Intermittent fasting, time-restricted eating, alternate-day fasting and meal timing approaches — understand the evidence, mechanisms and protocols, all contextualised for South African lifestyles and reviewed by registered dietitians.
Evidence-based content with peer-reviewed sources cited in each section.
Click any section below to expand the full content, including protocols, evidence and cited sources.
Fasting — the voluntary abstention from food for defined periods — is one of the oldest health practices, but modern science has only recently begun to understand its mechanisms. The key concept is metabolic switching: when the body depletes glucose from a fed state, it switches to burning fatty acids and producing ketone bodies. This switch activates cellular repair pathways (autophagy), reduces oxidative stress and inflammation, and improves insulin sensitivity. The emerging field of chrononutrition examines how the timing of meals interacts with our circadian rhythm — the body’s internal clock that regulates metabolism, hormones and digestion.
Research has exploded in the past decade. Mark Mattson (former NIH chief of neuroscience) and others have shown that intermittent fasting can improve multiple health markers. However, the evidence is nuanced — fasting is not magic, it does not suit everyone, and much of the benefit may come from simply eating less. Understanding the different protocols, who benefits, and who should avoid fasting is essential.
Key Mechanisms
• Metabolic switching: glucose to ketones
• Autophagy: cellular self-cleaning and repair
• Improved insulin sensitivity
• Reduced oxidative stress and inflammation
• Circadian alignment of metabolism
• Reduced overall caloric intake (often)
Main Protocols
• Time-restricted eating (e.g. 16:8)
• Alternate-day fasting
• 5:2 (two fasting days/week)
• Eat-stop-eat (24h fasts 1-2x/week)
• Prolonged fasting (24-72h)
• Meal timing and chrononutrition
Fasting is not starvation
Fasting protocols are time-limited and followed by adequate nutrition during eating windows. The goal is metabolic benefit, not calorie malnutrition. A healthy fasting practice maintains protein adequacy, micronutrient intake and overall energy balance across the week. Those who are underweight, pregnant, have a history of eating disorders, or certain medical conditions should not fast without medical supervision.
1de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541-2551.
2Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58.
3Manoogian ENC, Chow LS, Taub PR, et al. Time-restricted eating for the prevention and management of metabolic diseases. Physiol Rev. 2022;102(2):515-545.
Intermittent fasting (IF) is an umbrella term for dietary approaches that cycle between periods of eating and fasting. Unlike traditional diets that focus on what to eat, IF focuses on when to eat. The most common forms are time-restricted eating (daily eating windows like 16:8), alternate-day fasting, and 5:2. The 2019 NEJM review by de Cabo and Mattson — one of the most cited papers in the field — synthesised the evidence, noting that most IF benefits in humans come from improved insulin sensitivity, reduced oxidative stress, and often, a natural reduction in caloric intake.
What the Evidence Shows
Weight Loss
Meta-analyses (e.g. Harris et al. 2018; Patikorn et al. 2021) show IF produces weight loss of 3-8% over 6-12 weeks — broadly comparable to continuous calorie restriction. Much of the benefit comes from reduced caloric intake during eating windows. IF is an effective alternative to daily calorie counting for some, but not inherently superior for weight loss.
Insulin Sensitivity and Metabolic Health
IF improves insulin sensitivity and reduces fasting insulin and glucose. Sutton et al. (2018) showed that early time-restricted feeding improved insulin sensitivity, beta-cell responsiveness and blood pressure in men with prediabetes — even without weight loss. This suggests timing itself has metabolic benefit beyond calorie reduction.
Cardiovascular Markers
IF studies show improvements in blood pressure, resting heart rate, LDL cholesterol, triglycerides and inflammatory markers (CRP). The Mattson/de Cabo NEJM review documented these across multiple trial types.
Brain Health and Ageing
Animal studies show IF improves brain plasticity, cognitive function and lifespan. Human evidence is emerging but less robust — the mechanisms (ketones as brain fuel, BDNF production, reduced neuroinflammation) are well-established in animals. Clinical trials in cognitive decline are ongoing.
The honest summary
Intermittent fasting is a legitimate, evidence-supported approach — but it is not magic. For weight loss, it is broadly equivalent to continuous calorie restriction. Its unique advantages appear to be in metabolic health (insulin sensitivity, blood pressure) and its practical simplicity for some people. It does not suit everyone, and long-term adherence data is still limited.
1de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541-2551.
2Harris L, Hamilton S, Azevedo LB, et al. Intermittent fasting interventions for treatment of overweight and obesity in adults. JBI Database System Rev Implement Rep. 2018;16(2):507-547.
3Patikorn C, Roubal K, Arveiler D, et al. Intermittent fasting and obesity-related health outcomes. JAMA Netw Open. 2021;4(12):e2137174.
Time-restricted eating (TRE) involves consuming all daily calories within a defined window (typically 6-12 hours) and fasting for the remainder. The most popular form is 16:8 — a 16-hour fast with an 8-hour eating window (e.g. eating from 12:00 to 20:00). TRE is the most practical IF protocol for everyday life — it does not require calorie counting, fasting days or dramatic restriction, simply aligning meals within a window. The Sutton et al. (2018) Cell Metabolism trial was pivotal — it showed that early TRE (eating window 8:00-14:00) improved insulin sensitivity, blood pressure and oxidative stress in prediabetic men without weight loss, proving that timing itself has metabolic benefit.
Common TRE Protocols
16:8 — 16h fast / 8h eat (e.g. 12:00-20:00). Most popular, most practical.
14:10 — 14h fast / 10h eat. Gentler, good starting point.
18:6 — 18h fast / 6h eat. More aggressive, for experienced fasters.
Eating window in the morning/early afternoon (e.g. 8:00-14:00 or 7:00-15:00). Aligns with the circadian rhythm — insulin sensitivity is highest in the morning. Sutton et al. (2018) and Wilkinson et al. (2020) showed superior metabolic benefits from eTRE compared to late eating. Most beneficial for metabolic health.
Late Time-Restricted Eating
Eating window in the afternoon/evening (e.g. 12:00-20:00 or 14:00-22:00). More socially practical but less metabolically optimal. Late-night eating disrupts the circadian rhythm, impairs glucose tolerance and is associated with weight gain. If using late TRE, stop eating at least 3 hours before bed.
Why early eating is metabolically better
Insulin sensitivity and glucose tolerance follow a circadian rhythm — highest in the morning, lowest at night. Melatonin (released in the evening) impairs insulin action. Eating late dinner or night-time snacks occurs when the body is least equipped to handle glucose. This is why the Sutton et al. eTRE trial showed metabolic improvements without weight loss — timing aligned with circadian biology.
1Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress. Cell Metab. 2018;28(2):292-294.
2Wilkinson MJ, Manoogian ENC, Zadourian A, et al. Ten-hour time-restricted eating reduces weight, blood pressure, and atherogenic lipids. Cell Metab. 2020;31(1):92-104.e5.
3Chaix A, Manoogian ENC, Melkani GC, Panda S. Time-restricted eating to prevent and manage chronic metabolic diseases. Annu Rev Nutr. 2022;42:225-251.
Alternate-day fasting (ADF) and the 5:2 diet involve full or near-full fasting days interspersed with normal eating days. In ADF, you alternate between feast days (ad libitum eating) and fast days (0-500 calories). In 5:2, you eat normally 5 days/week and restrict to 500-600 calories on 2 non-consecutive days. These protocols are more intensive than TRE and require more discipline, but offer an alternative for those who prefer the feast/fasting structure to daily restriction.
The Key Evidence: Trepanowski et al. (2017)
The landmark JAMA Internal Medicine RCT by Trepanowski et al. (2017) randomised 100 overweight adults to ADF, daily calorie restriction, or no intervention for 1 year. The result: ADF and daily restriction produced equivalent weight loss (~6%) and equivalent improvements in cardiovascular markers. Crucially, the ADF group had a high dropout rate (38%) compared to daily restriction (29%) — suggesting ADF is harder to sustain. The conclusion: ADF works for weight loss, but is not superior to standard calorie restriction and may be harder to adhere to long-term.
Protocols Compared
Modified Alternate-Day Fasting
Alternating feast days (eat normally) and fast days (~500 calories). Varady et al. studies showed 3-7% weight loss over 8-12 weeks. The modified version (500 calories on fast days) is more tolerable than zero-calorie ADF and produces similar results.
5:2 Diet
Eat normally 5 days/week, restrict to 500-600 calories on 2 non-consecutive days. Popularised by Dr Michael Mosley. Easier to fit into a social life than ADF. Harvie et al. (2013) showed 5:2 produced weight loss and metabolic improvements comparable to daily restriction in breast cancer risk reduction context.
Eat-Stop-Eat
One or two 24-hour fasts per week (e.g. dinner to dinner). Popularised by Brad Pilon. Simpler concept than counting calories. Evidence is more limited — mostly derived from the broader IF literature rather than specific trials.
ADF is not for everyone
Full-day fasting is more demanding than TRE. Side effects can include hunger, irritability, fatigue, headaches and difficulty concentrating on fast days. Those with diabetes (especially on insulin or sulfonylureas), pregnant/nursing women, those with eating disorder history, and underweight individuals should not undertake ADF without medical supervision. Hypoglycaemia is a real risk for diabetics fasting without medication adjustment.
1Trepanowski JF, Kroeger CM, Barnosky AR, et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection. JAMA Intern Med. 2017;177(7):930-938.
2Varady KA, Bhutani S, Church EC, Klempel MC. Short-term modified alternate-day fasting: a feasibility study. Nutr J. 2009;8:54.
3Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers. Int J Obes. 2013;37(2):324-332.
Meal timing — when during the day you eat your meals — is a distinct concept from fasting duration. The emerging field of chrononutrition examines how meal timing interacts with circadian biology. The body’s metabolic processes follow a daily rhythm governed by clock genes: insulin sensitivity peaks in the morning, thermogenesis is higher earlier in the day, and melatonin release in the evening impairs glucose handling. This means the same meal eaten at noon vs 10pm has different metabolic effects. The 2017 American Heart Association scientific statement (St-Onge et al.) was the first major review to formally recognise meal timing as a cardiovascular risk factor.
Key Meal Timing Principles
Breakfast and Cardiovascular Health
Cahill et al. (2013, Circulation) — in a cohort of 26,902 men — found that men who skipped breakfast had 27% higher CHD risk than regular breakfast eaters. A 2019 meta-analysis (Bi et al.) confirmed breakfast skipping is associated with increased CVD and all-cause mortality risk. The evidence suggests breakfast supports metabolic health, though the optimal breakfast composition and size remains debated.
Late-Night Eating
Eating late — especially large meals close to bedtime — impairs glucose tolerance, raises postprandial lipids, disrupts sleep, and is associated with weight gain and higher CVD risk. The melatonin-insulin interaction means the body handles evening food poorly. Aim to finish eating at least 3 hours before bed. If a late dinner is unavoidable, keep it lighter and lower in refined carbs.
Meal Frequency
The old advice to eat small frequent meals is not well-supported. A 2019 meta-analysis (Schoenfeld & Aragon) found no significant difference in weight loss or metabolic outcomes between high and low meal frequency. Meal frequency should suit individual preference, hunger patterns and lifestyle — 2, 3 or 4 meals can all work. The total daily intake matters more than frequency.
Circadian Alignment
The principle: eat more earlier, less later. Front-load calories toward breakfast and lunch; keep dinner earlier and lighter. This aligns with metabolic peaks and supports better glucose control, weight management and sleep. The Sutton eTRE trial (2018) is the strongest evidence for this approach.
Practical chrononutrition
You don't need to fast to benefit from meal timing. Simply: eat a substantial breakfast, a good lunch, an earlier and lighter dinner, and avoid snacking after dinner. This circadian-aligned pattern captures much of the metabolic benefit of time-restricted eating without the strict fasting windows.
1St-Onge MP, Ard J, Baskin ML, et al. Meal timing and frequency: implications for cardiovascular disease prevention. Circulation. 2017;135(25):e96-e121.
2Cahill LE, Rimm EB, Rexrode KM, et al. The association of breakfast skipping with incident coronary heart disease. Circulation. 2013;128(4):337-343.
3Bi H, Chen Y, Zhang W, et al. Breakfast skipping and risk of all-cause mortality. Clin Nutr. 2020;39(10):3040-3047.
4Schoenfeld BJ, Aragon AA, Krieger JW. Effects of meal frequency on weight loss and body composition. Nutr Rev. 2015;73(2):69-82.
Feature
Intermittent Fasting
Time-Restricted
Alt-Day/5:2
Meal Timing
Protocol
Umbrella term
Daily window (16:8)
Fast days 1-2x/wk
Eat earlier, less late
Difficulty
Varies
Moderate
High
Low
Fasting duration
Varies
12-20h daily
24h or 500 cal days
Overnight (12h)
Weight loss
3-8%
3-5%
3-7%
Variable
Metabolic benefit
Good
Good (eTRE best)
Good
Moderate-good
Adherence
Varies
Good
Lower (high dropout)
Very good
Best for
General approach
Daily practicality
Structured fasters
Circadian health
Supervision
If diabetic
Generally self
Dietitian rec.
Self-directed
eTRE = early time-restricted eating. Weight loss figures are typical ranges from published trials, not guarantees.
Fasting has deep roots in South African religious and cultural traditions. Ramadan (observed by South Africa’s substantial Muslim community) involves a month of dawn-to-sunset fasting — essentially daily time-restricted eating with a reversed (evening) eating window. Lent and other Christian fasting traditions involve various forms of dietary restriction. These traditions provide a cultural framework that makes IF concepts familiar to many South Africans. The SA lifestyle — long commutes, early work starts, and braai/dinner culture — can both support and challenge fasting protocols.
SA-Specific Considerations
Ramadan — studied as a natural TRE model; metabolic benefits documented with caveats on eating window timing
Early work starts — many South Africans start work early; eTRE may suit morning-oriented schedules
Braai culture — evening social eating can conflict with early TRE; adapt with flexible windows
Commuting — long commutes can make breakfast timing tricky; plan portable options
Ramadan as a research model
Ramadan fasting has been extensively studied as a natural experiment in time-restricted eating. Studies show metabolic improvements (insulin sensitivity, lipids) during Ramadan, though some benefits reverse after Ramadan ends. Research by SA and international groups has informed both religious fasting guidance and broader IF science.
1Lessan N, Ali T. Metabolic and cardiovascular effects of Ramadan fasting. J Med Dis. 2019;11(1):e1.
2Kul S, Savaş E, Öztürk ZA, Karadağ G. Does Ramadan fasting alter body weight and blood biochemistry? Eur Rev Med Pharmacol Sci. 2014;18(14):2121-2126.
Who Should Not Fast (Without Medical Supervision)
The following groups should not undertake fasting without medical supervision: people with diabetes on insulin or sulfonylureas (hypoglycaemia risk), pregnant or breastfeeding women, children and adolescents (growth requires adequate nutrition), those who are underweight or have a history of eating disorders, those with adrenal fatigue or dysregulation, and people on medications requiring food intake. If in doubt, consult a doctor before starting.
Hypoglycaemia Risk
For people with diabetes (especially Type 1 or insulin-treated Type 2), fasting carries a real risk of hypoglycaemia — which can be dangerous or fatal. Any diabetic considering fasting must consult their endocrinologist or diabetes educator to adjust medication doses and monitor blood glucose. Never fast with diabetes without a plan for medication adjustment.
Eating Disorder Risk
Fasting protocols can trigger, worsen or disguise disordered eating patterns — particularly in those with a history of anorexia, bulimia or orthorexia. The structure of fasting can become a justification for restriction. Those with any eating disorder history should avoid fasting and work with a mental health professional and dietitian. Fasting should never be used as a weight control method by those vulnerable to disordered eating.
Over-Fasting and Malnutrition
More fasting is not better. Extreme protocols (multi-day fasts, OMAD — one meal a day — long-term) risk protein inadequacy, muscle loss, micronutrient deficiency, hormonal disruption (particularly in women) and metabolic adaptation. The eating window must contain adequate protein, fibre, healthy fats and micronutrients. If you are losing muscle, feeling fatigued, or losing your period (women), stop fasting and reassess.
Women and Fasting
Some research suggests women may be more sensitive to fasting stress, with potential effects on reproductive hormones and menstrual cycle. Women may benefit from gentler protocols (14:10 rather than 16:8), and should be alert to cycle changes. Pregnant and breastfeeding women should not fast. The evidence here is evolving — individual response varies.
When to see a professional
If you have diabetes, cardiovascular disease, kidney disease, a history of eating disorders, are pregnant or breastfeeding, or are on regular medications — consult your doctor and a registered dietitian before starting any fasting protocol. Fasting is a metabolic intervention with real effects; it should be undertaken with appropriate guidance, not as a casual experiment.
1Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46-58.
2Manoogian ENC, Chow LS, Taub PR, et al. Time-restricted eating for the prevention and management of metabolic diseases. Physiol Rev. 2022;102(2):515-545.
3American Diabetes Association. Nutrition therapy for adults with diabetes or prediabetes. Diabetes Care. 2019;42(5):731-754.
Whether you're exploring intermittent fasting for the first time, managing a health condition, or wanting to ensure your fasting protocol is safe and nutritionally adequate — connect with HPCSA-registered professionals across South Africa who can guide you safely.
Fasting is one of the oldest health practices — and modern science is revealing why it works. The key concept is metabolic switching: when the body depletes glucose, it switches to burning fat and producing ketones, activating cellular repair pathways, improving insulin sensitivity and reducing inflammation. HealthLab’s Fasting and Eating Patterns guide brings together the evidence — from the landmark de Cabo and Mattson NEJM review to the Sutton et al. early time-restricted feeding trial and the Trepanowski et al. alternate-day fasting RCT. We cover intermittent fasting, time-restricted eating (16:8), alternate-day fasting and 5:2, and the emerging field of chrononutrition and meal timing. South Africa has a rich fasting tradition — Ramadan, Lent and cultural practices — that makes these concepts familiar. Our evidence-based content, reviewed by HPCSA-registered dietitians, provides the honest, balanced and locally relevant guidance you need to understand whether fasting is right for you, which protocol to choose, and how to do it safely — with clear guidance on who should not fast and when to seek professional advice.
