The ketogenic primer

Keto is the diet everyone is talking about, but does it really work for weight loss? Fitness First editor Tony Sarno tries it out.

Now that several Hollywood stars are on it, the ketogenic diet has crossed from fitness fad into the mainstream. It promises to turn your body into a fat-burning machine, reduce inflammation, improve your metabolism and even boost cognition.

The ketogenic diet forces you to change fuel systems — from glucose (carbohydrates) to fat. The only time this happens in nature is when you’re starving and your body prioritises burning your fat reserves to spare your muscles as long as possible. The ketogenic diet effectively tricks your body into thinking it’s starving without actually starving.

It’s an interesting theory, but does it work? Here’s what happened to me when I tried the ketogenic diet.

The ketogenic diet

Theory: The ketogenic diet is a high-fat diet that bans carbohydrates to starve your body of glucose and make it burn fat instead. Many argue that our ancestors evolved largely with a ketogenic-like diet, thriving on fatty animal meats and low-starch plants and tubers.

Reality: Because calories in/calories out holds true for every diet, science struggles to prove that a ketogenic diet is superior to a low-fat one. Where the ketogenic diet appears to triumph is at suppressing hunger. Switching to a fat-powered metabolism and low-carb eating plan spares you the hunger cravings and blood sugar drops that a reduction in calories normally brings on. I found that the ketogenic diet does not magically melt your fat away if you don’t reduce your calorie intake as well, but it makes it much easier to live on fewer calories.


Theory: Ketosis is when your body burns fat for fuel instead of glucose. The simplest way to induce ketosis is to eat no more than 50g of carbs a day. When you burn fat your liver produces ketone bodies, little packets of fat energy that replace glucose. It can take from three days to two weeks to clear all the glucose from your system and go into ketosis.

Reality: It took me six days to start transitioning to ketosis because I didn’t realise that you also need to limit your protein intake, which can also convert to glucose. The recommendation is no more than 1g of protein per kg of bodyweight a day.

Reaching ketosis

Theory: As your body transitions into ketosis, your brain craves the glucose it doesn’t have and you can get symptoms that resemble a flu, including headaches, brain fog, fatigue and a general irritability. These can be accompanied by bad breath and constipation.

Reality: On day six of no carbs I felt like I had a rubber band around my head that was getting tighter and tighter as the day went on. At first I felt a bit spaced out, then I became forgetful, joking to my colleagues that I had “keto brain”. But to be honest, it didn’t feel great.

In ketosis

Theory: When you finally enter fat-burning mode, the brain fog and misery clear and you feel more alive and alert. Your energy seems to last longer without the peaks and troughs of carb-based energy. Some people claim the experience feels like a drug high.

Reality: I was really looking forward to the “high” experience, in the name of science, but sadly when I achieved ketosis it passed me by. I just realised that the brain fog was gone. There has been one weird effect while on ketosis: the likes on my tweets have gone through the roof, as if I’m tuning my posts better to the social zeitgeist.

Staying in ketosis

Theory: Keto enthusiasts claim they stay in ketosis indefinitely and are the healthiest they’ve ever been, but there are no long-term studies that prove this. Nutritionists also point out that good gut bacteria need carbs and fibre to thrive, and cutting these out will negatively impact on your gut health.

Reality: Staying in ketosis is hard work. Just two slices of bread will throw you out of it and getting back means all the misery of transitioning again. It’s not a sustainable lifestyle change without lots of discipline and preparation.

So, does it work?

Theory: According to health professionals claiming to be ketogenic diet experts, you should lose at least 0.5-1kg each week on the keto diet.   

Reality: I don’t want to sound like a sucker for the latest diet fad, but after just three weeks of ketosis, I’ve lost 5kg. Of course, that could be entirely fluid loss, but I also know that I’m just not eating as much without the carb cravings. I’ve also noticed that I’m having less peaks and troughs mentally and my energy is steady and even.

10 keto tips

  1. Banish carbs from your life. No more than 50g per day.
  2. Replace carbs with more fat, not more protein.
  3. Get your fat from meat and fish, avocados, olive oil, butter and nuts.
  4. Limit protein to no more than 1g per kg of bodyweight per day.
  5. Eat lots of vegetables, but stay away from starchy ones like potatoes.
  6. If you’re not sure whether a food is keto approved, Google is your friend.
  7. Meal prep, because you won’t find keto-friendly takeaway meals easily.
  8. When the brain fog kicks in, that’s when you’re transitioning to ketosis.
  9. You still need to drop calories for the diet to work. Big trap, this.
  10. Be vigilant, because a few too many carbs will kick you out of ketosis.