Protein during intermittent fasting: why it's crucial

Intermittent fasting isn't just about when you eat — it's also about what you eat in the time window you have. And of all the macronutrients, it's protein that works hardest while you fast.

Whether you follow the 16:8 schedule, do 5:2 or simply start eating later in the day: anyone who doesn't get enough protein loses not just fat, but also muscle mass. And that's exactly what you want to avoid.

Claudia Smet Updated on May 25, 2026

Quick answer

During intermittent fasting you eat within a limited time window. Protein is then especially important: it helps preserve muscle mass, keeps you full and stabilises blood sugar. Particularly for women over 40, a sufficient protein intake is not optional — it is essential.

In this article:
- Why protein is indispensable during IF
- How much you need daily
- The best protein sources for your eating window
- Specific focus point for women over 40
- Practical FAQ

Why is protein so important during intermittent fasting?

Protein is the building block of your body. It consists of amino acids and is essential for building and repairing cells, muscles and tissue. During the fasting period, your body uses stored energy — but if protein intake is too low, it will also start breaking down muscle tissue.

That is exactly what you want to avoid. Especially with intermittent fasting, where your daily intake is concentrated in a smaller time window, it is important to consciously plan sufficient protein.

Protein and weight loss during intermittent fasting

Protein has a high satiety effect: it keeps you fuller for longer than carbohydrates or fats. That is a major advantage during IF, because your eating window is limited and you are less likely to snack outside of it.

In addition, digesting protein itself has a calorie-burning effect — known as the thermic effect of food. Your body uses more energy to process protein than carbohydrates.

Four key benefits:

1. Stable blood sugar — protein reduces peaks and dips in your energy levels
2. Preservation of muscle mass — less muscle loss during weight loss
3. Long-lasting satiety — less hunger outside your eating window
4. Metabolic support — your metabolism stays more active

How much protein do you need during intermittent fasting?

As a general guideline, adults need approximately 0.8 to 1.2 grams of protein per kilogram of body weight per day.
For active people or those over 40, this increases to 1.2 to 1.6 grams.

With intermittent fasting, some calculation is needed: all that protein has to fit into a smaller window. With 16:8 you have 8 hours, so you spread your intake across two to three meals.

Practical example for 65 kg and 16:8:
→ Goal: ± 80 grams of protein per day
→ Spread over 3 meals: ± 25–27 grams per meal

The best protein sources for your eating window

Not all proteins are equal. For IF, filling and nutritious sources are ideal — no empty calories.

Animal sources:

Eggs (± 6g per egg)
Greek yoghurt (± 10g per 100g)
Chicken or turkey (± 25–30g per 100g)
Salmon or mackerel (± 20–22g per 100g)
Cottage cheese or quark (± 12g per 100g)

Plant-based sources:

Legumes such as lentils and chickpeas (± 8–9g per 100g)
Edamame (± 11g per 100g)
Tofu (± 8–10g per 100g)
High-protein pasta as a low-carb alternative

Practical tip: a protein shake can be helpful to reach your daily goal if your eating window falls short.

Protein-rich food Protein shakes IF recipes

Protein during intermittent fasting: key focus for women over 40

From your forties, body composition gradually changes. Oestrogen drops, muscle mass decreases slightly faster and fat distribution shifts — often towards the abdomen. This process is called sarcopenia and starts earlier than most people realise.

Good news: protein is one of the most effective ways to slow muscle loss. Combine that with intermittent fasting and you have an approach that makes sense both metabolically and hormonally.

Specific tips for women over 40:

Choose complete protein sources with all essential amino acids
Spread your intake across your meals rather than concentrating everything in one
Combine with resistance training for maximum muscle preservation
Pay attention to magnesium and vitamin D — both play a role in muscle and bone health

Zero 3 package IF products Protein-rich food Hormonal balance