Intermittent fasting: this is how 16:8 and other methods work

23 Mar

Intermittent fasting has gone from a niche phenomenon among biohackers to one of Sweden’s most sought-after dietary strategies. The concept sounds simple — eat during a limited window of time and fast the rest — but the methods, results, and individual variations are more complex than most Instagram posts care to imply. Here we go through the most common fasting schedules, what the research actually says about intermittent fasting for weight loss, who should avoid the method and how you combine fasting with exercise without losing muscle mass.

16:8 fasting — the most popular method and what studies show

The 16:8 method means that you fast for 16 hours per day and eat all food during an 8-hour window. In practice, for most people this means skipping breakfast: eating from 12 noon to 8 pm, then fasting until the next day at 12. The method is by far the most studied variant of intermittent fasting and the one that most people report being able to sustain long-term.

A randomized controlled trial published in JAMA Internal Medicine (2020) compared 16:8 fasting with free eating for 12 weeks in obese adults. The results? The fasting group lost an average of 0.94 kg more body weight — a statistically significant but modest difference. The researchers noted that some of the weight loss was due to loss of muscle mass, not just fat, which raised concerns.

How does 16:8 affect hunger and energy levels during the day?

For the first 3-7 days, most people experience increased hunger in the morning and slight irritability. Ghrelin (the hunger hormone) follows a learned rhythm and is released at times when the body expects food. After 1-2 weeks, ghrelin secretion adjusts to the new pattern, and most people report that morning hunger disappears completely.

Energy levels during fasting vary. Some experience increased mental clarity in the morning — likely linked to norepinephrine surges during fasting — while others feel foggy and unfocused until they eat. Individual differences in blood sugar regulation, sleep quality and stress levels explain this variation. Black coffee and tea without additives are allowed during the fasting period and can alleviate hunger and energy dips. The caffeine in coffee increases fat oxidation by 10-15% during fasting, which adds a small metabolic benefit in addition to the appetite-suppressing effect.

Other variations of intermittent fasting — 5:2, OMAD and alternate day fasting

16:8 is just one of several methods. Here are the most common options with pros and cons:

|Method|Schedule|Average weight loss (12 v)|Feasibility|

| 16:8 |16 h fasting / 8 h eating, daily|2–4 kg|High — easy to integrate into everyday life|

| 5:2 |5 normal days / 2 days of 500–600 kcal|3–5 kg|Medium — the restriction days can be tough|

|OMAD (One Meal A Day)|23 h fasting / 1 h eating|4–6 kg|Low — hard to get enough protein|

|Alternative Day Fasting (ADF)|Eat every other day, fast every other day|4–7 kg|Low — difficult to maintain socially|

The 5:2 method, popularized by British doctor Michael Mosley, involves eating normally five days a week and limiting intake to 500-600 kcal on two non-consecutive days. Research from the University of South Manchester (2011) showed that 5:2 produced comparable weight loss to continuous calorie restriction — but that more participants in the 5:2 group managed to stick with the method after 6 months, likely because they only needed to “be disciplined” two days per week.

OMAD (One Meal A Day) is the most extreme variant and means that the entire day’s caloric intake is eaten on a single occasion. The method provides rapid weight loss but carries significant risks: difficulty reaching sufficient protein intake in a single meal, risk of blood sugar peaks and troughs, and increased risk of eating disorder behavior. A study published in Nutrients (2023) found that OMAD participants lost more muscle mass in proportion to fat tissue compared to those who ate three meals per day, despite identical caloric intake. This method is not recommended for most people — and certainly not for those who do strength training and want to preserve muscle mass.

Alternative daily fasting (ADF) can produce impressive short-term results but is socially difficult to maintain. Saying no to dinner every other night puts a significant strain on family life and social relationships, causing most people to return to normal eating patterns within 2-3 months.

Fasting for weight loss — does it work better than regular dieting?

The big question: does intermittent fasting provide better weight loss results than simple calorie restriction? The answer from the research is clear: at the population level — no. A meta-analysis published in the Annual Review of Nutrition (2022) that included 27 randomized trials found no statistically significant difference in weight loss between intermittent fasting and daily caloric restriction over periods of 3–12 months.

What fasting provides is an alternative framework that some people find easier to follow. Instead of counting calories at every meal, you just need to look at the clock. This simplicity is the real benefit — not a magical metabolic effect.

However, there are groups that should avoid intermittent fasting altogether:

• Pregnant and lactating women — energy needs are increased and caloric restriction can negatively affect fetal and milk production.

• People with eating disorders (anorexia, bulimia, orthorexia) — periods of fasting can reinforce restrictive eating behavior and trigger relapse.

• Type 1 diabetics and type 2 diabetics on insulin or sulfonylurea — fasting can cause dangerously low blood sugar without dose adjustment.

• Children and young people under 18 — the body is growing and needs a regular supply of nutrition.

• People with low blood pressure — fasting can lower blood pressure further and cause dizziness and fainting.

If you belong to one of these groups but still want to structure your eating, a milder regimen of three fixed meals with no snacks can provide similar behavioral benefits without the risks of fasting.

However, fasting affects hormones in a way that may have health benefits beyond mere weight loss. During fasting, insulin levels drop (improves insulin sensitivity), norepinephrine levels increase (stimulates fat oxidation) and autophagy is activated (the cell’s “cleaning function” that breaks down damaged proteins). These effects have been observed in human studies but typically require at least 14–18 hours of fasting to reach clinical significance.

Do you want to combine fasting with other evidence-based strategies? Read our guide on how to lose weight fast and safely for a broader and more complete perspective.

Intermittent fasting and exercise — how do you avoid muscle breakdown?

Exercising while fasting requires some planning if you want to preserve muscle mass. Strength training in the fasted state is possible, but muscle protein synthesis is inhibited without amino acids in the blood. The best strategy is to put the workout at the end of the fasting period and break the fast with a protein-rich meal (at least 30 grams of protein) right after.

If you train early in the morning during a 16:8 period and don’t want to break the fast before 12, 10 grams of BCAA (branched chain amino acids) or EAA (essential amino acids) before exercise can reduce the catabolic effect — by only 40-50 kcal, which technically breaks the fast but preserves most of the metabolic benefits.

The protein intake per day is more important than the timing. Aim for 1.6–2.2 g/kg body weight spread over at least 3 meals during the eating window. Squeezing 120-160 grams of protein into 2 meals is possible but not optimal — muscle protein synthesis is maximized at 20-40 grams per occasion, and the excess is oxidized rather than used for muscle building.

Fasted endurance exercise (running, cycling, swimming at moderate intensity) is generally better tolerated and may even improve fat oxidation during the session. Studies show that regular fasted exercise adapts muscles to use fat as fuel more efficiently — a benefit for ultra-endurance athletes but less important for the average exerciser or gym-goer.

Regardless of the method: listen to your body. If you experience dizziness, weakness, or severely reduced performance during exercise in the fasted state, your body is signaling that it needs fuel. Forcing yourself through a bad session in the name of fasting does more harm than good in the long run. Read more about how fasting interacts with your basal metabolism in our metabolism guide.

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