Intermittent Fasting Guide: 16:8, 18:6, OMAD & Who Should Avoid
Intermittent fasting (IF) is less about what you eat and more about when you eat. By cycling between periods of eating and fasting, you may tap into fat stores more effectively and simplify your daily routine. But IF isn't one‑size‑fits‑all — schedules range from the popular 16:8 to the strict OMAD (one meal a day), and some people should avoid fasting altogether. This guide compares the main protocols, explains how our fasting calculator helps you pick and stick to a window, and honestly lists the health conditions where IF is off the table.
Why IF Works (When It Does)
During fasting, insulin levels drop, making it easier for the body to access stored fat for energy. The eating window naturally limits snacking, which often reduces total calorie intake without counting every bite. However, IF isn't magic — if you overeat during the window, weight loss won't happen. The benefits also include cellular repair processes (autophagy) that ramp up after about 16 hours of fasting, which is why schedules like 18:6 are popular for longevity, not just weight.
- 16:8 — Fast 16 hours, eat within an 8‑hour window (e.g., 12 pm to 8 pm).
- 18:6 — Fast 18 hours, eat within 6 hours (e.g., 1 pm to 7 pm).
- OMAD — One meal a day, roughly a 23:1 fast. Hard to sustain; can lead to nutrient gaps.
Step-by-step: Set Your Fasting Schedule
- Open the Intermittent Fasting Calculator tool.
- Select your desired protocol (16:8, 18:6, OMAD, or custom).
- Choose the time you want to start eating (e.g., 12:00 PM). The tool automatically sets the fasting start time and shows a live countdown to both the eating window open and close.
- You can also track how many hours remain in your fast — helpful for staying accountable during the final tough hours.
Who Should Avoid Intermittent Fasting
IF is not safe or suitable for everyone. Pregnant or breastfeeding women need consistent nutrient intake and should not fast. People with a history of eating disorders may find fasting triggers harmful patterns. Those on medications that require food (e.g., certain diabetes or blood pressure drugs) must consult a doctor before skipping meals. Additionally, underweight individuals (BMI < 18.5) and children/adolescents should avoid fasting. The TDEE Calculator can help determine whether you're getting enough calories within your eating window.
Frequently Asked Questions
Can I drink water or black coffee during the fast?
Yes, water, black coffee, and plain tea are allowed. They don't spike insulin and can help manage hunger. Avoid milk, sugar, or any caloric additives.
Will IF slow down my metabolism?
Short‑term fasting (16‑48 hours) does not lower metabolism; it may actually increase it slightly. Prolonged calorie restriction, not fasting itself, is what eventually drops BMR.
Can I exercise while fasted?
Many people do light to moderate exercise fasted. Heavy lifting or high‑intensity workouts may suffer without pre‑workout fuel — experiment and listen to your body.
What if I feel dizzy or get headaches during the fast?
Electrolyte imbalance is common. Add a pinch of salt to your water or consider an electrolyte supplement without sugar. If symptoms persist, shorten the fast.
Is it free and private?
Yes — the tool runs entirely in your browser, free, with no sign‑up and nothing uploaded to a server.
Intermittent fasting is a dietary pattern, not a medical treatment. If you have diabetes, low blood pressure, or any chronic illness, speak with your doctor before beginning any fasting regimen.
Try the Intermittent Fasting Calculator