Low-fat diet 'better' than low-carb diet for getting rid of body
fat - National Library of Medicine Behind The Headlines - Health
News from NHS Choices Fri, 14 Aug 2015 13:33:00 EST "Low-fat diets
'better than cutting carbs' for weight loss," says BBC News. But
wait, the Mail Online says: "Lowcarb 'is best for weight loss".
Confused? Traditionally, weight-loss diets were based on the
concept of eating a low-fat diet. But in recent years the idea of
low or no carbohydrate diets, such as the Atkins diet, have become
popular. A new diet lab rigorously enforced either a
low-carbohydrate or low-fat diet for 19 obese men and women over
six days. The six-day low-carb diet led to more weight loss than a
low-fat diet, but the low-fat diet led to more fat loss. And losing
fat tissue is generally better for your health. This means both the
BBC and Mail Online are technically correct. The study was small,
short term, and the diets were quite extreme. This makes them less
relevant to most people's everyday life or efforts to lose weight.
They aren't convincing enough to settle the low-carb versus lowfat
diet debate. But you could argue the whole low-fat versus low-carb
debate is a needlessly overcomplicated distraction from what should
be four simple words of advice – eat less, exercise more. The
principle of combining physical activity with reduced calorie
consumption is at the core of the NHS Choices weight loss plan – a
sustainable way to lose weight without relying on any gimmicks.
Where did the story come from? The study was carried out by
researchers from the US National Institutes of Health and was
funded by the same organisation. It was published in the
peer-reviewed science journal Cell Metabolism on an open-access
basis, so it is free to read online or download as a PDF (PDF,
1.7Mb). The BBC and Mail Online ended up with opposite headlines
based on the same study. The BBC went with "Lowfat diets 'better
than cutting carbs' for weight loss", while the Mail Online said
that, "Low-carb 'is best' for weight loss". This confusion is
understandable. The low-fat diet led to more fat loss – the outcome
the study was most interested in – but the low-carb diet led to
more weight loss overall.The researchers argue fat loss was more
important to the obese people in this study for long-term weight
loss. This implies the weight loss achieved in the low-carb diet
was not down to fat loss. Although this wasn't stated in the study,
you can lose weight via losses in muscle and water. Both the BBC
and the Mail carry a useful quote from an independent expert,
Professor Susan Jebb, who rightly says: "The best diet for weight
loss is the diet you can stick to". So outside of the highly
restricted diet lab, the bigger issue of weight loss is how to
stick to a diet in the long term. What kind of research was this?
This human laboratory study looked at the way two short-term diets
– one low in fat, one low in carbohydrates – affected a person's
metabolism and any weight loss. Your body gets energy from two main
sources: by burning fat and carbohydrate. Both are regulated by the
hormone insulin. Popular weight-loss diets often advise cutting
down either fat or carbohydrates, but there is debate about which
works best. This study wasn't focused on weight loss, but was
geared up to investigate how diet influences how the body burns fat
and carbohydrate. The researchers highlighted past randomised
control trials showing greater shortterm weight loss in obese
patients on low-carb diets. But they say there are problems with
these studies, meaning we don't know if they help you lose weight
by changing your metabolism for the better – lowering insulin
levels and causing more fat burning and energy expended as a whole
– or simply make you eat less overall. Low-carb diets can be higher
in protein and fat, which are filling, making you eat less. This
wasn't the case in this study. What did the research involve? The
study confined 19 obese adults (9 men, 10 women) to a "metabolic
ward", or diet lab, for two two-week periods while on low-fat or
low-carb diets. While in the diet labs, researchers meticulously
monitored and restricted their diets, energy intake and
expenditure, and used a host of biological measures to establish
whether they were burning fat or carbohydrates as their source of
energy. Each person spent five days in the diet lab on an
energy-balanced diet (50% carbohydrate, 35% fat, 15% protein)
before being randomly assigned to a diet that slashed their calorie
intake by 30% (around 800 calories a day lower) for a further six
days. This was achieved either through a 60% reduction in carbs
(low-carb diet) or an 85% reduction in fat (low-fat diet).
Throughout the six days they had no access to additional food or
drink, and even meetings with visitors were watched by nurses or
researchers to catch any potential cheating. After a two- to
four-week washout period where they could eat what they liked,
volunteers were readmitted totry the other diet. This repeated the
five-day balanced diet followed by the alternate six-day low-fat or
low-carb diet. Some physical activity was controlled – they all had
to do 60 minutes of walking on a treadmill each day – and the rest
was monitored using portable activity monitors worn on the hip. The
study excluded those who had large weight change (more than 5kg) in
the past six months, had diabetes, were menopausal, were pregnant
or breastfeeding, or had mobility problems. Two men dropped out of
the study after the first stint on the low-carb diet, so did not
contribute data to the subsequent low-fat diet part of the study.
What were the basic results? Both diets led to weight loss over the
six days, but those on the low-carb diet lost significantly more.
After six days the low-carb group had lost about 1.85kg on average
compared with around 1.3kg on the low-fat diet, around a half kilo
difference in just six days. The low-carb diet led to significant
changes in metabolic fuel selection. Insulin levels dropped, which
lowered carbohydrate burning by around 500 calories a day and
increased fat burning by around 400 calories a day. "Remarkably",
as the researchers put it, fat burning didn't change fuel selection
– both fat and carb burning remained unchanged. That said, the
low-fat diet resulted in a greater body fat loss compared with the
low-carb diet, despite being equivalent in calories. Short-term fat
loss was measured as the difference between the amount of fat taken
in and the amount of fat burnt (as measured by biological testing)
in the diet lab – this isn't the usual way. The normal
measurements, percentage body fat or fat mass, didn't change
between the groups. The authors used the short-term measure because
the study was too short to influence body fat percentage or fat
mass. The team built a mathematical model to predict changes in
metabolism and body weight before the study was conducted. They
adapted the model using the data collected and predicted what would
happen in the future. The model accurately predicted the results of
the short-term diets and predicted small long-term differences in
body fat. How did the researchers interpret the results? The
researchers say that, "Calorie for calorie, restriction of dietary
fat led to greater body fat loss than restriction of dietary
carbohydrate in adults with obesity. This occurred despite the fact
that only the carbohydrate-restricted diet led to decreased insulin
secretion and a substantial sustained increase in net fat oxidation
compared to the baseline energy-balanced diet." They added: "We can
definitively reject the claim that carbohydrate restriction is
required for body fat loss", saying that, "Fat loss is a more
important goal than weight loss in the treatment of obesity" and
that outside ofthe highly controlled diet lab "diet adherence is
likely the most important determinant of body fat loss". Conclusion
This well-designed diet lab study showed that a six-day
low-carbohydrate diet affects a person's metabolism far more than a
low-fat diet. The low-carb diet led to more fat burning and overall
energy expenditure via lower insulin levels, whereas the low-fat
diet didn't alter fat or carb-burning proportions, but led to more
fat loss. The study was well designed and rigorous, but included
only 19 people and lasted only a few weeks. This is a small number
of people, and a short amount of time, on which to base any
generalisations about most people. The results aren't convincing
enough to settle the low-carb versus low-fat diet debate. The diets
were pretty extreme and rigorously enforced under diet lab
supervision. Sticking to a diet that cuts energy intake by a third,
including fat intake by 85%, would be a big challenge for people
outside of the diet lab. But these large changes were necessary to
elicit a measureable effect in the short time the researchers had
available – they weren't meant to be directly applicable to outside
life. Smaller changes over the long term might work equally well,
something the researchers' mathematical modelling attempted to
predict. The researchers did make the point that the low-carb diet
they were investigating wasn't a low-carb diet in the traditional
sense. They kept carbs at 140g a day, when traditional low-carb
diets are said to contain less than 50g per day. But to do this
they would have had to increase fat and protein intake to balance
out calorie intake, which they didn't want to do as they wanted to
look at the sole effect of reducing carbs. This again highlights
that making overall conclusions on the effects of low-fat or
low-carb diets in general is not possible outside the strictly
controlled and specific dietary composition used in this study.
Professor Susan Jebb, Professor of Diet and Population Health at
the University of Oxford, hits the nail on the head, saying: "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." Starchy foods high in carbohydrate should make up
around one-third of everything we eat. This means we should base
our meals on these foods. Read more about a balanced diet. Want to
lose weight without resorting to gimmicks? The NHS has a
tried-and-tested weight loss plan.
Question 1: How can a study that only involves 19 people in
total provide insights?
Question 2: What were the major findings?
Question 3: Low carb diet restriction is required for body fat
loss. Reject or do not reject?
Question 4: Can the findings of this study be generalized to
most people? Why or why not?