肥胖指的是体脂肪过度堆积而对健康造成负面影响的身体状态[8]:6。肥胖最初的定义以身体质量指数(Body Mass Index, BMI)为指标,之后也加入了其他标准,例如评估腰臀比或其他心血管病症风险因子[22][23]:14。BMI与体脂肪率和体脂肪总量都密切相关[24]。儿童的健康体重因性别与年龄而有不同的标准,此标准需透过与同年龄性别的其他孩子(常模)比较来决定。举例而言,儿童与青少年的肥胖定义为BMI高于95%的同侪(95百分位以上)[25]。用于决定儿童与青少年肥胖与否的常模资料来自1963年至1994年,因此并没有反映近年平均体重的上升[26]。
肥胖症是最常见的可预防性致死因素(英语:Preventable causes of death)之一[43][44][45]。欧美的大规模研究指出身体质量指数(BMI)介于20到25公斤/平方米的非吸烟者[42][46]以及BMI介于24-27公斤/平方米的吸烟者在非吸烟和吸烟者群体中分别有最低的死亡风险,高于或低于这个范围死亡风险都会上升[47][48]。亚洲人则在BMI达到22到25公斤/平方米时死亡率就开始上升[49]。BMI大于32公斤/平方米的女性未来十六年内的死亡率高达常人的两倍[50]。肥胖每年带走111,909至365,000条美国人的生命[2][45];欧洲有7.7%的死亡(相当于约100万人)和过重相关[51][52]。平均而言,肥胖会减少6-7年的余命[2][53],当一个人的BMI达到30-35公斤/平方米间时,余命会减少2-4年[46],而严重肥胖者(BMI大于40公斤/平方米)则会减少十年余命[46]。
不同的区域与国家的人均饮食热量摄取(英语:Dietary energy supply)差别很大,并会随着时代而有明显改变。从1970年代早期到1990年代晚期,除了东欧地区外,全球的人均每日热量摄取(购买的食物量)都在上升。1996年,人均每日热量供应最高的国家是美国,高达3,654卡路里(15,290千焦耳),到了2003年更来到了3,754卡路里(15,710千焦耳)。1990年代晚期,欧洲人平均每人每日购买3,394卡路里(14,200千焦耳)的食物,而亚洲的发展中国家每人每日的食物购入量有2,648卡路里(11,080千焦耳),而撒哈拉以南非洲则是2,176卡路里(9,100千焦耳)[95]。总热量的摄取量与肥胖的发生有关[96]。
即使现在的饮食指引(英语:List of nutrition guides)相当普及[97],但饮食过量与饮食内容不健康的问题却未因此改善[98]。自1971年到2000年,美国的肥胖率从14.5%爬升到30.9%[99],而平均的热量摄取也在同期上升。女性的每日热量摄取从1971年的1,542卡路里(6,450千焦耳)到2004年的1,877卡路里(7,850千焦耳),平均上升了335卡路里(1,400千焦耳);而男性从1971年的2,450卡路里(10,300千焦耳)到2004年的2,618卡路里(10,950千焦耳),平均增加了168卡路里(700千焦耳)。饮食热量的上升主要来自于碳水化合物摄取量的上升,而非油脂摄取的上升[100],多出来的碳水化合物主要来自含糖饮料和马铃薯片[101]。现代美国年轻人的饮食中,含糖饮料占了每日热量摄取的近四分之一[102];一般认为碳酸饮料、调味果汁、调味茶、运动及能量饮品等含糖饮料在肥胖率的上升中居功厥伟[103][104],并造成代谢症候群与第2型糖尿病的风险上升[105]。
肥胖的发生与维持有许多种可能的病理生理机转[152],但这个研究领域一直乏人问津,在1994年时,弗莱德曼的研究团队才发现瘦素基因[153]。弗莱德曼的研究团队认为瘦素是影响饱足感的因子。实验中发现胖胖鼠(英语:ob/ob mouse)(一种意外发现,具有过度食欲并且极端肥胖的实验用小鼠)带有突变瘦素基因,因此展现肥胖的表现型,这个结果开启以瘦素来治疗人类肥胖的可能性,但不久后卡罗(英语:Jose F. Caro)的研究团队就发现无法在肥胖的人身上找到任何瘦素基因的突变;另一方面,在肥胖者身上的瘦素基因表现量上升,反而指出了肥胖者身上可能有瘦素抵抗现象[154]。在发现瘦素后,研究者陆续发现许多其他影响食欲、进食、脂肪储存、胰岛素抗性的激素,举例来说饥饿素、胰岛素、食欲素、双酪胺酸肽3-36(英语:peptide YY)、胆囊收缩素、脂联素等因子都是学者研究的对象。脂激素(英语:Adipokine)是由脂肪组织释放出的讯息因子,它们的作用会影响许多肥胖相关的疾病。
自文艺复兴时代开始,部分上层阶级开始炫耀自己的体胖,在英王亨利八世与亚历山大·达·布罗(英语:Alessandro dal Borro)的画像中就能看到这样的趋势[18]。知名画家鲁本斯经常描绘鲁本斯式的女性全身像,虽然表现上也强调多产,但在他画中的女性仍然维持沙漏状的身形[210]。到了十九世纪,整个西方世界对于肥胖的看法开始改变,在肥胖作为财富与地位的象征许多世纪以后,纤瘦取而代之成为新的追求目标[18]。
在许多国家,宠物肥胖(英语:Obesity in pets)是常见的问题。在美国,有23到41%的狗过重,当中有5.1%达到肥胖的程度;猫的肥胖率则稍高一些(6.4%)[238]。在澳洲,兽医统计的犬只肥胖率为7.6%[239]。犬只肥胖的风险与饲主是否肥胖相关,但猫只的风险则与饲主肥胖与否无关[240]。
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