The basic idea is simple: eat very few carbohydrates — under about 20-30g per day — and your body runs out of its usual fuel source. After a few days, it shifts to burning fat for energy instead, including producing molecules called ketones that your brain can use as fuel. That metabolic state is ketosis, and it’s what the diet is named after.
That’s the mechanism. Here’s what it means in practice.
What you eat: meat, fish, eggs, cheese, butter, cream, olive oil, avocados, leafy greens, most above-ground vegetables, nuts in moderation. What you don’t eat: bread, pasta, rice, potatoes, most fruit, anything with added sugar, beer, most processed food.
The Brussels and Belgian context makes some of this harder than it sounds. Belgian food culture is built on bread, potatoes, and frites. The default lunch at most offices is a sandwich. The default dinner at someone’s home is likely to include stoofvlees met frieten or some variation of it. The waffle is practically a national symbol. None of these are keto-friendly. You will need to actively navigate around them, and people will notice that you’re doing so.
For shopping, Delhaize and Colruyt both stock everything you need without requiring a special trip anywhere. Rob in Ixelles and Woluwe has a better selection of things like good olive oil and specialty cheeses if you want to eat well rather than just eat keto. The Marché du Midi on Sunday mornings is excellent for vegetables and fish. Halal butchers throughout Molenbeek and Saint-Gilles tend to have good quality meat at reasonable prices.
The first week is usually the hardest, and it has a name: keto flu. As your body depletes its glycogen stores, it also loses water and with it a significant amount of sodium, potassium, and magnesium. You may feel tired, get headaches, feel irritable, or have muscle cramps. This is almost entirely an electrolyte issue. Drink more water, add salt to your food, eat foods high in potassium (avocado, leafy greens, salmon), and consider a magnesium supplement. Most keto flu symptoms clear up within a few days when you stay on top of electrolytes.
After the first week or two, most people feel significantly better — often better than they did before starting. Energy tends to stabilise, hunger becomes easier to manage, and the constant need to eat every few hours usually diminishes.
A few things to watch: track your carbs at first, at least for a few weeks, because it’s easy to underestimate them. Read labels. Hidden carbs show up in places you wouldn’t expect — some deli meats, sauces, yoghurts. Aim for fat as your primary calorie source, enough protein to feel satisfied and maintain muscle, and as few carbs as possible.
It takes around two to four weeks to fully adapt. Give it that long before deciding whether it works for you.