So you've started a low-carb diet to reap the benefits, which include more energy, mental clarity, and increased focus (1).
You've reduced your carb intake, added more healthy fats, and added more vegetables, so when can you expect to start to notice changes?
The answer is, unfortunately, that it depends. Some people notice changes within two weeks, others may not notice a change until 3–4 weeks.
It depends on a few factors:
Your Diet Before Becoming Low-Carb
Were you binging on high-carb items weekly or daily? Did you have to make serious changes to adapt to this low-carb lifestyle? First off, congrats on making the change. However, because your body is so used to high-carb amounts, it may take longer for your body to adapt and for your mentation (clarity, focus, mindset) to change.
Your Energy Intake
If you aren't getting enough calories, you will still feel sluggish and fatigued. Low-carb can only do so much for your brain if you aren't feeding your body the calories it needs. If you are regularly exercising, you may need even more calories. How do you make sure you are getting enough? Use this niKETO calculator to make sure you are meeting your nutrition needs. In general, even if you are trying to lose weight, you shouldn't go below 1200 calories per day.
If you are setting goals for your carbs, fat, and protein, are you hitting them? Or do you notice you overdo it on the carbs? If so, try using a meal tracker to identify sources of hidden carbs. If you have been low-carb and are not seeing any changes, try lowering your carb allotment for the day. Experiment with small changes in your diet, you may be a few grams of carb away from reaping the benefits of low carb diets!
Ultimately, changes take time. Patience is important with any lifestyle change. You may notice small changes, like no longer needing that second cup of coffee at work. Most low-carb dieters say the changes aren't like a flipped switch; you don't wake up one day with new found energy.
It is usually a gradual series of changes leading to more consistent energy throughout the day. For example, you may find your workouts become easier, or you may notice yourself being more productive at work.
Be patient with yourself and allow your body to adapt to your new lifestyle!
(1) White, H. & Venkatesh, B. (2011) Clinical review: ketones and brain injury. Critical Care (London, England), 15, 219. http://europepmc.org/abstract/MED/21489321.