Which approaches is most effective for weight loss? Is there a difference at all when it comes to cardiovascular risk factors or diabetes?
The increasing prevalence of obesity and its associated health challenges has become a significant public health concern. A wide range of dietary strategies continues to be proposed as potential solutions for weight management. Among these, low-carbohydrate (LC) and high-carbohydrate (HC) diets frequently gain attention.
In the study, 22 individuals with obesity (body mass index: 36.5 ± 0.8), including 14 participants with impaired glucose tolerance but without overt diabetes, were evaluated. Participants were admitted to the General Clinical Research Center (GCRC) at Washington University Medical Center and randomly assigned to either an HC diet (carbohydrate intake ≥180 g/day) or an LC diet (carbohydrate intake ≤60 g/day). Following 48 hours of initial dietary adherence, participants were discharged to continue their respective diets at home until achieving a 7% reduction in body weight, typically over six weeks.
Upon reaching the target weight loss, caloric intake was adjusted to maintain the new weight. After approximately four weeks on the weight-maintenance diet, participants returned to the GCRC for further testing, including clamp studies and body composition analyses.
The Results
The findings offer valuable insights into weight loss dynamics and the influence of dietary composition. During the initial 48 hours in the controlled environment of the GCRC, participants in both groups experienced a weight loss of approximately 2 kg.
This rapid reduction, likely due to a loss of body water, occurred with an energy intake of about 1100 kcal/day, a substantial decrease from the estimated pre-hospitalization intake of ≥3000 kcal/day based on data from similar populations. Assuming stable energy expenditure, this reflects an estimated caloric deficit of approximately 4000 kcal over the two days.
The more significant weight changes occurred during the following six weeks. Both groups, adhering to either LC or HC diets, achieved an average weight loss of 7 kg, of which approximately 5,250 g (75%) was attributed to fat loss. This translates to a daily fat loss of about 125 g, corresponding to an energy deficit of approximately 1000 kcal/day. Once participants left the controlled setting, their daily energy intake likely increased to around 2000 kcal/day to sustain this rate of fat loss.
Interestingly, weight loss was comparable between the LC and HC groups after both 48 hours and the nearly 11-week period. Although some studies have shown greater short-term weight loss with LC diets, these differences tend to diminish with longer follow-up periods. A meta-analysis of related studies has concluded that the macronutrient composition of a calorie-restricted diet has minimal impact on long-term weight loss, which ultimately depends on the balance between calories consumed and calories expended.
The study underscores the importance of overall caloric balance rather than macronutrient distribution in sustained weight management.
Low Carbs Diet and Insulin Sensitivity.
The study revealed distinct improvements in insulin sensitivity, with notable differences between hepatic and peripheral effects. Insulin’s ability to suppress hepatic glucose production, reflecting hepatic insulin sensitivity, showed greater improvement in the LC group compared to the HC group, both after 48 hours and after approximately 11 weeks. Conversely, insulin’s stimulation of glucose uptake, which reflects peripheral (primarily skeletal muscle) insulin sensitivity, improved only after 11 weeks of dietary adherence.
Several factors may explain the enhanced hepatic insulin sensitivity observed with the LC diet. Metabolizing fatty acids or amino acids requires less insulin secretion than metabolizing glucose. This principle is reflected in insulin pump algorithms, where insulin dosing is calculated based solely on the carbohydrate content of meals. Additionally, 63% of participants in the study were glucose intolerant, indicating insufficient insulin secretion. Such individuals may manage better with a diet requiring less insulin, such as an LC diet, compared to an HC diet. Improved glycemic control, potentially through reduced glucose toxicity, has also been linked to improvements in insulin resistance.
As for the sequence of improvements, the liver’s higher sensitivity to insulin, compared to skeletal muscle, may explain why hepatic insulin sensitivity improved earlier. Small changes in insulin action are often detected first in the liver due to its heightened responsiveness, as seen in this study for both hepatic and peripheral insulin sensitivity.
Low Carbs Diet and Fatty Liver
A particularly notable finding from the study was the significant reduction in excessive intrahepatic triglyceride (IHTG) content observed in participants following the LC diet. Within just 48 hours, the LC diet reduced IHTG levels by approximately 30%, compared to about 10% in the HC group. Over the course of 11 weeks, both LC and HC calorie-restricted diets resulted in a reduction of IHTG content by roughly 40%.
The precise mechanism underlying the rapid clearance of IHTG with the LC diet remains unclear. One potential factor may involve the early improvement in hepatic insulin sensitivity, which was approximately three times greater in the LC group compared to the HC group. Research has established a strong link between insulin resistance and hepatic fat accumulation.
Insulin resistance is a critical driver of hepatic fat accumulation. Hyperinsulinemia associated with insulin resistance has been shown to markedly enhance de novo lipogenesis by upregulating the sterol regulatory element binding protein-1c in the liver. This, in turn, activates key enzymes involved in lipogenesis, including fatty acid synthase and acetyl CoA carboxylase.
While the exact reasons for the rapid IHTG reduction with the LC diet require further exploration, the substantial impact of LC diets on liver fat highlights their potential therapeutic value, particularly given the clinical significance of hepatic steatosis, which can progress to steatohepatitis and cirrhosis. The current lack of effective pharmaceutical options for addressing this issue further underscores the importance of such dietary interventions.
https://doi.org/10.1053/j.gastro.2009.03.019
Summary
For those aiming to lose weight, long-term adherence to a calorie-restricted diet appears to be the most critical factor. Evidence does not strongly support the superiority of LC over HC calorie-restricted diets for weight loss in the long term. Similarly, in cases of nonalcoholic fatty liver disease, reducing excessive nutrient intake is crucial for mobilizing liver fat. Any calorie-restricted diet that can be consistently maintained over the long term has the potential to improve weight, insulin resistance, and cardiovascular risk factors. However, for improving glycemic control in individuals with diabetes, LC diets appear to offer greater benefits compared to HC diets.