10
VOLUME 10 NUMBER 1 • MARCH 2013
REVIEW
SA JOURNAL OF DIABETES & VASCULAR DISEASE
too much of certain foods and chronic disease. In 1984, the ‘food
wheel’ was published and for the first time carbohydrates were
depicted to form the bulk of the food we were to eat. This was
replaced by the ‘pyramid’ in 1992, and then by the ‘mypyramid’
in 2005. In both pyramids, carbohydrates formed the main source
of food.
The latest incarnation is the ‘MyPlate’, replacing the pyramid with
two plates, one containing fruits, vegetables, grains and proteins
and then a second smaller plate for dairy, with carbohydrates again
forming the bulk. At this point it should be noted that there are no
trials showing that by following a ‘MyPlate’ diet, one can reduce
incidence of heart disease. Hence it reflects the summary of various
bits of evidence put together as a best-fit diet, with some interplay
between science and the food industry.
Given the above as a background, it is appropriate to look at
the evidence for various diets, but specifically the LCD vs LFD with
regard to weight, surrogate markers, diabetes and diabetes-related
complications. Focusing first on weight, the trials show that in the
short term, i.e. less than six months, the LCD is better than the
LFD, but that over the longer term, there is no difference.
12,13
Sacks
et al
. compared weight-loss diets with different compositions of
fat, protein and carbohydrates, and showed that over two years, it
made no difference in terms of weight loss.
14
With regard to metabolic parameters, the LCD has an advantage
with regard to raising high-density lipoprotein (HDL) levels, but fares
worse with regard to low-density lipoprotein (LDL) levels.
13
When it comes to diabetes prevention, the main studies in this
area have used a LFD, and have showed a 58% reduction in the
progression from pre-diabetes to diabetes.
15
There are no formal
large-scale studies looking at preventing diabetes using a LCD.
When it comes to dietary manipulation in patients with diabetes,
one of the earliest and most well-quoted studies is by Westman,
16
where he compared a LCD to a low glycaemic index diet. He showed
that over 24 weeks, the LCD reduced HbA
1c
levels by 1.55% as
opposed to the low-glycaemic index diet, which reduced HbA
1c
levels by 0.5%. More weight loss occurred in the LCD diet, but
the authors were statistically able to show that the improvement in
HbA
1c
level was not dependent on weight loss.
Further studies however by Davis
et al
.
17
, Iqbal
et al.
18
and
Guldbrand
et al
.
19
did not show a difference between a LCD and
LFD with regard to HbA
1c
level specifically over longer periods
such as two years. The Diabetes Excess Weight Loss (DEWL) trial
20
compared a high-protein diet with a high-carbohydrate diet and
showed that over two years, neither diet was superior to the other
with regard to HbA
1c
levels or other metabolic parameters such as
weight or lipid levels. There is therefore no good evidence to show
that a LCD diet is better than a LFD or any other diet with regard
to HbA
1c
lowering
Recently there has been much interest in reversing diabetes, as
this has been noticed in bariatric studies. There was an interesting
study by Lim
et al
.
21
that showed calorie reduction (600 calories
over eight weeks) in 11 patients with diabetes could reverse the
abnormalities in diabetes. The dietary breakdown in the study was
46.4% carbohydrate, 32.5% protein and 20.1% fat. The question
with this low-calorie intake is whether to call this a low-fat or low-
carbohydrate diet, as both could apply. It then begs the question
whether the main dietary benefits of diets are via the alteration of
the macronutrients or the total calorie content.
It can be argued that it is virtually impossible to design and
conduct an adequate dietary trial. The alteration of any one
component of a diet will lead to alterations in others. Dietary trials
cannot generally be blinded and changes in the diet of the ‘control’
population are frequently seen. It is also recognised that adherence
to dietary advice over many years by large population samples, as
for most individuals in real life, is poor and the stricter the diet,
the worse the compliance. Finally, the impact of weight loss and
exercise is difficult to separate from the change in macronutrients.
A review by Wheeler
et al
. in
Diabetes Care
on multiple studies of
diet and diabetes concluded similarly to the above.
22
Ultimately, the main reasons for treating diabetic patients are to
reduce macro- and microvascular complications, as well as incidence
of death and cancer. To date there are no studies showing a benefit
with regard to any of these outcomes. In keeping with this, a
Cochrane
meta-analysis concluded that no high-quality data on the
efficacy of diet alone exist for treatment of type 2 diabetes mellitus
and that no data were found on micro- or macrovascular diabetic
complications, mortality or quality of life.
23
Recently, the LOOK
AHEAD study,
24
which employed a low-fat diet (< 30% of kCal
from fat with < 10% saturated fat) along with a 7–10% weight
loss and an increase in exercise to 175 minutes per week, was
stopped in view of futility, as it showed no reduction in incidence
of cardiovascular disease.
With all this information, what do the current guidelines say? The
American Diabetes Association suggests the mix of carbohydrate,
protein and fat may be adjusted to meet the metabolic goals and
individual preferences of the person with diabetes.
25
Saturated fat
intake should be < 7% of total calories. For weight loss, either low-
carbohydrate, low-fat calorie-restricted, or Mediterranean diets
may be effective in the short term (up to two years).
The South African Guidelines (SEMDSA) prescribe that fat
intake should be restricted to < 35% of the total energy intake.
26
The saturated fat intake should be restricted to < 7% of the total
energy intake. Carbohydrates should make up 45–60% of the
total energy intake. Proteins should make up 15–20% of the total
energy intake. For weight loss, optional diets include the low-fat,
low-carbohydrate and Mediterranean diets.
In conclusion, LCDs help with weight loss and are better than
LFDs over the short term for weight loss, but are no better for
weight loss over the longer term. Over the long term, LCDs are no
better for metabolic parameters (except HDL) or diabetes. Current
carbohydrate intake is too high, but the question is ‘what is the
optimal amount?’ There are currently no outcomes data. Food
sustainability doesn’t support a universal low-carbohydrate diet.
My recommendation is that dietary advice should be individualised
and metabolic parameters monitored. With regard to weight loss,
the main emphasis is more about cutting calories than altering
macronutrients.
References
1.
Rollo J, Cruikshank W.
An Account of Two Cases of the Diabetes Mellitus
. Vol I
and Vol II. T Gillet for C Dilly. London, UK, 1797.
2.
Banting W. Letter on corpulence, addressed to the public, 1864.
3.
Atkins RC.
Dr Atkins’ Diet Revolution.
Bantam, 1972.
4.
Keys A, Taylor HL, Blackburn H, Brozek J, Anderson JT, Simonson E. Coronary
Heart Disease among Minnesota Business and Professional Men Followed Fifteen
Years.
Circulation
1963;
28
: 381–395.
5.
Dawber TR, Kannel WB, Revotskie N,
et al.
Some factors associated with the
development of coronary heart disease. Six years’ follow-up experience in the
Framingham Study.
Am J Public Health
1959;
49
(10): 1349–1356.
6.
Report of the National Cholesterol Education Program Expert Panel on Detection,
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Arch Intern Med
1988: 14836–14869.
