Intermittent fasting: benefits, risks, and what you should know before trying it
What will happen to your body if you start fasting today: lower cholesterol, lower blood pressure, and better blood sugar control.
Eating well sounds simple in theory, but in practice, it is rarely so. Between long workdays, meals away from home, and constantly changing routines, maintaining a balance between what we eat and what the body needs can become a real challenge. This is why intermittent fasting has gained prominence as one of the schemes to control weight and lead a healthier life. However, like any nutritional strategy, there are benefits and risks of intermittent fasting that are worth knowing before adopting it.
For Mary Hyer, a nutritionist specialized in cardiac rehabilitation at Mass General Brigham, “intermittent fasting is not a miracle formula for weight loss,” so if you are considering it as an option, it is advisable to understand what intermittent fasting is and how it works in metabolism, what limitations it has, and in which cases it is not recommended.
What is intermittent fasting, how does it work, and how to practice it?
In simple terms, fasting means going a period of time without eating. Intermittent fasting takes that everyday idea and turns it into a more intentional eating scheme, where moments for eating are alternated with breaks that give the body a breather between meals.
One of the most common questions is how long that fast should last. “There are different modalities of intermittent fasting,” explains Hyer. “From a scientific point of view, it is considered fasting when the period without food lasts at least 12 hours.” From there, each person can adjust it to their routine, schedules, and lifestyle, without unique formulas or universal solutions.
Beyond defining what intermittent fasting is, understanding how it works in metabolism helps explain why it has gained so much attention. Normally, the body obtains its energy from glucose derived from food. However, after about 10 to 12 hours without ingesting anything, those reserves begin to decrease, and the body activates an alternative mechanism: it starts using fat as an energy source. These metabolic changes in intermittent fasting place it at the center of the conversation when it comes to well-being, weight control, and metabolic health.
Intermittent fasting is not a one-size-fits-all or rigid formula. There are different schemes, and beyond following strict rules, the key is to choose the one that best fits your daily routine. This is relevant because the metabolic changes during fasting are activated progressively and depend on both the duration and consistency of the chosen scheme. Hyer shares what the most popular models are:
• Time-restricted eating (16:8 method). This involves fasting for 16 hours and concentrating food intake in an 8 to 10-hour window. These eating plans are among the simplest to follow, explains Hyer.
• 5:2 diet. In this model, you eat normally five days a week and, on the other two days, you reduce caloric intake by at least 75%. Its main advantage is flexibility, as it allows you to adjust fasting days to social commitments.
• Alternate-day fasting. This scheme proposes alternating a day of regular eating with a day of fasting. There is also a modified version, where on fasting days, only a small portion of the usual calories is consumed.
Who it is for and who it is not: benefits and risks of intermittent fasting
Although it has been shown that intermittent fasting helps with weight loss and improves some health markers, this does not mean it is a scheme for everyone. If you are thinking of trying it, consider the benefits and health risks.
A great advantage is that “it is very easy to do,” notes Hyer. “You do not need to track meals or count calories. It is something that can be easily incorporated into life.” Additionally, it offers a variety of benefits for heart health and overall health, including weight loss, lower cholesterol, lower blood pressure, better blood sugar control, reduced insulin resistance, better brain health, and fewer age-related diseases.
On the other hand, Hyer warns that it may not be advisable for those living with diabetes or suffering from heart, kidney, or liver diseases; for people with a history of eating disorders; as well as during pregnancy or breastfeeding. She also notes that those with low blood pressure, people over 65 years old, and those still in the growth stage—such as children, teenagers, and young adults—should avoid this type of scheme.
Key tips for a more balanced intermittent fasting
Adopting intermittent fasting is not just about choosing a schedule and sticking to it, but doing so consciously and in alignment with daily life. The specialist from Mass General Brigham suggests that, before starting, it is advisable to clarify, in addition to the benefits and risks of intermittent fasting, some basic principles that can make the difference between a good experience and a frustrating attempt.
To make intermittent fasting more balanced, Hyer believes it is key to choose a scheme compatible with your lifestyle, have clear goals, prioritize quality foods, stay well-hydrated, and plan meals. Additionally, she recommends starting gradually and avoiding extreme methods that are difficult to maintain over time.
As with most eating plans, it is always a good idea to consult your doctor before drastically changing your diet, says Hyer. This is especially relevant for people taking medications, living with a medical condition, or considering more restrictive fasting schemes.
That said, the specialist points out that, for most people, a fast of between 12 and 14 hours is usually a safe and manageable option. “It is always important to prioritize healthy foods,” she adds. “If well-planned, intermittent fasting can become a tool that helps promote more conscious habits and a healthier lifestyle.”
References:
1 Welton, S., Minty, R., O’Driscoll, T., Willms, H., Poirier, D., Madden, S., & Kelly, L. (2020). Intermittent fasting and weight loss: Systematic review. Canadian Family Physician, 66(2), 117–125. PMID: 32060194.