Intermittent fasting limits when food is eaten rather than what is eaten. The strongest evidence shows reliable weight loss and better blood-sugar control, plus liver and blood-fat benefits in those starting with metabolic problems. These effects look roughly equal to simply eating less. Longevity claims rest mainly on animal research. It is simple and free. (Full Review)

Protocol

Standard Approach

Time-restricted eating, 8–10 h window

e.g. 16:8 (16 h fasting, 8 h eating); no calorie counting required

Best Time of Day

Earlier eating window

finish dinner late afternoon/early evening; late-night eating discouraged

Alternate Approach

Alternate-day fasting

~25% of needs (~500 kcal) on alternating days; ranks highest for outcomes but harder to sustain

Time to effect

Weight & Blood Sugar

Within a few weeks

improvements typically begin within a few weeks

Full Effect

8–12 weeks

benefits accumulate over this period

Adaptation

1–2 weeks

early adaptation symptoms usually settle

Benefits

High: Weight and fat loss; improved blood sugar control

Medium: Improved liver health in fatty liver disease; improved blood lipids

Low: Reduced blood pressure and inflammation; preserved or improved body composition with adequate protein

Speculative: Longevity and cellular repair; cognitive and brain health

Contraindications

Pregnancy or breastfeeding

Underweight (BMI under ~18.5)

Current or past eating disorder

Children and adolescents

Type 1 diabetes

Advanced kidney, liver, or cardiovascular disease

Key Interactions

Glucose-lowering medications (insulin, sulfonylureas such as glipizide, glyburide)

Blood pressure medications

Medications requiring food (e.g., ibuprofen, metformin)

Supplements with additive blood-sugar-lowering effects (berberine, chromium, high-dose cinnamon)

Fat-soluble supplements (vitamins A, D, E, K, fish oil)

Ketogenic diet or prolonged exercise

Risk & Side Effects

High: Hunger, irritability, and early adjustment symptoms; loss of lean mass

Medium: Constipation and digestive changes; disordered eating and overeating patterns

Low: Hormonal and menstrual disruption; low blood sugar episodes in at-risk individuals

Speculative: Possible cardiovascular signal with very short eating windows; gallstones with prolonged fasting

Monitoring

Marker	Target	Why
Fasting glucose	70–85 mg/dL	Core marker of blood-sugar control
HbA1c	< 5.4%	Tracks longer-term blood-sugar trend
Fasting insulin	2–5 µIU/mL	Detects insulin resistance early
Triglycerides	< 80 mg/dL	Responsive to fasting and carbohydrate intake
ALT	< 25 U/L (men), < 22 U/L (women)	Flags liver fat and improvement in fatty liver
Body composition (lean vs. fat mass)	Maintained or improved lean mass	Detects muscle loss, a key fasting risk

Cadence: Baseline before starting (fasting, morning), then ~3 months after starting, then every 6–12 months once stable; closer follow-up in the first weeks for those on glucose- or blood-pressure-lowering medication.

Qualitative Assessment

Energy levels and freedom from persistent fatigue
Sleep quality and timing
Cognitive clarity and mood stability
Hunger and satiety control within and outside the eating window
In women, regularity of the menstrual cycle

Intermittent Fasting for Health & Longevity

Protocol

Benefits

Risk & Side Effects

Monitoring

Qualitative Assessment