Low fat versus low carbohydrates: An in-depth discussion into the impact they have on weight loss by Nutritionist Robert Hobson

For many of us, the New Year brings good intentions of reducing our alcohol intake, making use of neglected gym memberships and trying to lose some of the excess weight we’ve gained over the Christmas period!

The New Year also means a new wave of diet books and tips on the best and quickest methods to lose weight. At a basic level, every new diet regime provides a way of reducing your overall calorie intake, which should be partnered with more exercise (basic equation of calories in versus calories out).

One of the most debated topics in nutrition is fat versus carbohydrates and the impact they have on weight loss, as almost all weight loss diets are centred around the ratio of these two nutrients (as well as intake of proteins, which impact on factors such as satiety and muscle integrity).

The traditional dietary advice for losing weight is to follow a low fat diet and is favoured by the Department of Health. This is where you cut down on your overall intake of fat, which helps to reduce your overall calorie intake and lead to a steady reduction in body weight over time, an approach that many people find easier to maintain in the long-term, as opposed to short-term fashionable diets.

The low fat diet also requires sticking to a reduced number of calories following other key healthy eating messages such as reducing your intake of saturated fat and sugar, eating oily fish and plenty of high-fibre foods such as whole grains, pulses, fruits and vegetables, which can impact on long-term health. There is no recommendation in this diet to reduce your carbohydrate intake, but guidance suggests that half your calorie intake should come from foods rich in this nutrient.

The low fat diet has helped lots of people lose weight and is thought to be a more practical approach, especially when eating out as you can look for foods that use low fat cooking methods and check food labels for their calorie and fat content. This approach is also considered to be sustainable as you apply the basic principles of healthy eating to the way you choose foods in the long term.

However, some question the simplicity of ‘calories in and calories out’, as not all diets work the same way and some make use of manipulating how the body metabolises our main energy providers, carbohydrates and fats, which is the basis of low carbohydrate diets such as Atkins and Dukan. These diets have caught the public’s attention with the promise of quick weight loss, whilst being able to eat high-protein foods and those high in fat such as bacon, cheese and cream, whilst never feeling hungry. But how does this compare to the traditional weight loss message given out by the Department of Health.

Firstly, the science behind the low carbohydrate diet differs to that of low fat. When you eat a food rich in carbohydrate, the body breaks it down into glucose (the simplest of sugars). In response to raised blood sugar levels, the body releases insulin whose job it is to direct glucose out of the blood and into cells to be used for energy. What doesn’t get used for energy is stored in the liver and muscles as glycogen. Insulin also drives glucose into fat cells to be stored as triglycerides, which can occur when there is any excess of glucose in the blood.

When you remove virtually all carbohydrates from the diet such as the initial phase of the Atkins diet (prescribes just 40g of carbohydrate daily, which is the equivalent to about two slices of bread). In the absence of glucose from carbohydrates, the body is forced to use some protein and also fat stores for energy. Fats are converted to ketones that the body uses to supply energy to cells of organs such as the brain that usually favour glucose. In essence you body is burning fat, which is the basis for the weight loss and losing fat tissue is generally better for your health.

Low carbohydrate diets are effective, backed up by plenty of studies that report quicker weight losses and reductions in body fat compared to a traditional low fat diet (2, 3, 4). Researchers have even suggested that this may be a better approach for treating obesity. However, low fat diets can also help you to lose weight and studies have shown that both low fat and low carbohydrate diets can lead to significant weight losses (5). It’s also not fully clear how effective the weight losses are in the long term between the two approaches, although it has been suggested that both are similar over time.

Some health bodies including the Department of Health worry that low carbohydrate diets encourage the intake of saturated fat that may impact on your risk of heart disease, but research shows that a moderately low carbohydrate diet may benefit the heart as long as proteins and fats come from healthy sources (6).

Some of the unpleasant side effects of following a very low carbohydrate diet are fatigue, fogginess, irritability, headaches and smelly breath, which are the result of the body switching to ketones over glucose for energy, which in the long term can be damaging to your health. There is also the question of cutting out food groups such as fruits and other foods high in fibre, a nutrient linked to a reduction in heart disease risk and cancer and one that is already in short supply in most peoples diets in the UK.

Diets are considered by most to be temporary, meaning they have an end goal. However you decide to lose weight, bear in mind, Its important to think about how you are going to keep the weight off that you have worked so hard to lose. Try adopting small changes that can be followed through to the way you eat after you have lost the weight, whether that is to eat a few less processed carbs, smaller portion sizes or choose low fat varieties of dairy foods.

The decision of what diet to choose is really up to you and there is no right or wrong approach as sticking to either one will result in weight loss. Unfortunately the science and debating has detracted away from the fact that the most effective diet is going to be the one that you stick to. There is too much significance placed on the nutritional composition of diets when the biggest challenge is finding strategies that help people to adhere to them.


2. Hu T1, Mills KT, Yao L, Demanelis K, Eloustaz M, Yancy WS Jr, Kelly TN, He J, Bazzano LA (2012). Effects of low-carbohydrate diets versus low-fat diets on metabolic risk factors: a meta-analysis of randomized controlled clinical trials. Am J Epidemiol. Oct 1;176 Suppl 7:S44-54.

3. Foster GD, Wyatt HR, Hill JO, et al (2003). A randomized trial of a low-carbohydrate diet for obesity. N Engl J Med. 2003;348:2082-90.

4. Samaha FF, Iqbal N, Seshadri P, et al (2003). A low-carbohydrate as compared with a low-fat diet in severe obesity. N Engl J Med. 348:2074-81.

5. Una Bradley, Michelle Spence, C. Hamish Courtney, Michelle C, McKinley, Cieran N. Ennis, David R. McCance, Jane McEneny, Patrick M. Bell, Ian S. Young and Steven J. Hunter (2009). Low-Fat Versus Low-Carbohydrate Weight Reduction Diets
Effects on Weight Loss, Insulin Resistance, and Cardiovascular Risk: A Randomized Control Trial. Diabetes. Dec; 58(12): 2741–2748

6. Halton TL, Willett WC, Liu S, et al (2006). Low-carbohydrate-diet score and the risk of coronary heart disease in women. N Engl J Med. 355:1991-2002.