The Science of Intermittent Fasting: Benefits, Risks & What Blood Tests Reveal

Intermittent fasting (IF) has moved from fringe biohacking to mainstream health practice, and for good reason. A growing body of research demonstrates measurable benefits for metabolic health, inflammation, and cellular repair. But it's not a one-size-fits-all intervention, and blood tests are the best way to track whether it's working for you.

What Happens When You Fast?

When you stop eating, your body transitions through distinct metabolic phases. Understanding these phases helps explain why different fasting protocols produce different results.

The Fed-to-Fasted Transition (0–12 Hours)

In the first 4–6 hours after your last meal, your body is still processing food. Insulin is elevated, glucose is being used for energy, and fat storage is favoured. Between 6–12 hours, insulin begins to drop, glycogen (stored glucose in the liver) is gradually depleted, and the body starts shifting toward fat oxidation.

This is why a standard 12-hour overnight fast isn't particularly remarkable — you're really just completing the digestion and glycogen depletion phase. The metabolic magic begins after 12 hours.

Ketosis and Fat Oxidation (12–18 Hours)

Once glycogen stores are meaningfully depleted, the liver begins converting fatty acids into ketone bodies — an alternative fuel source for the brain and muscles. Ketones are not just an energy substrate; they also function as signalling molecules that reduce inflammation and oxidative stress. Blood ketone levels typically reach 0.5–1.0 mmol/L by 16–18 hours of fasting.

Autophagy: Cellular Cleanup (18–24+ Hours)

Autophagy — literally "self-eating" — is the process by which cells break down and recycle damaged proteins and organelles. It's essentially your body's quality control system, removing dysfunctional cellular components before they accumulate and cause disease. The 2016 Nobel Prize in Physiology or Medicine was awarded to Yoshinori Ohsumi for his work elucidating the mechanisms of autophagy.

Autophagy is upregulated during fasting, with significant activity beginning around 18–24 hours. However, the exact timing varies between individuals and is difficult to measure directly in humans.

What Blood Tests Reveal About Fasting Benefits

The effects of intermittent fasting are measurable through standard blood markers:

• Fasting insulin — Typically decreases by 20–31% with regular IF, indicating improved insulin sensitivity
• Fasting glucose — Modest reductions of 3–6% are common with time-restricted eating
• HbA1c — May decrease over 3–6 months of consistent practice
• hs-CRP — Reductions of 20–40% have been observed in clinical trials
• Triglycerides — Often decrease by 10–30%, reflecting improved fat metabolism
• LDL particle number — Some studies show reductions, particularly in pattern B (small, dense) particles

Common Fasting Protocols

• 16:8 — 16 hours fasting, 8-hour eating window. The most popular and sustainable protocol. Typical schedule: eat between 12pm and 8pm.
• 18:6 — Slightly more aggressive, with greater insulin reduction and earlier ketosis onset.
• 5:2 — Eat normally five days per week, restrict to 500–600 calories on two non-consecutive days.
• OMAD (One Meal a Day) — A 23:1 protocol. Effective but harder to maintain adequate nutrition.
• Extended fasts (24–72 hours) — Maximise autophagy but carry greater risk and should be medically supervised.

Who Should Be Cautious

Intermittent fasting is not appropriate for everyone. The following groups should consult a healthcare professional before starting:

• Pregnant or breastfeeding women
• People with a history of eating disorders
• Those taking diabetes medications (especially insulin or sulfonylureas)
• Individuals with adrenal insufficiency or very low cortisol
• Anyone underweight or with low muscle mass

Tracking Your Progress

If you're practicing intermittent fasting, blood tests are the most objective way to measure results. Test at baseline (before starting), then repeat at 8 and 12 weeks. Key markers to track: fasting insulin, fasting glucose, HbA1c, lipid panel (including triglycerides), and hs-CRP. This gives you concrete data rather than relying on how you feel — though that matters too.

References

1. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. doi:10.1056/NEJMra1905136
2. Ohsumi Y. Historical landmarks of autophagy research. Cell Res. 2014;24(1):9-23. doi:10.1038/cr.2013.169
3. Harvie MN, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers. Int J Obes. 2011;35(5):714-727. doi:10.1038/ijo.2010.171
4. Sutton EF, et al. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress. Cell Metab. 2018;27(6):1212-1221. doi:10.1016/j.cmet.2018.04.010