Postprandial triacylglycerols after diets varying in carbohydrate type and amount (the CARBFUNC study)
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Introduction: Obesity, the most prevalent nutritional disorder in the Western world today, continues to increase worldwide. During the last decades, poor quality diets characterized by refined grains and processed foods with added sugars, fats and salt have dominated the food supply, which likely plays a crucial role in the observed increase in obesity and overweight. Obesity associates with increases in both fasting and postprandial circulating triacylglycerols (TAG), the latter known as lipemia, and these TAG elevations have recently been described as independent risk factors for cardiovascular disease (CVD). Among dietary factors, carbohydrates are particularly known to increase circulating TAG, underscoring an important cross-talk between lipid and glucose metabolism. Not only quantity but also quality of carbohydrates may be important. The degree of cellular degradation, i.e., cellularity, of carbohydrate foods was recently proposed to define carbohydrate quality. However, no previous randomized controlled trial (RCTs) has tested the possible importance of carbohydrate cellularity relative to amount for obesity and related risk factors such as postprandial lipids. Objectives: The primary aim of this thesis was to determine possible carbohydratedependent adaptive changes in 4-hour postprandial circulating TAG after a standardized mixed-meal test, following a 3-month period on 3 diets varying greatly in carbohydrate quality and quantity. Secondary aims were to examine changes in fasting TAG, postprandial TAG patterns, correlations between change in 4-hour TAG and body weight dependent or independent of diets, change in fasting non-esterified fatty acids (NEFA) and relationship with TAG, relationship between 4-hour TAG and fasting glucose, and possible differences in diet responses between men and women. The current hypothesis of the thesis was; 3-month metabolic adaptation to a cellular and/or low-carbohydrate diet improves postprandial lipemia to a greater degree than an acellular carbohydrate diet, when measuring the response to a mixed meal challenge, or stated differently, that a more acellular carbohydrate diet to a lesser degree facilitates postprandial clearance of lipids from the circulation. V Methods: Two hundred and eighty obese people were recruited in the Bergen area by flyers, online ads, radio interviews and newspaper articles. Hundred and fifty subjects with BMI >30, aged 20-55 years without known co-morbidities and not smoking were randomized with stratification by sex to one of three planned diets; LFHC-AC (Low-fat high-carbohydrate – acellular, 45 energy (E)% carbohydrates, 38 E% fat, 17 E% protein), LFHC-C (Low-fat highcarbohydrate – cellular, 45 energy (E)% carbohydrates, 38 E% fat, 17 E% protein) and LCHF (Low-carbohydrate high-fat, 8 E% carbohydrates , 75 E% fat, 17 E% protein). Data at baseline and 3-months were collected at the Research Unit for Health Surveys in Bergen. Fasting and postprandial (30, 60, 90, 120 and 240 minute) TAG, fasting NEFA, fasting and postprandial glucose and anthropometric measurements were assessed. Adjusted and unadjusted differences between diets and within diets were analyzed with linear mixed models (LMEM) in R by a blinded statistician. The CARBFUNC study is approved by the Regional Committees for Medical and Health Research Ethics (REC) (2017/621) and registered in Clinical trials as NCT03401970. Results: Although, only the LCHF group showed a significant decrease in postprandial TAG at all measured time-points as well as in the fasted state, no significant group differences in absolute score change in postprandial TAG, fasting TAG or fasting NEFA were seen between the control group (LFHC-AC) and the LFHC-C diet or the LCHF diet after three months on the intervention. However, the total TAG AUC decreased substantially greater in the LCHF diet than in the LFHC-C diet, causing a significant difference between the two diets. A weak positive correlation was observed between change in 4-hour TAG and change in body weight independent of diets (Spearman´s Rho = 0.346, P = 0.014). A positive correlation was also seen within the LCHF diet (Spearman´s Rho = 0.549, P = 0.015). No significant correlation was seen between TAG and NEFA. NEFA increased significantly on the LFHC-AC and LCHF diets, but no significant differences were seen between any of the groups. There was no significant relationship between circulating postprandial glucose and postprandial TAG (lowest P = 0.072 within the LFHC-C group). For men across all groups combined, there was a statistically significant change in 4-hour TAG (P = 0.014), fasting TAG (P = 0.007) and fasting NEFA (P = 0.003), but this was not observed for women. Conclusion: Neither the LFHC-C nor the LCHF diet showed a significantly different change in 4-hour postprandial TAG levels compared to the LFHC-AC diet, suggesting that carbohydrate VI cellularity and amount of carbohydrates do not differentially affect postprandial handling of TAG. The hypothesis that adaptation to a high-fat, low-carbohydrate diet improves postprandial lipemia was however supported. The mean TAG concentrations decreased significantly for all postprandial timepoints (30-, 60-, 90-minutes, 2- and 4-hour) in addition to a significant decrease in the total AUC within the LCHF diet after three months, while no such decreases were seen for the LFHC diets.These results indicate that the participants particularly on the LCHF diet adapted by enhancing postprandial TAG clearance capacity. These effects associated with loss of body weight specifically within the LCHF diet and not within the LFHC groups. A correlation between loss of body weight and 4-hour postprandial TAG values were also seen for all groups combined. Fasting TAG concentrations were also reduced on the LCHF diet, in correlation with postprandial TAG levels. Overall, the data suggest a favorable adaptive effect of the LCHF diet to reduce both fasting and postprandial TAG concentrations. There were no correlations between fasting NEFA and fasting or postprandial TAG on any of the diets, but the statistical power may have been too low. There were no significant differences in change in NEFA on the different diets, although the levels increased significantly on the LFHC-AC and LCHF diet but not on the LFHC-C diet. Further, no clear relationship was found between change in postprandial circulating TAG and postprandial glucose after three months on the diets. Finally, no significant differences between gender were found for the variables 4-hour TAG, fasting TAG and fasting NEFA.
PublisherThe University of Bergen
SubjectObesityDietCarbohydratesTriacylglycerolsCellular carbohydratesAcellular carbohydratesHuman nutritionHuman ernæring
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