16/8 intermittent fasting restricts food intake to an 8-hour window each day, leaving 16 hours of fasted, low-insulin metabolism. It is the most studied, most sustainable, and most accessible IF protocol — backed by published trials in prediabetes (Sutton 2018), metabolic syndrome (Wilkinson 2020), and obesity (Cienfuegos 2020). For diabetes and prediabetes, 16:8 produces meaningful improvements in fasting glucose, insulin sensitivity, weight, blood pressure, and cardiometabolic markers when sustained over weeks to months. This guide covers the evidence, how to start, sample windows, what to eat and avoid, medication coordination, and how to track progress.
What “16/8” Means
The numbers refer to hours: 16 hours fasting + 8 hours eating = a 24-hour daily cycle. Within the 8-hour eating window, you eat normally (2 to 3 meals plus optional snacks). During the 16-hour fast, only water, black coffee, plain tea, and other zero-calorie liquids are allowed. The pattern repeats daily.
- Most common window: 12 PM to 8 PM. Skip breakfast, eat lunch and dinner. Socially easy.
- Early window: 8 AM to 4 PM. Eat breakfast, eat early dinner/late lunch. More metabolically optimal but socially harder.
- Compromise: 10 AM to 6 PM. Late breakfast, early dinner. Good balance.
- Late window: 1 PM to 9 PM. Skip breakfast, late dinner. Less metabolically ideal.
The Evidence Base for 16:8 in Diabetes
- Sutton et al. 2018 (Cell Metabolism): 8 prediabetic men on early 16:8 (eating 8 AM to 2 PM) showed improved insulin sensitivity, beta-cell function, blood pressure, and oxidative stress — without weight loss. Same calories, just earlier eating window.
- Wilkinson et al. 2020 (Cell Metabolism): 19 patients with metabolic syndrome on 10-hour eating window (similar to 16:8) for 12 weeks. Weight dropped 3 percent, blood pressure improved, atherogenic lipids decreased, A1C dropped 0.22 percentage points.
- Cienfuegos et al. 2020 (Cell Metabolism): Adults with obesity on 16:8 (and shorter windows) showed weight loss, improved insulin sensitivity, and improved cardiometabolic markers over 8 weeks.
- Gabel et al. 2018: 23 obese adults on 16:8 for 12 weeks lost 3 percent of body weight, reduced systolic blood pressure 7 mmHg, all without explicit calorie counting.
- Multiple meta-analyses: Time-restricted eating produces modest but consistent improvements in glucose, insulin, and weight across heterogeneous populations.
For the published mechanistic context, see Mattson et al.’s Effects of Intermittent Fasting on Health, Aging, and Disease in the New England Journal of Medicine.
How to Start: 4-Week Ramp
| Week | Schedule | Example | Focus |
|---|---|---|---|
| Week 1 | 12:12 | Eat 7 AM – 7 PM | Establish an overnight fast; no late-night snacking |
| Week 2 | 14:10 | Eat 9 AM – 7 PM | Delay breakfast by 2 hours |
| Week 3 | 16:8 | Eat 11 AM – 7 PM | Push breakfast to late morning |
| Week 4+ | 16:8 sustained | Eat 12 PM – 8 PM (or your chosen window) | Settle into long-term schedule |
Three Sample 16:8 Days
| Time | Standard 12-8 PM | Early 8 AM – 4 PM | Compromise 10 AM – 6 PM |
|---|---|---|---|
| Morning | Black coffee, water | Breakfast: 3-egg omelet, avocado | Black coffee, water |
| Mid-morning | Water | Water | Breakfast: Greek yogurt with chia and berries |
| Noon-1 PM | Lunch: Big salad with grilled chicken | Lunch: Salmon with vegetables | Lunch: Salad with protein |
| 3-4 PM | Snack: nuts or fruit | Last meal: leftover protein + greens | Snack: cheese or nuts |
| 5-6 PM | — | Black coffee or tea | Dinner: protein + vegetables |
| 7-8 PM | Dinner: salmon, vegetables, side salad | Water or tea | Water or tea |
| After window | Water, tea | Water, tea | Water, tea |
What to Eat in Your 8-Hour Window
The window matters as much as the fast. A 16-hour fast followed by 8 hours of refined-carb eating produces only modest benefit. Build the window around protein, fiber, healthy fats, and minimal refined carbs:
- Breaking the fast (first meal): Protein-led, with fat and fiber. Examples — eggs with avocado, Greek yogurt with chia and berries, salmon with greens. Avoid breaking the fast with cereal, pastries, or juice — these spike glucose hard after the fasted state.
- Mid-window meal: Salads with grilled protein, fish or chicken with vegetables, low-carb bowls.
- Closing meal: Protein + non-starchy vegetables + healthy fat. Lighter than your largest meal of the day if possible.
- Snacks: Nuts, hard-boiled eggs, cheese, olives, full-fat yogurt, berries. Avoid chips, crackers, pretzels, cookies.
- Hydration: Water with each meal. Aim for 2 to 3 liters daily total.
What’s Allowed During the 16-Hour Fast
| Beverage / Item | Allowed? | Notes |
|---|---|---|
| Water | Yes | Drink freely |
| Sparkling water (unsweetened) | Yes | Plain or with lemon |
| Black coffee | Yes | May enhance fat oxidation |
| Plain tea (green, black, herbal) | Yes | Unsweetened |
| Apple cider vinegar (1 tbsp in water) | Yes | May help glucose response on fast break |
| Salt and electrolyte powder (no sugar) | Yes | Helpful for headaches and energy |
| Coffee with 1 tbsp cream | Technically breaks strict fast | OK for most TRE purposes |
| Coffee with sugar or sweetened creamer | No | Triggers insulin response |
| Diet soda / artificial sweeteners | Discouraged | May trigger insulin response in some |
| Bone broth | Breaks fast | OK if symptomatic |
| Bulletproof coffee (butter + MCT) | Breaks fast | Preserves ketosis benefits |
| Gum (sugar-free) | Mild interaction | Generally OK in moderation |
Medication Considerations for 16:8
- Metformin: Generally safe. Take with the first meal in your eating window to reduce GI side effects. Move evening dose to last meal of window.
- Sulfonylureas: High hypoglycemia risk during the 16-hour fast. Typically requires dose reduction or shift to once daily with first meal of window. Discuss with your doctor before starting.
- Insulin (basal): May need dose reduction as fasting glucose improves. Endocrinologist input required.
- Insulin (mealtime): Move to window meals only. Lower doses likely needed within 1 to 2 weeks.
- SGLT2 inhibitors: Generally OK with 16:8 (the 16-hour fast is short enough to keep DKA risk low). Avoid combining with longer fasts.
- GLP-1 agonists (semaglutide, liraglutide): Compatible and synergistic.
- Blood pressure medications: Diuretics may need reduction within 2 to 4 weeks as BP often drops on IF.
What to Expect Week by Week
| Time Point | Common Experiences |
|---|---|
| Days 1-3 | Hunger spikes mid-fast; possible headache, irritability; salt water helps |
| Week 1 | Hunger pattern adjusting; ghrelin shifting to eating-window times |
| Week 2 | Hunger flattening; energy stabilizing; some weight loss (mostly water) |
| Week 3-4 | Fasting glucose typically drops 10 to 25 mg/dL; clearer mental state |
| Month 2 | Weight loss 4 to 8 pounds total; clothes fitting differently; blood pressure may drop |
| Month 3 | A1C drop 0.3 to 0.6 points at retest; insulin sensitivity markers improved |
| Month 6 | Sustained metabolic gains; weight loss often 10 to 20 pounds; medication reductions possible |
Common Pitfalls
- Compensatory overeating: Eating substantially more in the 8-hour window cancels the calorie benefit. Track intake for the first 2 weeks.
- Refined-carb refeed: Breaking the fast with bagels, cereal, or pastries spikes glucose hard.
- Late-night eating: Pushing the window to 2 PM to 10 PM cancels much of the circadian benefit.
- Inadequate sleep: Less than 7 hours of sleep blunts IF benefits significantly.
- Caffeine overuse: More than 3 to 4 coffees during the fast raises cortisol and disrupts sleep.
- Electrolyte neglect: Salt and magnesium prevent most adaptation symptoms.
- Not adjusting medication: The most common cause of hypoglycemia. Coordinate with your doctor.
Combining 16:8 With Other Strategies
- 16:8 + Mediterranean diet: Excellent sustainability and evidence base.
- 16:8 + low-carb (50 to 100 g/day): Stronger glucose control without strict keto.
- 16:8 + keto: The most aggressive insulin-lowering combination — see our IF + keto guide.
- 16:8 + resistance training: Preserves muscle and amplifies metabolic gains.
- 16:8 + post-meal walks: 15- to 20-minute walks after the first meal of the day significantly blunt glucose response.
Who Should Not Do 16:8
- Pregnancy or breastfeeding.
- History of anorexia, bulimia, or binge eating disorder.
- Underweight (BMI under 18.5).
- Type 1 diabetes without endocrinologist supervision.
- Children and adolescents.
- Anyone with frequent or severe hypoglycemia.
- Active cancer treatment (without oncologist input).
- Adrenal insufficiency.
Special Situations
- Travel: Shift the window to local time; do not maintain home schedule.
- Holidays: Allow looser timing for one day; return to schedule next day.
- Sick days: Pause IF — recovery needs consistent nutrition.
- Heavy training: Move window to include post-workout meal within 1 hour.
- Menstrual cycle: Some women feel hungrier in the luteal phase; relax to 14:10 if needed.
- Night shift work: Align the 16:8 window to your active hours rather than clock hours.
Tracking Progress
- Weekly fasting glucose for the first 4 weeks.
- Weight twice weekly (morning).
- Blood pressure weekly.
- A1C at month 3 and 6.
- Fasting insulin and HOMA-IR (often improve before A1C does).
- Energy, sleep, mood, hunger pattern (subjective, but important).
Related Reading
For complementary detail, see our guides on designing an intermittent fasting plan, the best intermittent fasting schedule, intermittent fasting times, and combining IF with keto. For broader context, see our diet and nutrition hub, A1C levels, and whether prediabetes is reversible.
The Bottom Line
16/8 intermittent fasting is the most studied, most sustainable, and most accessible IF protocol for diabetes and prediabetes. Eat within an 8-hour window each day; fast for the other 16 hours with water, black coffee, and tea. Published evidence supports A1C reductions of 0.3 to 0.6 percentage points, improved insulin sensitivity, weight loss, and blood pressure improvement when sustained over months. The most effective window is typically earlier (8 AM to 4 PM or 10 AM to 6 PM), but the most sustainable window for your life matters more. Ramp gradually from 12:12 to 14:10 to 16:8 over 3 weeks. Coordinate medication adjustments with your doctor before starting — sulfonylureas and insulin are the main concerns. Build the eating window around protein, vegetables, and healthy fats — not refined carbs — to capture the full metabolic benefit.
