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Carbohydrates, fat burning and fat balance. Confusion!

A change in body tissue mass is most closely related to energy balance over time. As such, reducing carbohydrates is one viable method to initiate fat loss if it leads to a prolonged energy/calorie deficit over time. But carbohydrates alone are not inherently fattening.

Of course, carbohydrates can contribute to fat gain if in an energy/calorie surplus: indirectly by blunting fat oxidation (the burning of fatty acids for fuel) or directly via ‘de novo lipogenesis’ (the process of converting carbohydrates to fats). De novo lipogenesis is, however, an energy-costly process and does not occur in most real-life scenarios. Dietary fat, on the other hand, does not need to be converted and is stored in adipose tissue far more efficiently.

At this point it is important to differentiate between fat burning (i.e. fat oxidation) and loss of body fat. For most people, the term ‘fat-burning’ will simply mean that you’re losing body fat. For those who have a deeper understanding of human physiology, fat burning can be viewed as slang for fat oxidation. Fat oxidation occurs when we oxidise fatty acids to generate ATP (adenosine triphosphate) which can be used for energy production.

Key point – just because you are burning more fat for fuel (eg, on a high-fat and/or ketogenic diet) does not necessarily mean we are burning the fat that is stored in our fat tissue and thus losing body fat. Remember, changes in body fat tissue are most closely related to energy balance over time and as a result body fat loss will be determined by net fat balance (fat storage minus fat oxidation). So, yes, although you do burn more fat on a high-fat diet, you have more of it to burn.

A classic study by Hall and colleagues in a tightly controlled setting showed that carbohydrate restriction was not physiologically advantageous for fat loss. The study took place in a metabolic ward and looked to compare the effect of different carbohydrate and fat intakes on energy intake, output, and substrate balance while holding calories and protein constant during hypocaloric conditions (30% energy deficit). Interestingly, net fat balance was reduced to a greater extent in the restricted fat group i.e. subjects who restricted fat intake lost more body fat, albeit ~36g/day.

A more recent, year-long randomised control trial found that a low-carb and a low-fat diet produced similar weight loss and improvement in health markers. Insulin production was also found to have no impact on predicting weight loss success. The results of this study contribute to a large body of evidence suggesting that, for fat loss, neither low-carb nor low-fat is superior – once total calorie intake and protein intake are equated.

If insulin and carbohydrate consumption alone were truly the cause of fat gain and therefore a major contributor towards the obesity-epidemic, then why are there numerous tribes across the world who eat high carbohydrate diets and are devoid of obesity-related diseases? Some examples include the Okinawans of Japan, Kitavans of the Pacific Islands, and Kuna Indians in Panama. The Kitavans, who consume a diet with almost 70% of calories coming from carbohydrates, are described ‘by extreme leanness (despite food abundance)’.

​With overwhelming evidence showing no meaningful difference between a low-fat diet and a low-carb for fat loss success, the question remains, why do so many people get fantastic fat loss results going on a low-carb diet? 

It is important to acknowledge that restricting carbs from the diet leads to a reduction in a whole host of calorie-dense goodies: eg, pizza, burgers, biscuits, pastries, doughnuts, to name just a few. But note – these foods contain dietary fat too! The combination of both dietary fat and carbohydrate together, positively impacts the palatability of a food. The tastier and/or more rewarding a certain food is, the easier it is to over-consume.

Restricting foods similar to the examples listed above, can often automatically improve the nutrient density/quality of the diet. Additionally, consuming less carbohydrate may have the added benefit of increasing total dietary protein intake. Increasing protein intake can lead to a spontaneous reduction in total calories, creating an energy imbalance. As a result, fat loss shortly ensues.

A good number of Bodyscan clients report switching to a keto diet before their first scan and some generate great results in their subsequent DEXA reports. But, as explained above, they are simply consuming far fewer calories as a result of cutting out carb-laden (and often fat-laden too) foods from their diet. It is ridiculous to think you can eat 5000 calories of dietary fat a day and not get fat, when your maintenance calories are only 2000. See this blog on the big insulin myth.

The truth is that cutting carbs can be a viable fat loss strategy for some, if it helps you eat less. If, however, cutting carbs negatively affects your performance, makes you feel miserable, moody, and always hunger, you should consider alternative options.

To summarise, adherence is the make or break of dieting success and the biggest predictor for long-term weight maintenance. Dietary strategies that optimise adherence should be the primary focus for those looking to lose body fat. This process can be enhanced by regular professional contact and guidance, to help implement bespoke behaviour and lifestyle modifications to support one’s goals.

A low-carbohydrate diet is simply a tool in the fat-loss toolbox and works wonders for some. Is it an absolute necessity for fat loss success for everyone, in every situation? Definitely not!

Kevin Garde
Nutritionist & Bodyscan Consultant
PRISM Nutrition

DEXA will prove your diet is working (or not!)

Dual-energy X-ray absorptiometry is the gold standard for measuring body composition and is highly consistent for measuring even small changes in body fat, as we explain in this video from our DEXA information series. Comparing DEXA to bio-impedance devices and skinfold calipers, it proves to be the most reliable between scans.

Therefore, if you’re wanting to see for yourself how your diet and exercise programme is working (or not working) for you, have a scan now and then one every three to four months to monitor your progress.

Bodyscan has two clinics in central London – Cornhill in the City (near the Bank of England) and at the northern end of Marylebone High Street. Full addresses are on our contact page.

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