It’s a truth universally acknowledged that losing weight is easy – it’s keeping it off that’s the real struggle. One of the explanations often given is that dieting makes your metabolism slow down so that you need fewer and fewer and calories just to stay the same weight – and if you start gradually eating a little more than this, almost unnoticed the weight creeps back on.
This may be because when you take in less food than your body is used to, it reacts as though to a famine and goes into survival mode – slowing down the rate at which you burn energy and storing what you do take in as fat to see you through the famine.
However, some research just out indicates that the content of the diet may play a crucial factor in helping to keep your metabolism busy – even if you are taking in fewer calories.
Low fat diets slow you down
The research shows that dieting definitely slows your metabolism down, but it slows down the MOST on a low fat diet and the LEAST on a low-carbohydrate diet. The difference in total energy expenditure (TEE) between these two diets was about 300 calories a day – the equivalent of what you would burn in one hour of moderate-intensity exercise!
This research is worth looking at because it was a detailed, complex and ‘controlled’ study carried out with overweight or obese people aged between 18 to 40 years. It was conducted over a period of four years (2006-2010) by a large team from several organisations: the Children’s Hospital Boston, Massachusettes, Brigham and Women’s Hospital, Massachsuetts, the US Department of Agriculture, Vanderbilt University and the New Balance Foundation Obesity Prevention Center.
The main outcome they were looking for was changes to levels of ‘resting energy expenditure’ – or REE – as a result of the diets. REE is your base metabolic rate – the amount of energy you need to run all your bodily functions even when you are simply lying down at rest. TEE is the total amount of energy you use in a day.
How did they do it?
They started the study by putting everyone on the same ‘run in’ diet which helped them achieve between 10% and 15% weight loss (the run-in diet had 45% energy from carbohydrates, 30% from fat and 25% from protein). There were 32 people at the run-in phase and 21 people who completed the whole trial.
After the run-in, the participants were then split into three groups each consuming a diet which was either (1) low fat, (2) low-GI or (3) low-carbohydrate, although the total amount of calories in each diet was the same (isocaloric)
- The low fat diet had 60% of energy from carbohydrate, 20% from fat and 20% from protein (based on the conventional advice for a healthy diet)
- The low-GI diet had 40% energy from carbohydrate, 40% from fat and 20% from protein
- The low-carbohydrate diet had 10% of energy from carbohydrates, 60% from fat and 30% from protein
Each group had their REE tested at the end of this phase of the diet both in hospital and under ‘free-living’ conditions.
The groups were then swapped to one of the other diets, and the trial was repeated, and then swapped again – so that everyone got to try all three diets. In effect, this type of experiment (cross-over) means that each person acts as their own ‘control’ to reduce the risk of confounding variables. The order of the diets was randomly assigned for each participant.
What were the results?
the low fat diet predicted weight regain, both because it slowed metabolism the most and also affected the production of the hormone leptin, which regulates appetite (leptin resistance or lack of leptin means you almost always feel hungry). This diet was based on whole grains, fruit and vegetables.
the low-carbohydrate diet (based on the Atkins diet) had the most beneficial effects on energy expenditure and also improved some other health measures associated with the metabolic syndrome.
the low-GI diet had similar, but smaller, beneficial effects to the low-carb diet.
However, the researchers warn that levels of cortisol (the stress hormone) and C-reactive protein (which is linked to inflammation) were raised on the low-carbohydrate diet, which they call a ‘restrictive regime’ – but the study was not able to identify why the low-carb diet should have this effect.
Because of this, they say that in the long-term, the GI diet might be better for people’s health.
I’d love to know if the low-carbers were eating the kind of stuff often associated with low-carb – such as highly-processed protein bars, soya flour and processed seed oils such as maize and sunflower oils and chemical sweeteners such as aspartame – all of these are deleterious to health in some way.
If they were eating real (unprocessed) foods which are naturally low carb like this (see Foodwiser Pyramid) I would take a large bet that they would not be in a stressed and inflammatory state!
Effects of Dietary Composition on Energy Expenditure During Weight-Loss Maintenance
Cara B. Ebbeling, PhD; Janis F. Swain, MS, RD; Henry A. Feldman, PhD; William W. Wong, PhD; David L. Hachey, PhD; Erica Garcia-Lago, BA; David S. Ludwig, MD, PhD
Journal of the American Medical Association
June 27, 2012