Low carb high fat, or high carb low fat? Which is best? Is either best??

Everyone knows someone doing a keto/primal/high fat low carb diet, but is it the best way to lose weight or fuel your next race (in particular endurance events)? You might be looking at this from a fat loss, health, or performance perspective, but it largely all boils down to the same conclusions.

Simply put, whatever your reason for considering high fat/low fat/high carb/low carb, stop thinking one type of diet is significantly better than the other. What is best for you depends on your preferences (and adherence to the diet) and your goals.

For health, total weight loss has the greatest influence on blood pressure and cardiac risk factors, and insulin sensitivity, regardless of type of diet. Total weight loss is underpinned by the ability to adhere to a diet, so it comes back to your preference and ability to stick to a type of diet to lose weight that matters [1,2].

For fat loss, as long as calories and protein are matched, the amount of your remaining intake that is carbs or fat doesn’t matter. It is personal preference. As long as you are in a calorie deficit you will lose weight, and fat. Neither a low carb high fat (LCHF) or high carb low fat (HCLF) is superior. For reference, low carb is defined as having a max of 45% of total calories from carbs or 50-150g/day. Very low carb, or ketogenic, is defined as 50g/day or less of carbs. As a rough guide, a total weekly calorie deficit of 3500 for 0.5kg fat loss/week.

A recently published study ‘Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion’ [3] found that both low fat and low carb diets produced similar weight loss and led to similar improvement in metabolic health markers. If you can stick to a HCLF diet better, and can remain in a calorie deficit on this, then that is the type of diet you should use.

For performance, don’t look to either low carb high fat (LCHF) or high carb low fat (HCLF) as the singular best diet. Both fat and carbs are important fuels for fuelling and recovery.

They both have advantages, dependent on your goals and the length of your event, but what is without doubt is that carbohydrates are crucial for optimal performance. I highlight this as it’s entirely possible to complete a sprint or even an ironman if you are on a HFLC diet, that is not in question, but to achieve your BEST performance, your body needs carbs to fuel the most effectively.

Do you want to just finish your race slowly thinking ‘I completed it’ or do you want to finish your  race in the fastest time you can knowing you gave yourself the best shot at it?  

For high intensity exercise performance carbohydrate is a critical and very important fuel. For endurance events it is fine to eat a lower carb diet… note not a no carb diet… carbohydrate still has an important role in endurance events, and more so if performance rather than completion is your aim.

So what is the best way to fuel your training and racing? The answer is periodised carbohydrate intake. Read on to find out what this means in practice.

Periodising your nutrition around training and racing

Metabolic flexibility/adaptation

This refers to the ability to quickly and efficiently switch between the fuel source your body is using. For low intensity exercise/at rest, we want to be fat adapted so that our body preferentially uses stored fat for energy. Converting fat to energy requires oxygen and so exercise intensity must reduce for this process to occur, hence it’s use in low intensity exercise or at rest. The benefit of this is that it preserves your glycogen stores. However, when undertaking high intensity exercise we want to be able to utilise our carbohydrate stores for energy. Carbohydrate is a more efficient fuel, providing energy quickly to be used by the working muscles as it can be metabolized without needing oxygen.

In order to enhance your body’s ability to switch between fuel sources there are ways to periodise your diet in relation to your training routine to enhance your metabolic flexibility. You may have heard of fasted training, or the terms train low or sleep low. These all refer to doing some training sessions with minimal carbohydrate stores, thus forcing your body to use fat stores for energy (the underlying biological processes are to do with increased mitochondrial biogenesis and cell signalling which result in increased lipid oxidation).

Fasted training – often the first session of the day in the morning after your overnight ‘fast’, so you might have a black coffee upon waking before the session, but no food. 

Train low – can be used for the second session of a day; you fuel for your first session of the day, and then after that session you have fat/protein to eat but minimal carbohydrate, and then do your second session later in the day without having refuelled on carbohydrate. Then you have a dinner including carbohydrates after session 2.

Sleep low – after finishing training for the day have a low carbohydrate dinner before going to bed.

The important point here is that for fasted training and train low you are utilising those strategies before recovery/easier/low intensity sessions. Before your key hard workouts you need to make sure you are properly fuelled to execute the session well. This structure of working in some low carbohydrate training into a diet that is not low carbohydrate has been termed ‘Fuel for the work required’ [4].  This periodisation means you get the benefits of low carbohydrate training without the consequences of a chronic low carbohydrate intake, which actually impairs performance and can impair immunity.

To sum up

So remember, it’s not about low carb/high fat or high carb/low fat, it’s about personalised, periodised nutrition that fits alongside your training plan, ensuring you have adequate energy to train effectively while also giving you the metabolic adaptations to ensure you are in good shape for your endurance events.


  1. Dansinger ML et al. Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction: a randomized trial. JAMA 293(1), 43-53 (2005).
  2. Truby H et al. Randomised controlled trial of four commercial weight loss programmes in the UK: initial findings from the BBC “diet trials”. BMJ 332(7553), 1309-1314 (2006).
  3. Gardner CD et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion The DIETFITS Randomized Clinical Trial. JAMA 319(7), 667-679 (2018).
  4. Impey SG et al. Fuel for the work required: A practical approach to amalgamating train-low paradigms for endurance athletes. Physiological Reports 4(10), 2016.

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