Intermittent Fasting: Principles, Cost, Benefits, and Risks

Andy Paras

4 Sep, 2025

Intermittent fasting has become one of the most popular dietary trends worldwide, praised for its potential health benefits, weight loss effects, and simplicity. By temporarily abstaining from food, this eating pattern aims to give the body a break from constant digestion, promote detoxification, and improve overall well-being. While the practice has roots in ancient spiritual and religious traditions, today it is embraced for its potential to enhance vitality, regulate metabolism, and support a healthier lifestyle. However, like any dietary approach, it requires careful planning and awareness of its possible risks.

By Andy Paras
Updated on 04/09/2025 at 23:37

By depriving the body of food for several hours, intermittent fasting aims to purify it and help shed a few kilos. What are the benefits? Are there any health risks? How should one start? Nutritionist Raphaël Gruman explains.

Contents

What is intermittent fasting?
How it works: Which intermittent fasting method is most effective? How to do it?
16/8 intermittent fasting: How long and how often per week?
Who is intermittent fasting for?
Allowed foods
Forbidden foods
Is intermittent fasting easy to follow? How to do it?
Health benefits of intermittent fasting
Health risks of intermittent fasting
Effects in case of cancer
Advantages of intermittent fasting
Disadvantages of intermittent fasting
Precautions before starting fasting

What is intermittent fasting?

For millennia, humans have fasted for spiritual or religious reasons. Today, this practice, now trendy, is undertaken for well-being and health.

"Intermittent fasting aims to cleanse the body of junk food, excesses, and toxins by giving a pause to the liver, kidneys, and skin," explains Raphaël Gruman, nutritionist.

The principle is to abstain from food for 12 to 16 hours, effectively skipping a meal and reducing calorie intake.

"During fasting, the body uses circulating glucose in the blood or stored in the liver to continue functioning," Gruman adds. "If fasting extends beyond three days, the body starts tapping into fat and protein reserves, mainly from muscles."

The result can be a body reset, weight loss, and a way to fight certain diseases.

Different types of intermittent fasting exist:

Fasting for 24 hours
16/8 method: eating during an 8-hour window and fasting for 16 hours
5/2 method: consuming 500–600 calories for two days and eating normally the rest of the week

How does intermittent fasting work? Which method is most effective?

Also called “fasting” in English, intermittent fasting involves not eating for 12–16 hours straight, effectively reducing from three to two meals per day.

For example, you eat within an 8-hour window and abstain from food for the remaining 16 hours — this is the 16/8 method.

"For instance, one might have dinner but skip breakfast the next morning, only eating again at lunch," explains Gruman. "However, it can be adapted: if you are physically active or prefer breakfast, you could skip dinner instead. The fasting schedule might then be breakfast, lunch, and snack, stopping food intake around 5 PM and resuming around 7–8 AM the next morning."

While solid food is restricted (lowering daily caloric intake), it is essential to stay well-hydrated — 1.5–2 liters of fluids such as water, tea, herbal infusions, or broth — to avoid fatigue or headaches.

When starting intermittent fasting, choose a fasting window that suits your daily rhythm. Some may fast in the morning, others at night. The key is selecting a calm time of day and avoiding temptations, such as cooking for family in the evening.

16/8 Intermittent Fasting: How long and how often?

There is no strict duration.

"For weight loss, I recommend a full week of intermittent fasting," Gruman says. "For digestive well-being, fasting one to two days per week is sufficient."

Regular physical activity and a balanced diet remain essential for good health.

Read also: How to lose weight effectively.

Who is intermittent fasting for?

Intermittent fasting is suitable only for healthy adults. It is not recommended for pregnant or breastfeeding women, children, adolescents (risk of growth delay), or elderly people (who may be further weakened).

Allowed Foods

During non-fasting meals, all foods are allowed: cereals, legumes, meats, fish, eggs, dairy, fruits, vegetables, even pastries and sweets. Healthy fats such as nuts, olive oil, sardines, and salmon are encouraged. Nuts are excellent for health, provide energy, and are rich in omega-3s. Olive oil should ideally be cold-pressed and organic. Proteins, whether animal or plant-based, are essential.

During fasting, only water, broths, and herbal teas are allowed. On fasting days, you may consume up to 25% of your usual intake.

Forbidden Foods

For main meals aimed at weight loss, sugary foods such as sweets, pastries, and candy — high-calorie bombs — should be avoided.

During the fasting period, solid foods are prohibited. "Coffee is also discouraged as it requires digestion by the liver," says Gruman.

Is intermittent fasting easy to follow? How to do it?

Yes, if done for short periods. Hunger can be managed by drinking fluids, which fill the stomach and curb appetite. Staying busy can also distract from hunger. Going to bed early makes skipping dinner easier.

"The body adapts by converting fat reserves into ketone bodies, which replace glucose," explains Gruman. "Ketones provide satiety, a sense of euphoria, and reduce the urge to eat."

Health Benefits of Intermittent Fasting

Increased vitality: Fasting helps remove toxins, improving fatigue, sleep (noticeable from day 1), energy, and skin health.

Weight loss: A 16-hour fast can result in around 1 kg per week. Fasting reduces fat mass, controls hunger, and improves satiety. This contributes to better blood pressure and cardiovascular health.

Reduced inflammation: Fasting may decrease pain, stiffness, and NSAID use in rheumatoid arthritis. It may also enhance concentration by promoting neuronal renewal, according to International Journal of Obesity.

Fasting may stimulate growth hormone secretion, which plays a role in cellular renewal.

Health Risks of Intermittent Fasting

Extended fasting without medical supervision can cause dizziness, headaches, muscle loss, dehydration, macronutrient deficiencies (especially protein), mineral deficiencies (e.g., iron), fatigue, anemia, sleep problems, irritability, and drowsiness.

It may also affect the heart — particularly rhythm disorders — as long-term restriction weakens the heart muscle. Consultation with a doctor or cardiologist is advised.

A minor but unpleasant effect is bad breath (halitosis).

Effects in Case of Cancer

Opinions on anti-cancer effects are mixed. Animal studies suggested fasting before chemotherapy may protect healthy cells while making cancer cells more vulnerable.

In 2017, France’s INCa concluded that available clinical evidence does not confirm benefits for cancer prevention or treatment. Fasting-related weight and muscle loss could worsen malnutrition and sarcopenia, negatively affecting prognosis.

However, a 2020 Nature study on 129 breast cancer patients showed those who fasted three days before and after chemotherapy responded better than those who ate normally.

"Fasting can support anti-cancer treatment," says Gruman, "but patients must be supervised by healthcare professionals due to the risk of malnutrition."

Periodic fasting should always be supervised for chronic illnesses, diabetes, or other health conditions.

Advantages of Intermittent Fasting

Short-term fasting with two to three meals per day does not cause imbalances, deficiencies, or fatigue.
Improves digestion, sleep, skin quality, energy, relieves rheumatoid arthritis pain, and supports brain function.
Promotes weight loss (~1 kg per week).
Compatible with vegetarian, vegan, gluten-free, lactose-free, salt-free, kosher, and halal diets.
Can be practiced for short periods.

Disadvantages of Intermittent Fasting

Long, unsupervised fasting can cause deficiencies, muscle loss, and weight regain. The Haute Autorité de Santé recommends losing no more than 1–2 kg per month.
May trigger disordered eating or sugar cravings.
Can be socially limiting (e.g., skipping dinner reduces social interactions).

Precautions Before Starting Fasting

Pregnant or breastfeeding? Heart disease or diabetes? Eating disorders? Consult a healthcare professional before intermittent fasting.

Expert Opinions:

Guerri, Dietitian Nutritionist:
"Intermittent fasting has many health benefits (detox, disease prevention, reconnection to hunger cues, increased energy), but it is not suitable for everyone. Medical supervision is necessary. It is safe and effective when fasting is occasional and non-fasting meals are balanced."

Antoine Avignon, Diabetologist Nutritionist:
"It is one of the diets with the most scientific evidence showing long-term benefits and anti-aging effects. Studies demonstrate positive effects on cardiovascular risk factors."

Conclusion

Intermittent fasting can offer numerous benefits, including weight management, improved metabolic health, and increased vitality, when practiced safely and under appropriate guidance. Short-term fasting is generally well-tolerated by healthy adults, but it may not be suitable for children, pregnant or breastfeeding women, elderly individuals, or those with certain medical conditions. As with any diet, balancing nutrition during eating periods and consulting healthcare professionals before starting are essential to ensure the practice is effective and safe. When done responsibly, intermittent fasting can be a valuable tool for improving health and supporting long-term wellness.

Intermittent Fasting: Principles, Cost, Benefits, and Risks