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Intermittent Fasting: Does It Work and How to Start

27 September 20246 min read By Dushyanta Dushyanta

Intermittent fasting tells you when to eat, not what. The most common version is the 16:8 method: a 16-hour overnight fast followed by all daily food inside an 8-hour window. The evidence is more boring than the marketing. When calories are matched, fasting produces similar weight loss to standard calorie restriction. For some adults it makes calorie control easier because eating windows reduce snacking and simplify meal planning.

A 2024 meta-analysis of 26 time-restricted eating trials (Panagiotou et al., Nutrients) reported average losses of 1.6 kg body weight, 2 cm waist circumference, and modest improvements in insulin, total cholesterol, and LDL. A 2019 NEJM review (de Cabo and Mattson) summarised the underlying physiology. Both papers support a measured, evidence-based view rather than the “miracle metabolic switch” framing of popular media.

-1.6 kg
Average weight loss in TRE trials
-2 cm
Waist circumference loss
26
Trials in the 2024 meta-analysis
~12 hr
Fasting hours before fat oxidation rises

Sources: Panagiotou et al. Nutrients 2024 (PMID 39519533). de Cabo and Mattson. NEJM 2019 (PMID 31881139).

What actually happens during the fast

Most adults are in a fed state for 16 to 18 hours of every 24, even when they sleep, because the last meal sits in digestion for several hours. The de Cabo NEJM 2019 review traced the physiology of extending the fasted window beyond the standard pattern.

Hours 0 to 4 after the last meal. Insulin rises with the meal then falls. Glucose stored as liver glycogen begins to be released to keep blood sugar steady.

Hours 4 to 12. Liver glycogen continues to release glucose. Insulin sits at a low steady level. Fat oxidation rises gradually.

Hours 12 to 18. Liver glycogen approaches depletion. The body shifts toward fat oxidation as the dominant fuel. Ketone production begins to rise in the liver, particularly beta-hydroxybutyrate.

Hours 18 to 24+. Ketone production rises further. Autophagy markers increase (the cellular recycling process). Growth hormone pulses rise. These changes are the basis for the “metabolic switch” that fasting advocates discuss. For most adults practising 16:8, only the early part of this window is reached daily.

What the trial evidence supports

  • Weight loss. Equivalent to calorie restriction at 6 to 12 months when calories are matched. The Panagiotou meta-analysis found 1.6 kg average loss, with greater drops in adults with overweight or obesity at baseline.
  • Insulin sensitivity. Improvements seen across short and medium-term trials, particularly in adults with prediabetes or metabolic syndrome.
  • Cardiometabolic markers. LDL, total cholesterol, and waist circumference drop modestly. Effects are larger when the eating window is earlier in the day (early TRE) rather than late evening.
  • Hunger control. Many adults report less appetite chaos and easier portion control once the window is established. Not universal; some adults find restricted windows trigger evening binges.

Who tends to benefit

  • Adults who prefer larger, less frequent meals.
  • Adults who do not enjoy breakfast.
  • Adults with insulin resistance, prediabetes, or metabolic syndrome (improvements consistent across trials).
  • Adults who want a simple rule (a time window) rather than calorie counting.
  • Adults who graze excessively in the evening and need a structural reason to stop.

Who should not try it without medical input

  • Anyone with a history of disordered eating. Restricting time windows can trigger relapse.
  • Pregnant or breastfeeding women.
  • Adults with type 1 diabetes (insulin coordination required).
  • Adults with type 2 diabetes managed with insulin or sulfonylureas (hypoglycaemia risk without prescriber adjustment).
  • Anyone under 18.
  • Adults with significant medical conditions or recent surgery should consult a GP before starting.

A 4-week starter plan

Week 1
12-hour fast (12:12)
Stop eating at 8pm, eat again at 8am. Easy entry. Water, plain tea, or black coffee during the fast.
Week 2
14-hour fast (14:10)
Stop at 8pm, eat at 10am. First few mornings may feel hungry. Resolves within four days.
Week 3 and 4
16-hour fast (16:8)
Eat between 12pm and 8pm. Two larger meals or three normal meals fit. Hit your protein target across both.
Maintenance
Earlier window is better
Where life allows, push the window earlier (10am to 6pm). The TRE data shows larger metabolic improvements when food sits earlier in the day.

Common mistakes

  • Eating low-quality food inside the window. Fasting is not a permission slip for processed food. Calorie quality still matters.
  • Skipping protein. Aim for 1.6 g per kilogram of bodyweight spread across the window. Compressed windows make this harder, so plan ahead.
  • Starting at 18:6 or 20:4 on day one. Build up across two to four weeks. Most adults who fail at fasting failed because they jumped too far too fast.
  • Under-hydrating. Water, plain tea, and black coffee through the fast. Lack of fluid amplifies hunger and headaches.
  • Late-window training. If you train hard, eat at least one meal containing 30 g of protein in the two hours after the session.

Frequently asked questions

Does black coffee break the fast?
No, in the practical sense. Black coffee and plain tea carry near-zero calories and do not raise insulin meaningfully. Add milk, sugar, syrups, or sweeteners and you are out of the fasted state. Most adults find black coffee actually reduces hunger during the morning.
Will I lose muscle if I fast?
Not at 16:8 with adequate protein and resistance training. Studies of 16:8 plus weight training in trained men found muscle preserved with fat loss when protein hit 1.6 g/kg and calories were not severely restricted.
Is fasting safe for women?
Most evidence in adult women supports 12 to 14 hour windows without issue. Tighter windows (16+ hours) can affect menstrual regularity in some women, particularly those with low body fat or high training loads. If your cycle changes, ease back to a 12 to 14 hour window.
When should I train?
Either fasted or fed works for most adults. Heavy strength sessions tend to feel better fed. Easy aerobic sessions tend to work fine fasted. Listen to performance and energy; the data does not strongly favour one over the other for general health and body composition.
Do I need to do this every day?
No. Many adults run 16:8 on weekdays and a more relaxed 12:12 on weekends, which still produces 5 days of metabolic benefit without socially awkward late dinners. Consistency over 3 to 5 days of the week outperforms 7 days of struggle.

Bottom line

Intermittent fasting is a tool, not a miracle. It works for adults who find calorie control easier with a time rule than with portion control. The metabolic gains are modest and consistent. The risks are manageable if you ease into it and protect protein and training. If it does not feel sustainable after 4 to 6 weeks, the wider point still holds: an earlier and shorter eating window beats grazing from 7am to 11pm for most adults' health.

Work with DT Fitness London

For a nutrition plan that fits your training and your real schedule, book a consultation at www.dushyantatomar.com.

Dushyanta Tomar, MSc Applied Sports and Exercise Physiology, CIMSPA Accredited Personal Trainer.

Sources

  1. Panagiotou K, Stefanou G, Kourlaba G, et al. The Effect of Time-Restricted Eating on Cardiometabolic Risk Factors: A Systematic Review and Meta-Analysis. Nutrients. 2024, vol 16, issue 21, page 3700. PMID 39519533. pubmed.ncbi.nlm.nih.gov/39519533
  2. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019, vol 381, issue 26, pages 2541 to 2551. PMID 31881139. pubmed.ncbi.nlm.nih.gov/31881139
  3. NHS. Healthy weight: eating habits. nhs.uk
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