Low–Carbohydrate Diets Health Advisory: 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 threefold increased risk of colon cancer.14,15
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. Studies of large populations published in subsequent years arrived at similar conclusions.16 In addition, meat–heavy diets are often low in dietary fiber, which protects against cancer.17 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.18,19 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.5,20–23 In some studies, about 30 percent of people on low–carbohydrate diets showed an increase in cholesterol levels, despite their weight loss.21,23
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.21 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.23 The effect of the diet on HDL (“good”) cholesterol levels is not consistent.5,6,20
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.21,23
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.19 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.24 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.
Furthermore, a study of nearly 30,000 women followed for 15 years found that coronary heart disease death was associated with intakes of red meat and dairy products when substituted for servings of carbohydrates. Coronary heart disease death was significantly reduced when animal protein was replaced with vegetable protein, leading the authors to conclude that "Long–term adherence to high–protein diets, without discrimination toward protein source, may have potentially adverse health consequences."33
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.24 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 40 or who have hypertension. Mild kidney impairment is also found in approximately 40 percent of individuals with diabetes.25 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.24
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.26
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.27
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.28,29 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.15 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.30 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:23
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