Intermittent Fasting: What the Science Really Says

In recent years, intermittent fasting has probably become the most discussed dietary approach in Portugal and worldwide. Some swear by its results. Some dismiss it completely. And some practice it without fully understanding what it's doing to their bodies. Science holds a more balanced position than any of these extremes — and more honest than most articles on the topic let on. What is known: autophagy is progressively activated with fasting, being more pronounced in fasts of 16+ hours. Its direct clinical relevance in healthy humans is still being investigated. Gut Health: An Emerging Area The Arizona State University study (published in Nature Communications) found that intermittent fasting combined with protein control increased the diversity of the gut microbiome and microbes associated with a healthier metabolic profile — results superior to those of simple caloric restriction. Cognitive Function: Preliminary Evidence Fasting may promote the production of BDNF (brain-derived neurotrophic factor) — a protein associated with neuronal health and protection against neurodegenerative diseases. Ketone bodies produced during fasting are also an alternative energy source for the brain. Many people report greater mental clarity during fasting periods — although controlled scientific evidence in humans is still limited. The Controversial 2024 Study: What You Should Know In 2024, a large-scale study analyzing data from over 19,000 adults raised a question that shook the enthusiasm around intermittent fasting: people who restricted eating to less than 8 hours a day showed a significantly higher risk of cardiovascular mortality than those who spread their meals over 12-16 hours. Before concluding that intermittent fasting is dangerous, it's important to contextualize: The study was observational — it doesn't prove causality, only association People with very short eating windows may have other uncontrolled health behaviors The study did not distinguish food quality within the eating window Other studies, including 2024 reviews, find positive cardiovascular effects of intermittent fasting in overweight people The honest conclusion: science is genuinely divided in this area. Intermittent fasting is not a panacea without risks — and people with cardiovascular conditions or diabetes should be cautious and consult a doctor before adopting more restrictive protocols. Table: What Science Says About the Benefits of Intermittent Fasting Benefit Level of Evidence Notes Weight loss ★★★☆☆ Moderate No superior to conventional caloric restriction Insulin sensitivity ★★★★☆ Good Consistent evidence in multiple studies Inflammation reduction ★★★★☆ Good Inflammatory markers reduce in 2-4 weeks Cardiovascular health ★★★☆☆ Mixed Contradictory results — depends on individual profile Autophagy ★★★☆☆ Promising Real, but clinical evidence in humans limited Gut health ★★★☆☆ Promising Microbiota improvement in recent studies Cognitive function ★★☆☆☆ Preliminary Evidence mainly in animal models Longevity ★★☆☆☆ Preliminary Solid data in animals; human inconclusive Who It Might Be Right For Intermittent fasting is not a universal approach. It can be a good option for those who: Have difficulty counting calories and prefer a simple "when to eat" rule Naturally don't have an appetite in the morning and already have a late breakfast Seek to improve insulin sensitivity and metabolic health Want to simplify their eating routine without complex meal plans Have a lifestyle that allows consolidating meals into a time window without significant social impact Who It Is Not Recommended For Pregnant and breastfeeding women — increased nutritional needs People with a history of eating disorders — fasting can exacerbate problematic patterns Type 1 diabetes or insulin use — risk of hypoglycemia Children and adolescents — growth needs incompatible with temporal restriction People who are underweight or malnourished Those with gastric problems (gastritis, ulcers) — prolonged fasting can worsen them People with high stress or anxiety — can increase cortisol levels How to Start Gradually (If You Decide to Try It) The smoothest transition is to start with the 12:12 protocol — which for many people simply means stopping eating after dinner and not snacking before breakfast. It's a low-impact change that already offers some of the metabolic benefits of fasting. Then, if you adapt well, you can gradually progress to 14:10 and eventually 16:8 — always listening to your body and without forcing an abrupt transition. During the eating window, the quality of what you eat is fundamental. Intermittent fasting with ultra-processed food cancels out the metabolic benefits — and possibly worsens the situation. A note on supplements during fasting: natural ingredients like EGCG, L-Carnitine, and Guarana — present in Kyoslim — are generally compatible with intermittent fasting protocols, as they contain no calories. Taking them before the eating period can support metabolism during the active window. Always consult your healthcare professional if you have doubts. Frequently Asked Questions About Intermittent Fasting Does coffee break a fast? Black coffee without sugar, sweeteners, or milk does not break a fast from a metabolic standpoint — it does not stimulate an insulin response and does not interrupt autophagy. Tea and water follow the same logic. Coffee with milk or sweeteners represents caloric intake and technically breaks the fast. Can I exercise while fasting? Yes, and there is research suggesting that fasted aerobic exercise can increase fat oxidation. However, for high-intensity exercise or strength training, performance may be compromised — especially in the first few days of adaptation. Experiment with moderate intensity and adjust according to your body's response. Does intermittent fasting cause muscle mass loss? Muscle mass loss is a legitimate concern, especially in prolonged fasts. Adequate protein during the eating window and strength training are the most important factors to minimize this risk. Intermittent fasting combined with controlled protein intake — as in the Arizona State University study — better preserved muscle mass than simple caloric restriction. How long until I feel the benefits? Improvements in insulin sensitivity and glucose regulation can be observed in days to weeks. Reductions in inflammatory markers typically appear in 2-4 weeks. Benefits in body composition require consistency over weeks to months. Is intermittent fasting sustainable long-term? It largely depends on the individual. Some adapt easily and maintain it for years. Others find the protocol socially incompatible or psychologically difficult. Long-term adherence is the most predictive factor of success — and if fasting creates anxiety around food, a different approach may be more suitable. Important Health Information Disclaimer: This article is for informational purposes only and does not constitute medical advice. Intermittent fasting may not be suitable for everyone. Consult your doctor or nutritionist before starting any fasting protocol, especially if you have existing health conditions, take medication, or belong to any of the risk groups mentioned. Results vary by individual. The statements in this article have not been evaluated by regulatory authorities. The products mentioned are not intended to diagnose, treat, cure, or prevent any disease.

Intermittent Fasting: What the Science Really Says

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