The keto diet has become popular in recent years, thanks to its potential for quick weight loss and new research on its effects on blood sugar and energy.
A 2023 review in the Journal of Clinical Endocrinology & Metabolism found that ketogenic diets can help people lose weight and improve insulin sensitivity when done properly.
Because keto is very different from typical diets, with strict carb limits and high fat intake, it’s important to learn the basics before you begin.
If you’re thinking about starting the keto diet, you might be curious about how it works, what foods you can eat, and how hard the change will be. This guide explains the science, offers practical food tips, and outlines what to expect in your first weeks, so you can decide whether keto aligns with your health goals and lifestyle.
What Is the Keto Diet?
The ketogenic diet is a way of eating that is high in fat, moderate in protein, and very low in carbohydrates. It was first created in the 1920s to help children with epilepsy. Now, people mostly use it for weight loss, blood sugar control, and better metabolic health.
For beginners, the usual macronutrient breakdown is:
- 70-75% of calories from fat
- 20-25% from protein
- 5-10% from carbohydrates (usually 20-50 grams per day)
Cutting carbs this much makes your body look for a new energy source. Instead of using glucose from carbs, your liver turns fat into ketones, which your brain and muscles use for fuel. This change is called ketosis, and it’s the main feature of the keto diet.
Unlike other low-carb diets that let you eat some carbs, keto requires you to stick closely to your carb limit to stay in ketosis. Even one high-carb meal can stop your body from making ketones for a while, so being consistent is especially important at the start.
How the Keto Diet Works
When you eat carbohydrates, your body breaks them down into glucose, which enters your bloodstream and triggers the release of insulin. Insulin helps shuttle glucose into cells for immediate energy or stores it as glycogen in muscles and the liver.
However, when carbohydrate intake drops below a certain threshold—typically under 50 grams daily—your glycogen stores become depleted within 24-48 hours.
When this happens, your body looks for another energy source. Your liver starts turning fatty acids into ketones: beta-hydroxybutyrate (BHB), acetoacetate, and acetone. These ketones travel in your blood and can fuel your brain, which usually depends on glucose.
Research from Harvard Medical School says this change usually takes two to four days, but it can vary depending on your activity, health, and how many carbs you ate before.
Once you’re in ketosis, your body becomes highly efficient. Once you reach ketosis, your body gets better at burning fat for energy. Many people lose weight quickly at first on keto, partly because they lose fat and partly because they lose water as glycogen stores are used up.
Being in ketosis can also make you feel less hungry and give you steadier energy during the day, likely because of lower insulin and the filling nature of fat.
What to eat on a Keto Diet
When you start this diet, understand which foods fit within your macronutrient targets. The key is choosing nutrient-dense, whole foods that provide healthy fats, adequate protein, and minimal carbohydrates.
Healthy Fats (Your Primary Fuel Source)
- Avocados and avocado oil – Rich in monounsaturated fats and potassium
- Olive oil and coconut oil – Ideal for cooking and dressings
- Butter and ghee – Preferably from grass-fed sources
- Nuts and seeds – Almonds, walnuts, chia seeds, flaxseeds (in moderation due to some carb content)
- Fatty fish – Salmon, mackerel, sardines, which also provide omega-3 fatty acids
- Full-fat dairy – Cheese, Greek yogurt, heavy cream (watch for hidden carbs in some dairy products)
Protein Sources (Moderate Intake)
- Meat and poultry – Beef, pork, chicken, turkey (preferably unprocessed)
- Eggs – One of the most versatile keto staples
- Seafood – Shrimp, crab, lobster, white fish
- Plant-based proteins – Tofu and tempeh, though portion control is important due to carb content
Low-Carb Vegetables
- Leafy greens – Spinach, kale, arugula, lettuce
- Cruciferous vegetables – Broccoli, cauliflower, Brussels sprouts, cabbage
- Other low-carb options – Zucchini, asparagus, bell peppers, mushrooms, cucumber
These vegetables give you important fiber, vitamins, and minerals without adding too many carbs. The NHS notes that fiber intake can drop on keto because you eat fewer grains, so eating plenty of non-starchy vegetables helps keep your digestion healthy.
Foods to Avoid on a Keto Diet
To stay in ketosis, you need to cut out or greatly reduce foods that are high in carbs. Even foods that are usually considered healthy, like fruit and whole grains, can quickly push you over your daily carb limit.
High-Carb Foods to Eliminate
- Grains and starches – Bread, pasta, rice, oats, quinoa, cereals
- Sugary foods – Candy, pastries, cakes, cookies, ice cream, soda
- Most fruits – Bananas, apples, oranges, grapes (small portions of berries like strawberries and raspberries are acceptable)
- Legumes – Beans, lentils, chickpeas, peas
- Starchy vegetables – Potatoes, sweet potatoes, corn, carrots (in large amounts)
- Low-fat and processed foods – These often contain added sugars and hidden carbs
Beverages to Limit or Avoid
- Sugary drinks – Fruit juice, sweetened coffee drinks, energy drinks
- Alcohol – Beer and sweet cocktails are high in carbs; dry wine and spirits, in moderation, are sometimes acceptable, but alcohol can slow ketone production
The strictness of these strict rules can make starting keto feel tough at first. But many people find that after they get used to checking nutrition labels and spotting hidden carbs, planning meals gets easier.
Keto Symptoms
In the First Few Weeks getting into ketosis doesn’t always go smoothly. As your body switches from using carbs to fat for energy, you might feel some symptoms known as the “keto flu.” Despite the name, it’s not a real illness but just a short adjustment period.
Common Early Symptoms
In the first three to seven days, you might feel tired, get headaches, feel irritable, have trouble focusing, feel sick, or get muscle cramps.
These symptoms happen because your body is using up its glycogen, losing water and electrolytes, and getting used to a new energy source.
Research in Frontiers in Nutrition shows that imbalances in electrolytes, especially low sodium, potassium, and magnesium, play a big role in keto flu symptoms.
To feel better, make sure you drink enough water and replace lost electrolytes. You can add a little salt to your meals, eat foods high in potassium like avocados and leafy greens, and think about taking a magnesium supplement (usually 300-400mg daily).
If you take medicine for blood pressure or diabetes, talk to your doctor before adding more salt, since keto can change your blood pressure and blood sugar.
Energy and Hunger Changes
After the first week or two, most people say they think more clearly, have steady energy without blood sugar crashes, and feel less hungry.
This is because ketones give your brain a steady fuel source, and fat helps you feel full. A study in the American Journal of Clinical Nutrition found that people on keto often eat fewer calories without feeling hungry, likely because of this effect.
However, your athletic performance might drop for the first few weeks, especially during high-intensity workouts that need quick energy from glucose. Your endurance can improve once your body fully adapts to burning fat, but this can take 4 to 6 weeks.
Who should Avoid the Keto Diet
While the keto diet can help with weight loss and metabolic health, it’s not right for everyone. Some health conditions and life stages mean you should be careful or avoid keto altogether.
1. Conditions Requiring Caution
If you have type 1 diabetes, the keto diet carries a risk of diabetic ketoacidosis—a dangerous condition where ketone levels become excessively high.
People with type 2 diabetes who take insulin or blood sugar-lowering medications should only attempt keto under close medical supervision, as the diet can cause blood sugar to drop too low.
If you’ve had kidney stones, liver disease, or pancreatitis, be careful with keto because its high fat content and possible rise in uric acid can make these problems worse.
Also, anyone with a history of eating disorders should avoid strict diets like keto, since they can lead to unhealthy habits.
2. Pregnancy and Breastfeeding
The keto diet is usually not recommended if you are pregnant or breastfeeding. Babies and infants need enough carbohydrates to grow well, and some research links ketosis during pregnancy to possible developmental issues, though more studies are needed.
3. Medication Interactions
Keto can change how your body handles some medicines, especially those for blood pressure, diabetes, and cholesterol.
If you take prescription drugs, talk to your doctor before starting keto. Your doctor might need to change your doses as your metabolism shifts.
Getting Started: Practical Tips
If you’ve chosen to try the keto diet and it fits your health needs, a few simple steps can help make the change easier.
Plan Your Meals in Advance
The first week is usually the toughest because you’re figuring out which foods fit your macros and fighting cravings for your usual carb-rich meals.
Planning your meals ahead of time removes the guesswork and helps you avoid grabbing high-carb foods. Try making a simple three-day meal plan that you can repeat as you get used to keto.
Track Your Macros
At least initially, using an app like MyFitnessPal or Cronometer to track your food intake can help you stay within your carb limit and get enough fat and protein.
Most people can stop tracking so closely once they get a feel for portion sizes, but at first, it’s easy to eat too many carbs or not enough fat by mistake.
Aim for at least eight glasses of water daily, and consider adding extra salt to your meals or drinking bone broth to maintain sodium levels.
Magnesium and potassium supplements or foods rich in these minerals can also help prevent cramps and fatigue.
Be Patient with Your Body
Your body won’t switch to ketosis right away, and it needs time to adjust. If you eat too many carbs one day, don’t give up on the diet.
Just get back on track with your next meal. Being consistent over time is more important than being perfect every day.
Making Keto Work for You
Starting keto takes commitment, planning, and an open mind about food. It’s not a quick fix and may not work for everyone. But if you can stick to the carb limits and focus on healthy fats, keto gives you a different way to manage your metabolism, not just cut calories.
To succeed on keto, you need to prepare, be patient, and pay attention to how your body feels. If you still have negative symptoms after two weeks, or if the diet feels too hard to keep up, it may be time to rethink if keto is right for you. Good health means finding a plan that fits your goals and your life, and no one diet works for everyone.
If you decide to continue, choose whole, nutrient-rich foods, pay attention to your body, and ask a healthcare provider or dietitian for help if you need it. The best diet is one you can stick with that supports both your physical and mental health.
