While the popularity of low-carbohydrate (CHO) diets has been waning in recent years, the scientific evidence indicating these fad diets can have untoward effects on the human body continues to mount.
A recent study compared the impact of feeding either a high- CHO diet (20% fat & 60% CHO) or a high-fat diet (60% fat & 20% CHO) to 24 overweight or obese subjects.1 The men and women subjects were randomly assigned to receive one of the two diet for 8 weeks. The level of energy restriction on the two diets was the same (-500kcal/day) so it is not surprising that the weight loss on the two diets was also very similar. On both diets there was also a similar improvement in glucose tolerance associated with lower fasting insulin levels and improved insulin sensitivity. This too should not be very surprising given the similar energy restriction and weight loss achieved on the two diets and the fact that insulin resistance primarily develops in response weight gain rather than the ratio of fat to CHO in the diet. Perhaps a bit surprising was the similar drop in total and LDL-Cholesterol on the two diets despite the higher saturated fat content of high-fat diet compared to the low-CHO diet. However, the subjects remained calorie-restricted throughout the study and this results in increased mobilization of body fat stores blunting the impact of differences in dietary fat composition. The significantly greater drop in fasting triglyceride (TG) level on the low-CHO vs. the lowfat diet is consistent with most earlier data. However, it should be noted that postprandial TG levels increase much more with each high-fat meal relative to a isocaloric high-CHO meal. This may be important as growing evidence links elevated postprandial TGs to CVD risk much more than fasting TG levels. Indeed a single high-fat meal has been shown to impair the ability of arteries to dilate. In this study the authors used a measure of artery stiffness known as the augmentation index (AI) to examine any subtle changes in artery stiffness resulting from the two diets. They observed a significant difference in the AI on the two diets with the AI increasing from an average of 12.3 to 14.5 on the high-fat diet but falling from an initial 17.0 to 13.3 after 8 weeks on the high-CHO. The authors conclude "The difference in AI may imply a negative effect of low-CHO diets on vascular risk."
If high-fat diets do gradually stiffen arteries over time they may be increasing the risk of cardiovascular disease even during the active weight loss phase when fasting blood lipids often appear no worse than those observed on a low-fat diet.
Another study examined the impact on mood and cognitive function in 118 mostly obese (mean BMI = 33.7) subjects 24- 64y who were randomly assigned to consume either a very-low- CHO diet (<40g/day) diet or a more conventional "low-fat" diet (30% fat, 46% CHO, & 24% protein) for one year.2 Most earlier studies have shown improvement in mood and very modest or no changes in mental function. However, these studies lasted no more than several weeks. One study that lasted 24 weeks did find greater improvement in mood on a low-fat diet than on a very low-CHO diet. The results of this study confirmed that over the first 8 weeks both diets improved various measure of mood to a similar extent. However, various measures of mood deteriorated over the next several months on the ketogenic diet but not on the conventional "low-fat" diet. Despite a similar loss of weight on the 2 diets those on the very low- CHO diets scored more depressed, angry, and confused after a year than those assigned to the more conventional diet.
Low carbohydrate diets may work in the short term for weight loss because calorie intake is reduced. But these studies show that low-carb and ketogenic diets have detrimental long term adverse effects on a patient’s cardiovascular health. In addition very low-carb diets may improve mood in the short term (as do high-CHO diets) but in the long term frequently leave people more depressed, angry and confused. There seems little reason to subject people to the adverse effects of very low-CHO diets when there are far healthier alternatives..
By James J. Kenney, PhD, RD, LD, FACN
1. Diabetes 2009;58:2741-8
2. Arch Intern Med. 2009;169:1873-80
For more information on low-carb diets, go online to foodandhealth.com and click on CPE courses. Scroll down to weight loss and you can read Dr. Kenney’s comprehensive research review on fad diets. He has been writing and updating this paper since 1999 and it cites over 100 references. CFFH subscribers who are RDs can take this online CPE course for a discounted rate.
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.