Intermittent fasting is an eating pattern where you cycle between periods of eating and periods of not eating. Unlike traditional diets that focus on what you eat, intermittent fasting focuses on when you eat. The most popular version, called 16:8, involves eating all your meals within an eight-hour window and fasting for the remaining 16 hours of the day.
How Intermittent Fasting Works
The core idea is simple: by extending the gap between your last meal of one day and your first meal of the next, you give your body time to shift from burning recently eaten food to burning stored energy. After roughly 12 hours without food, your body begins drawing more heavily on fat stores for fuel. Longer fasts push this process further, which is why different protocols set different fasting windows.
During the fasting period, your insulin levels drop significantly. Insulin is the hormone that tells your cells to absorb sugar from your blood, and it also signals your body to store fat. When insulin stays low for an extended stretch, your body gets better access to its fat reserves and your cells can perform internal cleanup processes, recycling damaged components and repairing themselves.
The Most Common Fasting Schedules
There are several ways to structure an intermittent fast, and the right one depends on your lifestyle and goals.
- 16:8 (daily time-restricted eating): You eat during a six- to eight-hour window each day and fast for the remaining 16 to 18 hours. Most people accomplish this by skipping breakfast and eating between noon and 8 p.m., though any window works. This is the most popular method because it fits naturally into a typical schedule.
- 5:2: You eat normally five days a week and limit yourself to one 500 to 600 calorie meal on the other two days. The two low-calorie days don’t need to be consecutive.
- Alternate-day fasting: You alternate between regular eating days and fasting days, where you either eat nothing or eat very little (around 500 calories). This is one of the more aggressive approaches.
- OMAD (one meal a day): You eat all your daily calories in a single meal, typically within a one-hour window. This is the most extreme daily version and can be difficult to sustain long term.
What It Does for Weight Loss
A large network meta-analysis published in The BMJ, covering dozens of randomized clinical trials, found that all major intermittent fasting methods produce meaningful weight loss compared to eating without restrictions. In trials lasting less than 24 weeks, alternate-day fasting led to an average weight reduction of about 3.4 kilograms (roughly 7.5 pounds) compared to unrestricted eating. Time-restricted eating, like 16:8, produced smaller but still measurable losses.
In longer trials of 24 weeks or more, the most structured fasting strategies led to weight reductions ranging from about 1.9 to 3.6 kilograms compared to eating freely. Importantly, when researchers compared intermittent fasting head-to-head with traditional calorie restriction, the long-term results were essentially the same. Neither approach had a clear advantage over the other for keeping weight off. The real benefit of intermittent fasting, for many people, is that limiting when you eat is simpler than tracking every calorie.
Effects on Blood Sugar and Insulin
Beyond weight loss, intermittent fasting appears to improve how your body handles blood sugar. A meta-analysis in Frontiers in Nutrition found that multiple fasting protocols, including time-restricted eating and twice-weekly fasting, improved insulin resistance in people with type 2 diabetes compared to a regular diet. They also lowered fasting blood sugar levels significantly.
These improvements likely come from a combination of weight loss and the metabolic changes that happen during fasting itself. When you go without food for 14 to 18 hours, your body spends less time in a high-insulin state, which helps your cells become more responsive to insulin over time. For people who already have blood sugar issues, this can be a meaningful shift.
What You Can Have During a Fast
Water, black coffee, and plain tea are all fine during a fasting window. They contain essentially no calories and don’t trigger an insulin response. Sparkling water and water with a squeeze of lemon are also generally considered safe.
What does break a fast is anything that triggers a significant insulin response. Protein powder, branched-chain amino acid supplements, and any supplement containing maltodextrin, pectin, cane sugar, or fruit juice concentrate will all spike insulin and pull your body out of a fasting state. Adding cream or sugar to coffee counts too. If you’re fasting for metabolic benefits rather than just calorie control, keeping your fasting window truly calorie-free matters.
Common Side Effects in the First Week
Most people experience some adjustment symptoms when they first start fasting, particularly during the first five to seven days. Headaches, fatigue, irritability, and mild nausea are the most common complaints. These happen partly because your body is adapting to using stored energy instead of a constant supply of food, and partly because fasting can shift your fluid and electrolyte balance.
If you notice muscle cramps, brain fog, or unusual tiredness, low electrolytes are a likely culprit. Adding a pinch of salt to your water or drinking an electrolyte supplement during your fasting window can smooth the transition considerably. These symptoms are temporary for most people and tend to resolve within one to two weeks as your body adapts to the new eating schedule.
Who Should Avoid Intermittent Fasting
Intermittent fasting is safe for most adults, but it’s not appropriate for everyone. People with a current or past eating disorder should avoid it, as the rigid rules around when to eat can reinforce harmful patterns. Pregnant or breastfeeding women need a steady supply of nutrients and calories that fasting windows can disrupt. People at high risk of bone loss and falls, including older adults with osteoporosis, may also be harmed by fasting, since it can accelerate muscle and bone breakdown if protein intake drops too low.
People taking medications that require food, particularly diabetes medications that lower blood sugar, need to coordinate with their prescriber before starting any fasting protocol. Combining blood sugar-lowering medication with an extended fast can cause dangerously low blood sugar levels.
How to Start
The easiest entry point is a 14:10 schedule: fast for 14 hours and eat within a 10-hour window. If you finish dinner at 7 p.m. and eat your first meal at 9 a.m., you’re already there. After a week or two, you can narrow the window to eight hours (16:8) if it feels manageable.
What you eat during your eating window still matters. Fasting doesn’t cancel out a diet of highly processed food. The people who see the best results pair their fasting schedule with meals built around whole foods: vegetables, protein, healthy fats, and complex carbohydrates. Eating enough protein is especially important, since your body needs amino acids to maintain muscle mass during periods without food. Aim to get your usual protein needs met within your compressed eating window rather than cutting portions across the board.
Consistency tends to matter more than perfection. Sticking with the same eating window most days helps your hunger hormones adjust to the new schedule, which is why the first week is hardest and why it gets noticeably easier by week two or three.