In this trial, which enrolled overweight and obese participants, composing the healthful diet of low GI and fat-restriction foods did controll weight wit increased HDL cholesterol levels, reduced LDL cholesterol levels or plasma triglyceride levels by 1 to 2 mmol/L and etc. Figure 2 shows improvements in waist circumference, blood lipids and other indicators of body weight and cardiovascular risk factors. They also demonstrate the advantages in effectively promoting low GI and fat-restriction diet to the overweight/obese population, using recommendations that, when followed, increase weight loss without cardiovascular risk. It indicates an urgent need for innovative approaches to support the implementation of current dietary advice17.
Our findings are supported by clinical trials of typical low-carbohydrate diets, that showed compared to currently-recommended low-fat diets, low-carbohydrate diets produced greater weight loss and more favorable changes in body composition, high-density lipoprotein cholesterol, and triglycerides18 − 20, while similar or lesser reductions in low-density lipoprotein cholesterol, blood pressure, blood glucose or insulin18, 21. Previous cohort studies in adults had shown that GI score was inversely associated with HDL cholesterol22. It is also meaningful that every diet studied in previous trials and this trial lowered LDL cholesterol levels of the participants from baseline when they were eating provided diets.
Moreover, our study suggested that dietary GI maight be more relevant to women than to men. Women generally lose weight more slowly and display differences in postprandial glucose and fat oxidation, which might influence the rate of weight loss. These explain the difference in indicators between male and female participants before and after intervention.
We chose a 3-months duration of the intervention periods based on results of previous studies, which suggested that 3-months was sufficient to detect changes in our outcomes. A controlled trial of 6 weeks found significant beneficial effect of low-fat diet on serum cardiovascular risk factors23. These results do not suggest that the effects of our diets become apparent only after a longer duration of intervention.
In this study, we hypothesized that a low glycemic index would cause weight loss and body composition improvement, and we would also observe the impact on CVD risk factors. A study reported implementation of a low GI diet in overweight children was feasible on the basis of a 12-week nutritional intervention by giving brief instructions on categorizing food24. Thus, if a low GI and low fat diet did relate to anthropometric or cardiovascular disease outcomes, we hope this relationship would be seen in the present group of southwest Chinese.
Replacing starches with low GI foods in daily meals was relatively difficult to implement in the early stage but well accepted by nearly all subjects. Learning to choose foods with a lower glycemic index and changing the eating habits a little, people coud improve the quality of their diet and stick to it for a long time.
Actually, previous dtudies in other populations showed mixed results25 − 27. A meta-analysis of 28 trials found that lowering glycemic index did not affect HDL cholesterol or triglyceride levels, while could lower LDL cholesterol level only if fiber content was also increased28. In fact, the effect of dietary fiber on lowering LDL cholesterol was well recognized. A high-carb diet with a low glycemic index mentioned in the article might contain more fiber and less dietary cholesterol. Therefore, we speculated whether the emergence of different clinical research results of the effect of low GI diet on CVD risk factors was related to different levels of dietary fiber and fat content. In our intervention analysis study, the diet of the subjects chosed low-GI foods, and the energy supply ratio of fat was strictly controlled below 30%, which excluded the effect of fat on CVD risk factors to a certain extent.
Analysis indicated that approximately 60%-70% effects of dietary on CVD risk factors were not explained by differences in weight loss, and plausibly due to different macronutrient concentrations in the diet27. This finding is important, because it indicated that obese adults who lose weight on a low-carb diet could improve cardiovascular status and blood lipid levels to the same or higher levels than a low-fat diet.
The recommended LC diet is composed of carbohydrates with low GI food, while the amount of carbohydrates is still uncertain. It is worth considering comprehensively GI is just one of the attributes of a carbohydrate food. Further, nutrients often cluster. The effects of glycemic index, if any, might actually result from other nutrients, such as fiber, potassium, and polyphenols, which favorably affect health. Even though the consumption of low GI foods is advocated, the independent benefits of GI are uncertain, especially when persons are already consuming a health diet rich in whole grains, vegetables, and fruits. In a recent prospective cohort study for 25 years exploring association between carbohydrate consumption and mortality, diets composed of 50–55% carbohydrates, regardless their plants or animal source, were associated with lowest risk of mortality29. When comparing LC diets, higher mortality rates were associated with animal-based protein and fats, while lower mortality rates were noticed among individuals who consumed diets with plant-based protein and fats. The advantages of increased plant food consumption should be emphasized. The optimal macronutrient composition the carbohydrate-to-fat (C/F) ratio for patients with obesity was still unclear. Therefore, while considering the intake of food GI, we also need to pay attention to the diet pattern, especially the composition of fat, fiber and phytonutrients in the diet.
Besides, previous epidemiological studies had demonstrated that saturated fatty acids and trans fatty acids was associated with an increased risk of CVD, while polyunsaturated and monounsaturated fats could reduce the risk30. In this study, we only focused on the total amount of fat during the intervention. Although foods with high saturated fat content (such as cream, chicken skin, fat poultry, fat livestock, etc. in Table 1) were restricted, we did not distinguish the intake of saturated fatty acid (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) respectively, which might interfere with the results of the study.
Those who were given advice might suffer gastrointestinal side effects. Although the diets prescribed were recommended for long-term health, it was possible that the acute increase in fiber from cereals, fruits, and vegetables and control in carbohydrate might cause some abdominal discomfort. Sudden reductions in carbohydrate intake might lead to symptoms like dizziness, fatigue and exercise intolerance, which tended to improve spontaneously in few days to weeks after such diet. These problems would not reappear when participants became accustomed to such diets. Although the participants were explicitly told in the informed consent before the research, some participants who could not tolerate these symptoms gave up persistence and fell off in the first few weeks.
Adherence In The Present Study
Usual eating habits were resistant to change without specific personal and environmental supports. The success of dietary recommendations were affected the intensity of the intervention. Further emphasis of compelling reasons was required on the longer-term health advantages of sustaining a good diet, as had been confirmed in long-term follow-up of part of the anticipants. Educational programs on food choices for health maintenance might also be helpful. Meanwhile, more emphasis should be placed on overcoming obstacles related to food preparation methods and food choices when dining out. In this trail, as long as the participants are willing, each meal coud be sent to the nutritionist by taking pictures, receiving supervision and comments, effectively solving the problem of diet compliance. Timely diet advice and monitoring feedback significantly increased interaction and persistence.
This was the first study in southwest China that investigated how low GI and fat restriction diet relate to overweight/obesity and markers of cardiovascular disease. One particular strength of our study was that it encouraged specific food consumption by providing both dietary advice and supervision. It was also in the context of glycemic index and fat portfolio-restricted dietary approaches, and determined the effects on body weight and risk factors for CVD. Participants received extensive knowledge of the low GI diets. Through online instant feedback intervention, its completion rate and diet adherence rate were high. Uniform guidelines were used in all consultations, and nutritionists did not turn a blind eye to the intervention process. Dietary sessions for the whole trial were intermittently observed for consistency by independent registered dietitian consultants who were not the regular part of the study staff.
The significance of this study was to study the effects of diet on weight and CVD risk by choosing foods with low glycemic index and fat restriction. However, further research is needed to determine the optimal composition of fat/carbohydrate and saturated/unsaturated fat, etc., which could be the optimal dietary composition to reduce the risk of cardiac metabolism. This study also had important public health implications in the setting of a high prevalence of excessive refined carbohydrate consumption and an epidemic of obesity and CVD worldwide.
Although this study had multiple strengths, there were some limitations. First, we only focused on the total amount of fat during the intervention. Although foods with high saturated fat content were restricted(such as cream, chicken skin, fat poultry, fat livestock, etc. in Table 1), we did not distinguish the intake of SFA, MUFA and PUFA respectively, which might interfere with the results of the study. Further analysis needs to be done in the future.
A second limitation was the provision of food guide, which might have beneficial effects in increasing intake of the desired foods for the participants. However, the changes observed in risk factors were not significant. As the participants were generally healthy at baseline, changes might have been more difficult to detect, which limited to see the effect. Besides, the clinical significance of small changes in some novel CVD risk factors is unknown.
Last, the sample size we recruited to participate in this cohort study was not large enough, lacking of control groups, and the observation time was short. The impact on long-term effects was unknown. The results of this study were not generalizable to the general population of the whole southwest China. Literature was still lacking for well-designed randomized controlled trials to compare low-fat versus low-carbohydrate diets without confounding effects of from the behavioral aspects of eating. The ideal amount of carbohydrates, fats, and protein in an optimal diet for weight loss without CVD risk is still uncertain. Further research on the effects of a low GI diet on weight loss and CVD risk is warranted.