Cut fat not carbs to lose more weight, say US researchers

Carbs Bread and Butter
Bread and butter - but cutting which one is best for weight loss?

Scientists at the National Institute of Health (NIH) in the United States have found that a diet low in fat leads to greater weight loss than a low carb diet.

A team of researchers, led by Dr Kevin Hall, of the National Institute of Diabetes and Digestive and Kidney Diseases in the US controlled the diets of 19 obese men and women, who were voluntarily confined to a hospital ward. The team scrutinised the respiration and exercise levels of the patients and subsequent weight loss and discovered that cutting fat as opposed to carbs resulted in more body fat loss among the subjects.

The study, published in the journal Cell Metabolism, found that the fat restricted diet resulted in 80% more fat loss than the carb restricted diet. Body fat loss was measured after a period of six days, with an average 463g of fat lost compared to 245g lost on the carb restricted diet.

The low carb diet - where proponents cut out or drastically limit foods such as pasta, white bread and foods containing refined sugars - has grown increasingly popular in recent years, sworn by celebrities and bolstered by the belief that cutting out carbohydrates alters the body's metabolism. A diet scarce in carbohydrates does result in lower insulin levels which then forces the body to dip into its own fat reserves, commonly referred to as 'fat burn'.

This study appears to demonstrate that despite this metabolic effect, cutting fat still results in greater body fat loss in the short-term.

Dr Hall told the BBC News site: "If it's easier to stick to one diet than another, and to ideally do it permanently, then you should choose that diet."

He added that those who find a low fat diet works for them are not "at a metabolic disadvantage" as some may have previously thought.

Low fat versus low carb

The effects of both low fat and low carb diets were analysed and compared following a five-day balanced diet which provided 2,700 calories consisting of 50% carbohydrates, 35% fat and 15% protein to establish a baseline. The study subjects then received both the low fat and the low carb diet during two separate inpatient stays spaced 2-4 weeks apart. During the diet, the subjects overall calorie intake was cut by 30% - with restrictions on either fat or carbs. Protein quantities remained static.

Body fat loss was calculated as the difference between daily fat intake and fat oxidisation whilst residing in a metabolic chamber for 23 hour periods during the baseline period and also during the low fat/low carb diet periods, with the results compared. Scientists utilised various methods in order to determine the exact chemical processes that were taking place inside the body, including urine analysis and exhaled oxygen and carbon dioxide levels.

'Behavioural strategies'

Prof. Susan Jebb, Professor of Diet and Population Health at the University of Oxford, described the study as "well conducted", but cautioned: "Life in the confines of a metabolic ward with strictly limited access to food can only address the metabolic response to food and not the behavioural response to the environment. The investigators rightly conclude that given the minimal metabolic differences between low fat or low carb diets, the best diet for weight loss is the diet you can stick to. The real challenge for science is not the nutritional composition of the diet, but the behavioural strategies to promote adherence. All diets "work" if you stick to an eating plan that cuts calories, whether from fat or carbohydrate, but sticking to a diet is easier said than done, especially given the prolonged time it takes to lose weight."

Prof. Nick Finer, Honorary Professor, National Centre for Cardiovascular Prevention and Outcomes, UCL Institute of Cardiovascular Science, says: "The study elegantly confirms that certainly in the short-term ‘calorie for calorie, restriction of dietary fat led to greater body fat loss than restriction of dietary carbohydrate’ in these obese adults and that theories that carbohydrate restriction are needed to lose body fat are wrong and that reducing insulin (as is seen with low-carbohydrate diets) is not necessary to increase fat-burning.

"In practice, evidence points to reduced fat diets being the most successful long-term, while increasing protein may enhance satiety or fullness. What really determines the success or failure of weight loss diets is how well people can adhere to them and this goes beyond just their nutrient composition involving issues of palatability, convenience, hunger, personal beliefs."

Dr Hall and his team now plan to analyse brain scans taken during the research to examine the relationship between different foods and reward systems within the brain.


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