Health Concerns
No published studies have addressed the long-term effects
of low-carbohydrate diets. The longest studies have
followed dieters for only 12 months, which is not sufficient
to assess whether dieters are at risk for the problems
seen in studies of general populations consuming large
amounts of meat, fatty dairy products, and the cholesterol,
saturated fat, and animal protein they contain. However,
long-term studies of the general population following
a variety of diets and short-term studies of individuals
on low-carbohydrate diets raise important concerns,
which are outlined below:
1. Colon cancer. Colon cancer is one of the most common
forms of cancer in North America and Europe and is among
the leading causes of cancer-related mortality. Long-term
daily intake of meat, particularly red meat, such as
beef, pork, or lamb (as is common in Western countries),
is associated with approximately a three-fold increased
risk of colon cancer.12,13
The 1997 report of the World Cancer Research Fund and
American Institute for Cancer Research, entitled Food,
Nutrition, and the Prevention of Cancer, concluded that,
based on available evidence, diets high in red meat
are probable contributors to colon cancer risk. In addition,
meat-heavy diets are often low in dietary fiber, which
protects against cancer.14 Low-carbohydrate diets typically
include red meats among their foods recommended for
daily consumption, but no studies have yet been conducted
to see whether low-carbohydrate dieters do indeed have
the same increased long-term cancer risk seen with other
populations eating meat-heavy diets.
2. Heart disease. Generally speaking, weight loss tends
to reduce cholesterol levels, while saturated fat and
cholesterol tend to raise them.15,16 Consequently, the
effect on cholesterol levels of a low-carbohydrate weight-loss
diet that includes saturated fat and cholesterol can
vary from person to person.5,17-19 In some studies,
about 30 percent of people on low-carbohydrate diets
showed an increase in cholesterol levels, despite their
weight loss.18,19
In a low-carbohydrate diet study conducted at Duke University,
funded by the Atkins Center for Complementary Medicine,
LDL (“bad”) cholesterol levels fell in 29
of the 41 study completers, as would be expected from
weight loss along with the various supplements used
in the study. However, LDL levels rose in 12 participants
by an average of 18 mg/dl (the increases ranged from
4 to 53 mg/dl). One participant had an LDL increase
from 123 mg/dl to 225 mg/dl (normal LDL values are typically
described as <100 mg/dl, although some investigators
have called for lower limits).
The participant was then treated with a “cholesterol-lowering
nutritional supplement,” and the LDL dropped to
176 mg/dl, which is still far above recommended levels.18
In a subsequent Duke University study, two low-carbohydrate
diet participants dropped out of the study because of
elevated serum lipid levels (one had an increase in
LDL cholesterol from 182 mg/dl to 219 mg/dl in four
weeks; the second had an increase from 184 mg/dl to
283 mg/dl in three months), and a third developed chest
pain and was subsequently diagnosed with coronary heart
disease. In 30 percent of participants, LDL cholesterol
increased by more than 10 percent.19 The effect of the
diet on HDL (“good”) cholesterol levels
is not consistent.5,6,17
We recommend caution when reading favorable press accounts
of the effect of low-carbohydrate diets on cholesterol
levels. The two Duke University studies cited above
are sometimes cited as evidence that low-carbohydrate
diets reduce LDL (“bad”) cholesterol and
increase HDL (“good”) cholesterol. However,
these studies did not test a low-carbohydrate diet alone.
Rather they tested the diet along with regular exercise
and various nutritional supplements, including flax
oil, borage oil, fish oil, vitamin E, chromium picolinate,
and a “multivitamin formula” containing
niacin, vitamin C, and other nutrients. Exercise and
supplements would be expected to influence cholesterol
levels on their own, apart from the effects of the diet.18,19
One particular danger of the press promotion of low-carbohydrate
diets is the suggestion that meats and dairy products
that are high in saturated fat and cholesterol do not
pose the risks that scientists have long said they do.
However, abundant evidence shows the risks of such foods.16
In fact, some evidence suggests that even a single fatty
meal (e.g., a ham-and-cheese sandwich, whole milk, and
ice cream) may adversely affect the compliance of arteries,
increasing the risk of heart attacks after meals.20
Low-carbohydrate diet promoters have argued that the
risks of diets high in saturated fat and cholesterol
may be disregarded when the diet is also very low in
carbohydrate. However, no long-term studies have tested
this conjecture.
3. Impaired kidney function. Studies of the Atkins diet
and other low-carbohydrate, high-protein diets have
not been of sufficient duration to evaluate their potential
to affect kidney function. However, reason for concern
comes from studies of the general population, in which
diets high in animal protein are associated with reduced
kidney function over time. Harvard researchers reported
that animal protein intake is associated with decline
in kidney function, based on observations in 1,624 women
participating in the Nurses’ Health Study.21 The
good news is that the damage to the kidneys was found
only in those who already had reduced kidney function
at the study’s outset.
The bad news is that as many as one in four adults in
the United States may already have reduced kidney function,
and the percentage is considerably higher for those
over forty or who have hypertension. Mild kidney impairment
is also found in approximately 40 percent of individuals
with diabetes.22 This suggests that many people who
have kidney problems are unaware of that fact and do
not realize that high-protein diets may put them at
risk for further deterioration. The kidney-damaging
effect was seen only with animal protein. Plant protein
had no harmful effect.21
The American Academy of Family Physicians notes that
high animal protein intake is largely responsible for
the high prevalence of kidney stones in the United States
and other developed countries and recommends protein
restriction for the prevention of recurrent kidney stones.23
4. Complications of diabetes. In diabetes, kidney and
heart problems are particularly common. The use of diets
that may further tax the kidneys and may reduce arterial
compliance is not recommended.
No studies of low-carbohydrate diets have been of sufficient
duration to assess their potential long-term effects
on individuals with diabetes. Because controlling blood
cholesterol levels and protecting kidney function are
essential for individuals with diabetes, health authorities
recommend choosing diets that are rich in vegetables
and fruits, while limiting saturated fat, cholesterol,
and animal protein.24
5. Osteoporosis. High intake of animal protein is known
to encourage urinary calcium losses and has been shown
to be associated with increased fracture risk in research
studies involving various populations.25,26 Two studies
have examined the effects of low-carbohydrate diets
on calcium losses. A Duke University study showed that
urinary calcium losses rose significantly in individuals
following a low-carbohydrate, high animal-protein diet
for six months.18 Similarly, the loss of calcium was
demonstrated in a low-carbohydrate diet study at the
University of Texas.
In the maintenance phase of the diet, urinary calcium
losses were 55 percent higher than normal. The researchers
concluded that the diet presents a marked acid load
to the kidney, increases the risk for kidney stones,
and may increase the risk for bone loss.27 No studies
of low-carbohydrate, high-protein diets have yet been
of sufficient duration to measure long-term bone loss.
6. Other adverse effects. The following adverse effects
were noted in a six-month study of a low-carbohydrate
diet, in addition to the effects on cholesterol levels
noted above:19
Constipation 68 percent
Headache 60 percent
Bad breath 38 percent
Muscle cramps 35 percent
Diarrhea 23 percent
General weakness 25 percent
|
