Keto: The Truth About the Ketogenic Diet and Whether It Actually Works for Fat Loss

Few diets have generated as much noise in the last decade as the ketogenic diet. If you spend any time at all on fitness social media, you have almost certainly encountered someone swearing that keto is the only way to lose fat, the secret to mental clarity, and the cure for everything from diabetes to brain fog. The passion behind the keto movement is real. But passion and evidence are not always the same thing. I want to give you an honest, evidence-based assessment of the ketogenic diet so you can decide for yourself whether it belongs in your life.

What the Ketogenic Diet Actually Is

The ketogenic diet is a very low-carbohydrate, high-fat, moderate-protein dietary approach that was originally developed in the 1920s as a therapeutic intervention for epilepsy in children ⁽¹⁾. It works by forcing the body into a metabolic state called ketosis, where the absence of carbohydrate-derived glucose means the liver begins converting fatty acids into molecules called ketone bodies, which are then used as an alternative fuel source by the brain and muscles ⁽²⁾. To achieve and maintain ketosis, carbohydrate intake typically needs to be restricted to somewhere between 20 and 50 grams per day, which is an extremely small amount. For context, a single medium banana contains roughly 27 grams of carbohydrate.

What many people do not realise is that a true ketogenic diet is not just low-carb. It is also relatively low in protein, because the body can convert excess protein into glucose through a process called gluconeogenesis, which can knock you out of ketosis ⁽³⁾. This means a strict keto diet derives roughly 70 to 80 percent of total calories from fat, 15 to 20 percent from protein, and only 5 to 10 percent from carbohydrate. That is a very specific macronutrient ratio, and most people who claim to be “doing keto” are not actually following it with anywhere near this level of precision.

The Short-Term Appeal and Why People Lose Weight Quickly

I will not deny that people lose weight quickly on keto. They absolutely do. But it is essential to understand why. When you drastically reduce carbohydrate intake, your body depletes its glycogen stores. Glycogen is stored in muscle and liver tissue, and every gram of glycogen is bound to approximately three grams of water ⁽⁴⁾. The average person stores between 400 and 600 grams of glycogen, which means that glycogen depletion alone can account for a weight loss of 1.5 to 2.5 kilograms in the first week. That is not fat loss. That is water loss. It looks dramatic on the scale, and it feels motivating, but it creates a misleading impression of what the diet is actually doing.

Beyond the water loss, the ketogenic diet does create a calorie deficit for most people, and that is the real reason fat loss occurs. When you remove an entire macronutrient group and restrict yourself to high-fat foods, appetite tends to decrease. This is partly due to the satiating effect of dietary fat and protein, and partly due to the appetite-suppressing effects of ketone bodies themselves ⁽⁵⁾. Reduced hunger means reduced calorie intake, and reduced calorie intake means fat loss. This is not unique to keto. Any dietary approach that successfully reduces calorie intake will produce fat loss, whether that approach involves cutting carbs, cutting fat, or simply eating less of everything.

What the Research Actually Shows About Keto and Fat Loss

This is where things get interesting, because the research does not support the claim that keto is superior to other diets for fat loss when calories and protein are matched. A meta-analysis published in the British Journal of Nutrition, which analysed 13 randomised controlled trials comparing very low-carbohydrate ketogenic diets to low-fat diets, found that both approaches produced similar long-term weight loss outcomes when total calorie intake was controlled ⁽⁶⁾. Another tightly controlled metabolic ward study conducted by researchers at the National Institutes of Health found no significant difference in body fat loss between a ketogenic diet and a high-carbohydrate diet when calories were precisely matched ⁽⁷⁾. The keto diet did not burn more fat. It did not provide a metabolic advantage. It simply provided a different framework for eating less.

This is a point I make repeatedly with my clients: the best diet for fat loss is the one you can actually stick to. A systematic review published in the Journal of the American Medical Association examined the effectiveness of major branded diets and concluded that dietary adherence was the strongest predictor of weight loss success, irrespective of the specific macronutrient composition of the diet ⁽⁸⁾. If keto helps you eat less and you enjoy it, it can work. But it is not magic, and it is not superior.

The Downsides of Keto That Deserve Honest Discussion

There are several practical and physiological downsides to the ketogenic diet that proponents rarely discuss. The first is the impact on training performance. If you train with any kind of intensity, whether that is resistance training for muscle building or high-intensity interval work for fat loss, your muscles rely heavily on glycogen as a fuel source. Depleting glycogen stores and failing to replenish them compromises your ability to train hard, and if your training quality drops, your results will follow. Research published in the Journal of Sports Medicine and Physical Fitness found that athletes on a ketogenic diet experienced significant reductions in high-intensity exercise performance compared to those consuming adequate carbohydrates ⁽⁹⁾.

The second issue is fibre. The ketogenic diet severely restricts most high-fibre foods including whole grains, legumes, most fruits, and many vegetables. As I discussed in detail in my carnivore diet article, fibre plays a critical role in gut health, cardiovascular health, and long-term disease prevention ⁽¹⁰⁾. Eliminating or drastically reducing fibre intake is not something I would recommend for anyone pursuing long-term health.

The third issue is social and practical sustainability. Keto is an extremely restrictive diet in a world where carbohydrates are everywhere. Eating out becomes difficult. Social occasions become stressful. Travelling with the diet is a logistical challenge. For my clients who are busy professionals, parents, or frequent travellers, this level of restriction is simply not practical over the long term. And a diet you cannot sustain is a diet that will ultimately fail you.

For vegetarians and vegans, keto presents an additional challenge. The diet relies heavily on animal-based fats and proteins, and while a plant-based keto diet is theoretically possible using foods like avocado, coconut oil, nuts, seeds, and tofu, it is extremely restrictive and makes hitting adequate protein intake very difficult. As a lifelong vegetarian myself, I can tell you from personal experience that keto is one of the hardest dietary frameworks to follow on a plant-based diet, and the restriction it imposes is simply unnecessary when other approaches deliver the same results with far more flexibility.

Where Keto Can Be Genuinely Useful

I want to be balanced about this because keto is not without merit. The evidence supporting its therapeutic use in epilepsy management is robust and well-established ⁽¹⁾. There is also emerging research suggesting potential benefits for individuals with type 2 diabetes, as the carbohydrate restriction naturally lowers blood glucose and can improve insulin sensitivity in the short term ⁽¹¹⁾. For individuals with specific metabolic conditions, a ketogenic approach under medical supervision can be a valuable tool.

I also find that a short-term, lower-carbohydrate phase (not necessarily full ketosis) can be a useful kickstart for some clients at the beginning of a fat loss phase. It simplifies food choices, reduces reliance on processed carbohydrates, and creates a rapid initial drop on the scale that builds confidence and momentum. I have written about this approach in my article on why I start most fat loss clients on a short-term carb reduction phase. The key word there is short-term. Using carbohydrate restriction as a temporary tool is very different from adopting keto as a lifelong dietary identity.

What I Would Recommend Instead

If your goal is fat loss, the principles that actually matter are well-established and do not require you to eliminate carbohydrates from your life. You need a sustained calorie deficit. You need adequate protein intake to preserve lean muscle mass and support satiety, ideally around 1.6 to 2.2 grams per kilogram of body weight per day ⁽¹²⁾. You need sufficient fibre from vegetables, fruits, and whole grains to support your gut health and keep you feeling full. You need healthy fats in appropriate amounts. And you need a plan that fits your life, your food preferences, and your schedule so that you can actually stick to it week after week, month after month.

Whether you eat meat, follow a vegetarian diet, or are fully plant-based, these principles apply universally. The specific foods you choose to meet these targets will differ based on your preferences and your body, but the underlying framework is the same. That is why I do not subscribe to any single dietary ideology. I subscribe to evidence and to finding what works for each individual client.

The Bottom Line

The ketogenic diet is a legitimate dietary tool that works for fat loss through calorie restriction, not through any unique metabolic magic. It is not superior to other well-structured dietary approaches when calories and protein are equated. It comes with meaningful trade-offs in terms of training performance, fibre intake, micronutrient variety, and long-term sustainability. And for many people, it is far more restrictive than it needs to be to achieve their goals.

If you have tried keto and it worked for you, I am not here to tell you it was wrong. But if you are considering it because you think it is the only way to lose fat, or because someone on social media told you carbohydrates are poison, I would encourage you to look at the evidence more broadly. There are simpler, more sustainable, and equally effective paths to the body you want.

If you want help finding yours, get in touch and I will build a plan around your life, your goals, and the foods you actually enjoy eating.