飲⾷中脂肪含量在飲⾷限制與體重循環中對肝臟的影響

Abstract

飲⾷限制（Dietary restriction）是指在不引起營養不良的前提下減少⽇常⽣活總熱量攝取之百分之20％ ⾄40％，已被廣泛證實可延⾧多種物種的壽命。除了延壽效果之外，飲⾷限制亦被認為能透過維持粒線體功能，改善與⽼化相關的疾病，如代謝性脂肪肝病(metabolic dysfunction-associated steatotic liver disease, MASLD)。飲⾷限制也可作為減重的⼿段。然⽽在實際應⽤中，⾧期且持續實施飲⾷不易維持，飲⾷限制的中斷易導致體重出現波動，進⽽導致體重循環(weight cycling)的出現。 僅管飲⾷限制的益處已被廣泛探討，但多數⾧期研究是以標準低脂飼料為基礎，與現代⼈偏向西⽅化、⾼脂肪含量的飲⾷型態有所差異，因此尚不清楚在⾼脂飲⾷背景下實施飲⾷限制是否仍能帶來正⾯健康效益。若能釐清不同脂肪含量飲⾷下飲⾷限制的效果，並進⼀步了解由攝⾷量變化引起的體重循環如何影響健康，將有助於制定更合適的飲⾷策略。 本研究旨在探討中等脂肪與⾼脂飲⾷背景下，⾧期飲⾷限制及體重循環對⼩⿏肝臟健康的影響。以中等脂肪含量的繁殖飼糧與⾼脂飼糧餵飼⼩⿏，建⽴飲⾷限制的動物模型；同時透過任飼（ad libitum）與飲⾷限制兩種餵飼⽅式交替，模擬體重循環的狀態。 在第⼀個實驗中，餵飼C57BL/6 公⿏中等脂肪飼糧，並分為四組：⾃由進⾷（AL）組、連續飲⾷限制（DR）組，以及兩組體重循環組別（S-AL組及S-DR組）。S-AL 組別起始於⾃由進⾷⽽S-DR 則起始於飲⾷限制，後續更換兩種飲⾷⽅法（⾃由進⾷或飲⾷限制）數次來達到體重循環。S-AL 及S-DR 兩組別分別在⾃由飲⾷引起體重復重及飲⾷限制引起之體重下降後犧牲。在中等脂肪飲⾷中，連續飲⾷限制可有效降低體重與肝臟脂肪堆積；S-DR ⼩⿏與DR組表現相似，也顯⽰有利於體重與肝脂控制效果。相較之下，S-AL ⼩⿏雖肝臟脂肪堆積下降，但卻出現加劇的肝臟細胞凋亡與纖維化，並伴隨粒線體融合、⽣成與⾃噬相關蛋⽩表現下降。這些結果顯⽰，在中等脂肪飲⾷模式中，體重循環後，若體重復重反⽽可能導致更嚴重的肝損傷；反之，體重減輕階則能保留其健康效益。 在第⼆個實驗中，探討⾼脂飲⾷下的影響。C57BL/6 公⿏分為四組：標準飼料⾃由進⾷（CTR）組、⾼脂飲⾷⾃由進⾷（H）組、⾼脂飲⾷飲⾷限制組（HDR），以及體重循環（WC）組。HDR與WC組⼩⿏體重雖低於H組，但⾎糖未改善且依然引起肝臟脂肪變性、纖維化及⾮酒精脂肪肝（nonalcoholic fatty liver disease, NAFLD) 評分依然偏⾼，此外，兩組肝臟中的ATP 含量下降，粒線體動態與⾃噬相關蛋⽩（如 MFN2、LC3、p62）表現也下降。此結果指出，⾼脂飲⾷下之飲⾷限制及體重循環，仍會引起肝臟損傷，並可能損害粒線體功能。 總結⽽⾔，透過飲⾷限制降低熱量攝取雖有效降低體重，然⽽其對肝臟之影響取決於飲⾷脂肪含量。此外，即便中等脂肪飲⾷限制可對肝臟帶來益處，然⽽中斷飲⾷限制造成體重回升，則會導致更嚴重的肝臟損傷。因此，控制飲⾷中脂肪含量並預防體重回升，是減緩肝臟損傷的關鍵。

Dietary restriction (DR), characterized by a reduction of 20% to 40% in habitual caloric intake without inducing malnutrition, has been extensively demonstrated to extend lifespan across a wide range of species. Beyond its reported effects on lifespan extension, DR has also been implicated in mitigating age-associated diseases, including metabolic dysfunction-associated steatotic liver disease (MASLD), through the preservation of mitochondrial function. DR can also be a strategy to lose body weight. Nevertheless, the long-term implementation of continuous DR remains a considerable challenge, as it is frequently disrupted in practical settings, thereby giving rise to weight cycling patterns. Although the benefits of dietary restriction have been widely investigated, most long-term studies are based on standard low-fat diets, which differ from the modern Westernized dietary pattern characterized by higher fat content. Therefore, it remains unclear whether implementing dietary restriction under a high-fat dietary background can still confer positive health effects. Clarifying the effects of dietary restriction with different dietary fat content and further understanding how weight cycling caused by variations in feeding patterns impacts health would contribute to the development of more appropriate dietary strategies. This study aimed to investigate the effects of long-term dietary restriction and weight cycling on liver health under moderate- and high-fat dietary backgrounds. Mice were fed either a moderate-fat breeding diet or a high-fat diet to establish a dietary restriction animal model. Additionally, alternating between ad libitum feeding and dietary restriction was applied to simulate weight cycling. In the first experiment, C57BL/6 male mice were fed a moderate-fat breeding diet and divided into four groups: ad libitum feeding (AL) group, continuous DR (DR) group, and two weight cycling groups, which were sacrificed either at the end of the ad libitum phase (S-AL) or at the end of the restriction phase (S-DR). Continuous DR under a moderate-fat diet showed beneficial effects on body weight and hepatic lipid accumulation. S-DR mice showed similar beneficial effects on body weight and liver lipid content to the DR group. In contrast, compared to AL mice, S-AL mice exhibited reduced hepatic lipid content but showed exacerbated apoptosis and fibrosis, as well as reduced expression of mitochondrial fusion, biogenesis, and mitophagy markers. These findings reveal that, in the moderate-fat breeding diet model, termination in the AL phase is associated with more severe hepatic damage, whereas termination in the DR phase shows the healthy benefits. In the second experiment, the effects of high-fat diets were investigated. C57BL/6 male mice were assigned to four groups: chow diet ad libitum (CTR) group, high-fat diet ad libitum (H), restricted high-fat diet (HDR) group, and weight cycling (WC) group with high-fat diet. Both the HDR and WC groups exhibited lower body weight compared to the H group, but blood glucose levels were still sustained, and pathological alterations were provoked. Hepatic steatosis, fibrosis, and NAFLD scores remained elevated. Additionally, the HDR and the WC groups showed reduced ATP contents and altered expressions of mitochondrial dynamics and autophagy-related proteins in the liver. These findings indicate that DR under high-fat conditions induces liver injury and may impair mitochondrial function, regardless of restriction pattern. Together, although reduced caloric intake contributes to the metabolic benefits of DR, dietary fat plays a more critical role. These findings highlight the importance of dietary fat and the prevention of weight regain to reduce hepatic injury.