reproduces much of the effect

suggests the operative variable is nutrient composition rather than energy as such.[^solonbiet2014]

without reducing calories at all, which suggests that the operative variable is nutrient composition

rather than energy as such.[^solonbiet2014]

## The primate studies

Two long-running rhesus macaque experiments produced opposite headline results, and reconciling them

Two long-running rhesus macaque experiments produced opposite headline results, and the effort to

is the most instructive episode in the field.

reconcile them is the most instructive episode in the field.

```compare

{

}

```

A joint reanalysis by both teams concluded that the divergence came largely from the

A joint reanalysis by both teams concluded that the divergence came largely from the controls and

controls and the diets rather than from the restriction.[^mattison2017] The NIA control animals were

the diets rather than from the restriction.[^mattison2017] The NIA control animals were already

already portion-fed and lean, so the study compared moderate restriction with mild restriction, not

portion-fed and lean, so that study compared moderate restriction with mild restriction, not with

with overfeeding. The Wisconsin control diet was higher in sucrose. The lesson is that "caloric

overfeeding. The Wisconsin control diet was higher in sucrose. In practice, then, "caloric

restriction extends lifespan" may in practice mean "avoiding chronic overfeeding on a poor diet

restriction extends lifespan" may mean "avoiding chronic overfeeding on a poor diet extends

extends lifespan", which is a considerably weaker and more ordinary claim.

lifespan", a considerably weaker and more ordinary claim.

## Human evidence

CALERIE remains the only randomized controlled trial of sustained restriction in healthy,

CALERIE remains the only randomized controlled trial of sustained restriction in healthy, non-obese

non-obese adults. Participants were asked for 25 per cent restriction over two years and achieved

adults. Participants were asked for 25 per cent restriction over two years and on average achieved

roughly half that on average, which is itself a finding about adherence. The intervention improved

roughly half that, which is itself a finding about adherence. The intervention improved

cardiometabolic risk markers, reduced inflammatory markers, and produced changes in thymic tissue

cardiometabolic risk markers, reduced inflammatory markers and produced changes in thymic tissue

consistent with reduced immune aging.[^spadaro2022] It also reduced bone mineral density and lean

mass.

A secondary analysis applying [[epigenetic-clock|DNA-methylation clocks]] found a small slowing of

one pace-of-aging measure and no significant change in the other clocks tested.[^waziry2023] The result is

one pace-of-aging measure and no significant change in the other clocks tested.[^waziry2023] The

frequently reported as evidence that restriction slows human aging. It is better described as

result is often reported as evidence that restriction slows human aging. A more accurate description

evidence that a two-year dietary change moves one composite predictor by a small amount, with the

is that a two-year dietary change moved one composite predictor by a small amount, and the

relationship between that predictor and any clinical outcome still unestablished — the general

relationship between that predictor and any clinical outcome is still unestablished. That general

problem addressed under [[aging-biomarkers]] and [[biological-age]].

problem is addressed under [[aging-biomarkers]] and [[biological-age]].

> [!caution] What has not been shown

> No human study has tested whether caloric restriction extends lifespan, and none realistically can:

> No human study has tested whether caloric restriction extends lifespan, and none realistically

> the trial would run for decades with an intervention almost nobody sustains. Human evidence is

> can: the trial would run for decades with an intervention almost nobody sustains. Human evidence

> restricted to surrogate measures over a few years.

> is limited to surrogate measures over a few years.

## Variants

Because sustained restriction is difficult, most current interest is in schedules that reproduce parts

Sustained restriction is difficult, so most current interest is in schedules that reproduce parts of

of the signal intermittently.

the signal intermittently.

*Time-restricted eating* confines intake to a window of roughly eight to ten hours. Randomized trials

*Time-restricted eating* confines intake to a window of roughly eight to ten hours. Randomized

in humans have generally found weight loss no greater than matched calorie reduction, and at least one

trials in humans have generally found weight loss no greater than with matched calorie reduction,

found a concerning share of the loss came from lean mass.[^lowe2020]

and at least one found that a concerning share of the loss came from lean mass.[^lowe2020]

*Alternate-day and periodic fasting* produce larger metabolic swings. In the largest controlled

lifespan study in genetically diverse mice, both fasting schedules and graded restriction extended

lifespan, with the most restricted group living longest; notably, the degree of metabolic improvement

lifespan, and the most restricted group lived longest. The degree of metabolic improvement did not

did not predict which animals lived longer, while traits related to resilience and genetic background

predict which animals lived longer, whereas traits related to resilience and genetic background

did.[^difrancesco2024] That dissociation undercuts the common assumption that a favourable metabolic

panel is the mechanism of benefit.

*Fasting-mimicking diets* use several days per month of a low-calorie, low-protein formulation

intended to trigger fasting responses while permitting some food. Trials report improvements in

cardiometabolic risk factors; claims about reduced biological age rest on the same surrogate measures

cardiometabolic risk factors; claims of reduced biological age rest on the same surrogate measures

whose validity is in question.

## Tradeoffs and risks

Restricted animals are cold-intolerant, slower to heal wounds, and more vulnerable to some infections

Restricted animals are cold-intolerant, slower to heal wounds, and more vulnerable to some

and to physical stress. In humans the documented costs of sustained restriction include loss of bone

infections and to physical stress. In humans the documented costs of sustained restriction include

density and lean mass, reduced strength, cold sensitivity, persistent hunger, reduced libido, and

loss of bone density and lean mass, reduced strength, cold sensitivity, persistent hunger, reduced

menstrual disruption — with implications for [[reproductive-longevity|reproductive function]] that

libido and menstrual disruption. The implications for

are rarely discussed alongside the longevity claims. Severe restriction interacts badly with the

[[reproductive-longevity|reproductive function]] are rarely discussed alongside the longevity

sarcopenia of later life, so the population most likely to want a geroprotector is the population

claims. Severe restriction interacts badly with the sarcopenia of later life, so the population most

least suited to this one.

likely to want a geroprotector is also the population least suited to this one.

The effect is also not universal. In panels of genetically diverse mice, restriction shortens life in

The effect is also not universal. In panels of genetically diverse mice, restriction shortens life

a minority of strains. There is no reason to assume humans are uniform where mice are not, and no

in a minority of strains. There is no reason to assume humans are uniform where mice are not, and no

biomarker exists to identify who responds.

> [!debate] Does the benefit shrink with lifespan?

> Restriction adds a large proportional gain in short-lived species and a smaller, contested one in

> Restriction gives a large proportional gain in short-lived species and a smaller, contested one in

> primates. One reading is that it works by triggering a famine-response programme whose payoff scales

> primates. One reading is that it triggers a famine-response programme whose payoff scales with how

> with how quickly an organism's reproductive schedule can be deferred, in which case a species that

> quickly an organism's reproductive schedule can be deferred; if so, a species that already lives

> already lives eighty years has little left to gain. This bears directly on how much weight

> eighty years has little left to gain. The question bears directly on how much weight

> [[blue-zones|dietary explanations of exceptional longevity]] deserve.

## Outlook

The practical trajectory has moved away from restriction itself and toward compounds that engage its

The practical work has moved away from restriction itself toward compounds that engage its

downstream signalling without requiring hunger — mTOR inhibitors, AMPK activators, and the wider set

downstream signalling without hunger: mTOR inhibitors, AMPK activators and the wider set of

of nutrient-sensing drugs discussed under the [[geroscience-hypothesis]]. Separately,

nutrient-sensing drugs discussed under the [[geroscience-hypothesis]]. Separately,

[[glp-1-receptor-agonists]] have made sustained reductions in intake pharmacologically achievable,

though they act mainly on excess adiposity rather than holding a lean animal below its normal

though they act mainly on excess adiposity and do not hold a lean animal below its normal intake.

intake. Comparative work on

Comparative work on [[negligible-senescence|exceptionally long-lived species]] and on the genetics

[[negligible-senescence|exceptionally long-lived species]] and on the genetics of restriction

of restriction response may eventually explain why some individuals benefit and others do not.

response may eventually explain why some individuals benefit and others do not.

What remains unresolved is the counterfactual that matters most for people: whether restriction adds

The counterfactual that matters most for people remains unresolved: whether restriction adds

anything to [[healthspan]], in a well-nourished adult of normal weight, beyond what

anything to [[healthspan]] in a well-nourished adult of normal weight, beyond what

[[exercise-and-aging|regular exercise]] and the avoidance of overfeeding already deliver. CALERIE was

[[exercise-and-aging|regular exercise]] and the avoidance of overfeeding already deliver. CALERIE

not designed to answer that, and no trial currently planned will.

was not designed to answer that, and no trial currently planned will.

## See also