This study is remarkable because of its large group of participants (more than 609 participants) of long duration (12 months) and careful dietary control.
A one-year randomized clinical trial found that a low-fat diet and a low-carbohydrate diet produced similar weight loss and improvements in metabolic markers of health.
In addition, the production of insulin and the evaluated genes had no impact on the prediction of success or failure of weight loss. Therefore, you should choose your diet according to personal preferences, health goals and sustainability.
Previous studies comparing low-fat diets with low carbohydrates have shown that individual weight loss can vary widely within assigned dietary groups.
The reasons for these individual responses are not well understood, leading scientists to hypothesize that perhaps insulin sensitivity or certain genetic components could explain the success or failure of different diets.
The present study evaluated whether differences in genetics or insulin production could help predict the success of weight loss in participants who carry out a low-fat or low-carbohydrate diet for a year.
Who was studied?
This randomized clinical trial (RCT) assigned 609 participants to a low-fat diet or a low-carbohydrate diet for 12 months.
In total, 263 men and 346 premenopausal women free of important health conditions (ie without diabetes, cancer, heart disease, high cholesterol, etc.) were included in the study. The average BMI was 33 (class I obesity) and the average age was 40 ± 7 years.
During the course of the study, each subject was instructed to attend 22 sessions of dietary advice with a registered dietitian; the average attendance was 66% for both groups.
During the first two months of the study, the low-fat group was instructed to consume only 20 g of fat per day and the low-carbohydrate group only 20 g of carbohydrate per day.
However, they were not expected to remain at these levels indefinitely: at the end of this 2-month period, they began to add fats or carbohydrates to their diet until they felt they had reached the lowest intake level that they could maintain sustainably.
None of the groups was able to stick to the very low starting doses: by month 3, the low-fat group was already consuming an average of 42 g of fat per day, while the low-carb group was consuming an average of 96.6 g of carbohydrates per day.
It is possible that some in the low carbohydrate group may have been in ketosis during these first two months due to the very low carbohydrate intake prescribed.
While the low-carbohydrate group was able to achieve reduced carbohydrate intake during the trial (≈115 g / day), only a very small minority reported consuming ≤50 g / day, the intake threshold typically required to remain in ketosis .
Although no caloric intake targets were assigned, both groups were instructed to consume high quality whole foods and beverages.
Specifically, they received instructions to “maximize vegetable intake, minimize the consumption of added sugars, refined flours and trans fats; and focus on whole foods that were minimally processed, rich in nutrients and prepared at home whenever possible. “
During the study a total of 12 randomized and unannounced multidirectional dietary withdrawals were taken during the study to assess the food intake.
With this method, an interviewer asks people to remember all the foods and beverages they consumed in the last 24 hours. If you are curious, you can try a 24-hour multi-pass retreat here.
Dietary compliance was also corroborated by changes in blood lipids and in respiratory exchange ratio (RER: this may indicate if you are burning fat or carbohydrates primarily).
What was studied?
The first primary hypothesis tested is a possible link between the genotype pattern and the type of diet for the success of weight loss.
All the participants were evaluated for 15 genotypes, including 5 genotypes “low in fat” (with the hypothesis of obtaining better results with a low fat diet), 9 genotypes “low in carbohydrates” (with the hypothesis of obtaining better results with a low-carb diet) and 1 “neutral” genotype.
The second main hypothesis being tested is a possible link between insulin secretion and type of diet for weight loss success.
At the beginning of the test and in months 3, 6 and 12, all participants completed an oral glucose tolerance test (OGTT) to measure insulin production.
An OGTT is a test that can measure your glucose and / or insulin levels in the blood after you have consumed a fixed amount of carbohydrates (typically 75 g of glucose).
Other measured results included changes in:
The body composition (evaluated by DXA).
Levels of cholesterol.
Fasting glucose and insulin.
Energy expenditure at rest.
Total energy expenditure.
What was the results?
In total, 481 participants completed the full test, which translates into a dropout rate of 21%, which is not unexpected for a diet study of this duration.
Although there were no significant dietary differences between the groups at the beginning (before the dietary interventions began), there were significant differences in months 3, 6 and 12 with respect to the percentage of intake of carbohydrates, fats, proteins, fiber and added sugars .
In addition, the intake of saturated fats was significantly reduced in the low-fat group, while the overall glycemic index was lower in the low-carbohydrate group.
Although both groups saw reductions in glycemic load, the decrease was much greater in the group with low carbohydrate content.
The study showed no significant difference in weight loss between the low-fat and low-carbohydrate groups.
At 12 months, the low-fat group had lost 11.7 lbs (5.3 kg) and the low-carbohydrate group 13.2 lbs (6.0 kg); this difference of 1.5 lbs during 12 months (0.125 lbs / month) is not statistically significant or clinically relevant.
In addition, within each group, differences in genotypes or insulin secretion did not make a significant difference in weight change.
This suggests that neither the genotype tested in this study nor the amount of insulin produced during the OGTT can predict the success of weight loss with a low-fat or low-carbohydrate diet.
Ironically, a possible confounder that masked an interaction might have been that both diets were based on whole foods.
If, for example, the low-fat diet consisted mainly of soft drinks and refined grains, the resulting resistance to insulin might have had an effect on the change in weight.
Both groups were able to improve certain health markers (BMI, percentage of body fat, waist circumference, blood pressure and insulin in fasting and glucose levels), although no significant differences were observed between the groups.
At the 12-month mark, low-density lipoprotein cholesterol (LDL-C) had decreased significantly in the low-fat group (-2.12 mg / dL), while it had increased in the low-carbohydrate group (+ 3.62 mg / dL).
However, the low-carbohydrate group also experienced a significant increase in high-density lipoprotein cholesterol (HDL-C) (+2.64, versus +0.40 mg / dL in the low-fat group). and greater reductions in triglycerides (-28.20 vs -9.95 mg / dL in the low-fat group).
The energy expenditure at rest (REE) was not significantly different between the groups at any point. By month 12, REE had decreased significantly since the beginning of the study for both groups (-66.45 kcal for low-fat, 76.93 kcal for low-carb).
The total energy expenditure (TEE) was not significantly different between the groups or in comparison with the initial value. Finally, although a little more than 10% of each group improved their metabolic syndrome during the trial, there were no significant differences between the diets.
What does this study tell us?
The results of this study contribute to a large amount of evidence indicating that, for weight loss, neither low fat nor low carbohydrate is superior (provided there is no difference in caloric intake or protein intake).
In this trial, the overall caloric intake was almost identical between the groups throughout the intervention period, and the low-carbohydrate group consumed only a little more protein (an additional average of 12.5 g / day).
The results of weight loss in this study are repeated both in the short term, in strictly controlled clinical trials and in the long term, less controlled clinical life trials. While the present study is a free-living test, it offers some advantages that are not seen in most others:
The program offered intensive dietary advice and guidance throughout the duration of the study. Many free-life trials provide instruction and / or support in advance, after which the participants are left to their own resources.
He confirmed the dietary intake of the participants through random dietary reminders of multiple passes of 24 hours, reinforced by lipid panels and RER tests. Most long-term diet trials simply use 24-hour reminders or food frequency questionnaires.
The study strongly encouraged participants to eat healthy diets rich in whole foods and not fill their pantries with low-fat or low-carb junk foods.
It is one of the largest studies of its kind, which reduces the likelihood that a result is due to a random error (also known as “noise”).
While our understanding of the interactions between genetics and diet continues to grow, this trial has tested 15 genotype patterns that are suspected to influence the success or failure of weight loss in low-fat or low-carbohydrate diets.
While the measure of insulin production used in this trial could not predict weight changes, the authors note that, based on other studies, fasting insulin measurements may be worth investigating more as predictors of weight loss. .
Finally, not all participants adhered perfectly to the assigned diet, which reduces our ability to establish a direct relationship between genotype, insulin production and dietary intervention.
However, the results of the study are still very suggestive of a relationship, and the authors plan to do more analyzes that take dietary adherence into account.
What should be considered of this?
An important aspect of this test that we must consider, and that is often overlooked, is inter-individual variability.
Studies try to discern a global effect and, often, do not report anything else, even when individual responses are everywhere.
A second important aspect to consider is adherence. At the beginning of the study, all participants were instructed to consume ≤20 g of fat (if they were in the low-fat group) or ≤20 g of carbohydrates (if they were in the low-carbohydrate group) during the first two months.
Then they could increase their intake of fats or carbohydrates to levels they felt they could sustain indefinitely. By the end of the trial, the vast majority had not been able to maintain such low levels.
The final dietary withdrawals reported an average daily fat intake of ≈57 g (low fat group) and a mean daily carbohydrate intake of ≈132 g (low carbohydrate group).
The applicability in real life matters a lot when extrapolated from the results of a study. The results of this study send a clear message that, when choosing a feeding style, sustainability is a component whose importance can not be underestimated.
There is no “better diet”. Diets low in fat and low in carbohydrates can work to lose weight; The healthy diet that will work for you is the one you can stick to.
When it comes to losing weight, neither a low-fat diet nor a low-carb diet is inherently superior.
Neither insulin production nor the genotypes evaluated had any noticeable effect on the success or failure of weight loss.
Choose a style of eating that suits your food preferences, health goals, lifestyle. The most important thing is to choose a feeding style that you can maintain.