Intermittent fasting: what should you definitely avoid eating?

Intermittent fasting is a dietary practice that involves alternating, on a daily or weekly basis, between periods of eating and periods of fasting (without calorie intake). The goal is not to drastically restrict calorie intake, but to give the body a sufficiently long digestive break so that it can activate certain physiological mechanisms that promote metabolic balance.

Which protocol is the most common?

The most common protocol is 16/8: which involves fasting for 16 hours and eating within an 8-hour window. For example, a person might choose to skip breakfast and eat between 11 a.m. and 7 p.m.

Some people prefer to start gently with a 12-hour intermittent fast, which is easier to incorporate into daily life, before moving on to a 14/10 or 16/8 schedule.

How should you structure your day?

There is no single intermittent fasting program, but rather several ways to structure your days. The key is to create a plan tailored to your pace, your constraints and how you feel. It’s also possible to practice intermittent fasting in the evening by eating dinner early or skipping the evening meal. This approach works well for those who feel more energetic in the morning.

Beyond the time window, the central goal is not to eat less, but to eat better, prioritising the nutritional quality of foods. The idea is to favour a diet rich in fibre, micronutrients, healthy fats and high-quality protein, while reducing fast-acting sugars, processed foods and snacking.

Note

Intermittent fasting is part of a holistic approach: returning to a more mindful, rhythmic diet that better respects the body’s natural rhythms.

Intermittent fasting aims to provide the body with periods of digestive rest, which are conducive to beneficial processes such as detoxification, regulation of blood sugar levels, the mobilisation of energy reserves, or even autophagy (a natural process of cellular cleansing). However, certain eating habits can undermine these benefits:

• Abruptly interrupting the metabolic processes triggered by fasting: resuming eating too abruptly, with foods high in sugar or saturated fat, can disrupt the physiological adjustments made during fasting. The body, which had entered “digestive rest” mode, is suddenly overloaded.

• Causing harmful blood sugar spikes: Eating foods with a high glycemic index (pastries, white bread, sugary drinks, etc.) quickly leads to a rapid rise in blood sugar levels. This stimulates a significant release of insulin, often followed by reactive hypoglycemia, which causes feelings of fatigue and early hunger.

• Fueling cravings or compulsive eating: If the body is deprived for too long and then exposed to an unbalanced diet, this can contribute to a disrupted relationship with food: uncontrollable sugar cravings, compulsive eating, or alternating between deprivation and overeating.

• Disrupting digestion or sleep: Heavy or high-calorie meals late in the day can disrupt digestion, cause bloating or acid reflux, and impair sleep quality. An overloaded digestive system slows down the process of falling asleep and can lead to waking up during the night.

During the fasting phase, the goal is to avoid triggering a metabolic response linked to calorie intake. Even small amounts of food or drink can interrupt the desired processes (digestive rest, lowered insulin levels, activation of autophagy). Here are the categories to strictly avoid:

Sugary drinks

Even a small glass of fruit juice or a sweetened plant-based beverage contains enough carbohydrates to break the fast. These drinks stimulate insulin production, restart the digestive process, and block the benefits associated with fasting. Examples to avoid: orange juice, sweetened almond milk, sports drinks, sweetened iced teas.

Concentrated or fatty vegetable broth

While it may seem harmless, vegetable broth—especially when made from a stock cube—can contain salt, fat and calories. It’s best to avoid it or opt for a homemade, fat and salt-free broth made entirely from vegetables.

Even “healthy” light snacks

A handful of nuts, a piece of fruit, or a square of dark chocolate are often seen as light snacks. However, they provide enough calories, carbohydrates, or fats to restart digestive and metabolic processes. Even if you don’t feel like you’re really eating, the fast is broken.

Drinks containing hidden calories

Certain so-called “health” drinks, such as smoothies, kombucha, juices, or drinks enriched with superfoods, may seem compatible with fasting. However, they often contain sugar, soluble fibre, or plant-based proteins that stimulate digestion.

Although intermittent fasting is primarily based on managing meal times, the content of your meals remains crucial. Certain foods can disrupt digestion, throw blood sugar levels off balance, or counteract the expected effects of fasting. Here are the ones to avoid or limit, especially when breaking the fast, when the body is most sensitive:

Fast-acting and refined sugars

White bread, pastries, puffed cereals, store-bought jams, and sugary fruit juices are quickly absorbed by the body. The result: a spike in blood sugar followed by a sharp drop, which can lead to fatigue, irritability, and early cravings. Over time, this fluctuation can disrupt insulin regulation and create an “energy roller coaster.”

Instead, opt for carbohydrate sources with a low or moderate glycemic index, such as oatmeal, whole fruits, or legumes.

Ultra-processed foods

Salty snacks, processed foods, diet sodas, commercial protein bars… These products often contain additives, artificial sweeteners, flavour enhancers and low-quality fats that offer little benefit to the body. They can also create a form of food dependency and disrupt natural hunger and satiety signals.

Tip: Read the ingredient list. The longer and more incomprehensible it is, the more you should avoid the product.

Alcohol, even in small amounts

A glass of wine or a beer may seem harmless, but alcohol slows down your metabolism, strains your liver—which is already working hard after fasting—and stimulates hunger. It also impairs sleep quality, which undermines the restorative effects of fasting.

During a period of intermittent fasting, it’s best to avoid alcohol, or limit it to very rare occasions, ideally well before breaking the fast.

High-fat or fried foods

Fried foods, processed meats, full-fat cheeses, dishes with heavy sauces... These foods require significant digestive effort, especially after a period of digestive rest. They can cause bloating, nausea, or a feeling of heaviness, which runs counter to the comfort and expected benefits of fasting. It’s better to opt for healthy fats: avocados, extra-virgin olive oil, seeds, small fatty fish…

Coffee on an empty stomach

Coffee, especially when consumed on an empty stomach, can irritate the digestive lining, increase heart palpitations, or cause excessive nervousness. This is particularly true for people who are anxious or prone to digestive issues. You can replace it with a warm herbal tea (rosemary, fennel, chamomile), chicory, or simply hot water with ginger and a few drops of lemon.

1. Stay well-hydrated during the fasting period: drink water regularly, as well as unsweetened herbal teas or fat-free, strained broth. This supports metabolism and prevents dehydration.

2. Move within your limits: gentle physical activity such as walking, yoga, or moderate exercise can support the metabolic effects of fasting. Always listen to your body.

3. Take time to rest: Fasting is about refocusing and finding balance. Allow yourself some quiet time, especially if you feel tired.

4. Break your fast with a meal that’s easy to digest: simple foods, high in fibre, light proteins and healthy fats.

5. Avoid overeating at the end of the day: Focus on regularity and listen to your body’s signals of fullness.

Intermittent fasting is not recommended for pregnant women, children or adolescents, people with eating disorders, or anyone with chronic health conditions without medical advice.

Intermittent fasting should never become a rigid constraint or a new form of restriction. If you experience excessive fatigue, dizziness, headaches, irritability, or compulsive eating, it is essential to adjust your approach or even stop.

Fasting that is poorly aligned with your natural rhythm can disrupt your sleep, performance, or mood. If you are very active early in the morning, for example, skipping breakfast may not be appropriate. It is important to choose an eating window that fits your daily routine (work, physical activity, parenting, etc.).

Intermittent fasting is neither a miracle solution nor the latest fad diet. It can be a valuable tool if practiced mindfully, in tune with your body’s needs, and without focusing on performance. Take the time to observe how you feel and your energy levels, and adjust accordingly based on what works best for you.

Marion has a Doctor of Pharmacy and is a naturopath and coach. After twelve years in the cosmetics industry, she now helps clients achieve greater overall balance and adopt a more mindful lifestyle, with a particular focus on sustainable nutrition. She also created a podcast where she shares insights to promote a lifestyle philosophy that respects both health and the environment.