Intermittent Fasting and Training: What Works and What is a Myth

Intermittent fasting became popular as a weight loss strategy and gained almost mystical contours in internet content. Current science allows for a more sober view: it is a useful tool for some profiles, it is neutral for many, and the most touted benefits need nuance. This article separates what is practical from what is fad.

What is intermittent fasting

Direct answer: Intermittent fasting is an eating pattern that alternates periods of intake with periods of total or almost total calorie restriction. The most common protocols are 16/8 (16 hours fasting, 8 hour window), 14/10 (mildest), 20/4 (most restrictive), and 5:2 (5 normal days, 2 days of severe restriction).

Main protocols

Protocol	Fast	Window	Typical audience
14/10	14h	10h	Beginners, women
16/8	16h	8h	Most Popular, Healthy Adults
18/6	18h	6h	Experienced, moderate cut
20/4	20h	4h	Forwards, high discipline
5:2	2 dias/semana	~500 kcal on restriction days	Alternative to diary
ADF (alternate day)	1 dia comendo, 1 dia restrito	Experimental alternative	Clinical context

What the Current Evidence Really Shows

Recent meta-analyses (2022–2025) comparing intermittent fasting with distributed calorie restriction show:

• Weight loss: similar when calories are matched
• Visceral fat loss: slight advantage in some studies for fasting, but not consistent
• Maintenance of lean mass: similar if total protein is adequate
• Metabolic markers (glycemia, insulin sensitivity): fasting may have a slight advantage in some populations
• Adherence: personal preference — some find fasting easier, others classic distribution

In other words: if the strategy favors individual adherence, fasting is a good tool. If it generates stress or uncontrolled hunger, it is not suitable.

Plausible benefits of intermittent fasting

• Facilitates creation of a calorie deficit (less window, less opportunity to overeat)
• Simplifies planning (fewer meals to organize)
• Reducing late-night snacking and snacking
• Possible slight improvement in markers of metabolic health
• Increased basal autophagy (clinical relevance still under study in humans)

Exaggerated benefits

• “Metabolism speeds up” — not to clinical magnitude
• “Cures insulin resistance” — improves, not cures
• “Burns localized fat” — no strategy does that
• “Dramatic autophagy in 16 hours” — significant effect appears with longer fasts
• “Activates growth hormone” — modest elevation, no practical impact on hypertrophy

Fasting and strength training: can it be combined?

Yes, with planning. Two main strategies:

Strategy 1: train within the window

• Eat before training (1 to 3 hours)
• Training and eating post-workout afterwards
• More compatible with heavy training
• Recommended for priority hypertrophy

Strategy 2: training fasted

• Training at the end of the fasting period
• Break fast with post-workout meal
• Acceptable for cardio and light training
• In heavy training, performance may drop

How to achieve protein in an 8-hour window

Anabolic resistance and absorption limit per meal require reasonable distribution:

Person	Total protein/day	3 refeições na janela	4 refeições na janela
60 kg (1,8 g/kg)	108 g	36 g por refeição	27 g
70 kg (2,0 g/kg)	140 g	47 g	35 g
80 kg (2,0 g/kg)	160 g	53 g	40 g
90 kg (2,0 g/kg)	180 g	60 g	45 g

3 to 4 meals within an 8-hour window are practical and efficient for hypertrophy.

What breaks the fast and what doesn’t

In strict protocols:

Break fast:

• Any solid food
• Liquids with calories (juices, milk, proteins, sports drinks)
• BCAA and Calorie Pre-Workout

Does not break fast (in practical 8/16):

• Water
• Pure coffee
• Unsweetened tea
• Salt water (in specific cases)

Some more restrictive interpretations (related to maximum autophagy) even exclude coffee. For the practicality of the deficit-focused 16/8, coffee and tea are acceptable.

Who benefits most

• People who tend to snack at night
• Work schedules that facilitate concentrated meals
• Who prefers fewer, but larger, meals
• Healthy adults with no history of eating disorders
• Profiles that have difficulty with diets counted gram by gram

Who should avoid

• History of eating disorders
• Women with irregular cycle or amenorrhea
• Pregnant or breastfeeding women
• Type 1 or type 2 diabetics on insulin (requires medical supervision)
• Athletes with high competitive intensity (low energy availability is a risk)
• Growing Teenagers

Common mistakes

Use a window as an excuse to overeat. If you consume 4000 kcal in 8 hours, losing weight is still difficult.

Fasting with very low protein. Lean mass is lost; It’s not the fault of fasting, it’s due to inadequate nutrition.

Skip breaking the fast properly. Starting with industrialized bread and juice can generate an unnecessary glycemic spike; prefer protein + fiber + good fat.

Combine extreme fasting with very heavy training. Recovery suffers.

To think that 16/8 is a miracle. It is a food restriction tool; without a calorie deficit, there is no loss.

Tips used by personal trainers

• Test 10/14 for 2 weeks before moving to 8/16
• Woman in glute focus: 16/8 usually works well; 20/4 tends to be too restrictive
• Training 30 to 60 minutes before the first meal is usually a great point for those who want to train on a light fast.
• Monitoring sleep, mood and menstrual cycle in the first 4 weeks helps assess compatibility
• For those who do a longer cut, cycling 16/8 with 14/10 (alternating weeks) reduces cumulative restriction

Key Takeaway Points

• Intermittent fasting is a tool; it’s not magic
• The main benefit in weight loss comes from the calorie deficit that facilitates
• Compatible hypertrophy if total protein is reached in the window
• 16/8 Protocol is the most practical for healthy adults
• Not for everyone — evaluate background, genre, and purpose

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Additional reading:

• Caloric deficit: how to calculate
• Fasted training: pros and cons
• Pre-workout meal: what to eat
• Daily protein: how much do you need per kg