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Cutting Back on One Amino Acid Increases Lifespan of Middle-Aged Mice Up to 33%
A new study in mice found limited intakes of one particular essential amino acid slowed the impacts of ageing and even lengthened their lifespan.
Scientists are now wondering if these findings could help people improve their longevity and quality of life.
Isoleucine is one of three branched-chain amino acids we use to build proteins in our bodies. It is essential for our survival, but since our cells can't produce it from scratch, we have to get it from sources like eggs, dairy, soy protein and meats.
But there can always be too much of a good thing. Earlier research using data from a 2016-2017 survey of Wisconsin residents found dietary isoleucine levels were linked with metabolic health and that people with higher BMIs were generally consuming much greater quantities of the amino acid.
"Different components of your diet have value and impact beyond their function as a calorie, and we've been digging in on one component that many people may be eating too much of," says metabolism researcher Dudley Lamming from the University of Wisconsin, US, who was involved in both studies.
"It's interesting and encouraging to think a dietary change could still make such a big difference in lifespan and what we call 'healthspan,' even when it started closer to mid-life."
A genetically diverse group of mice was fed either a diet containing twenty common amino acids as a control, a diet where all amino acids were reduced by about two-thirds, or a diet where only isoleucine was reduced by the same amount.
The mice were around six months old at the start of the study, which is the equivalent age of a 30 year old person. They could eat as much as they wanted, but only from the specific kind of food provided to their group.
Restricting dietary isoleucine increased the lifespan and healthspan of the mice, reduced their frailty, and promoted leanness and glycemic control. Male mice had their lifespans increased 33 percent compared to those whose isoleucine was not restricted, and females had a 7 percent increase.
These mice also scored better in 26 measures of health, including muscle strength, endurance, blood sugar levels, tail use, and hair loss.
The male mice in this group had less age-related prostate enlargement, and were less likely to develop the cancerous tumors that are common in the diverse mice strains.
Curiously, the mice given low isoleucine food also ate significantly more calories than the others. But rather than gaining weight, they actually burned more energy and maintained leaner body weights, even though their activity levels were no different.
The researchers think restricting isoleucine in humans, either by diet or pharmaceutical means, has the potential to yield similar anti-ageing effects - although, as with all mice studies, we won't know for sure until it's actually tested in humans.
注释：
Isoleucine: n
表示"异亮氨酸"，如：For example rice is low in isoleucine and lysine.例如，米饭中的异亮氨酸和赖氨酸就非常少。
frailty: n
表示" 脆弱；意志薄弱"，means "the state of being weak in health or body ; moral weakness"，如：Despite his age and frailty, he did not die. 尽管他年老虚弱，但他没有死。One of the frailties of human nature is laziness.
人性的弱点之一是懒惰。
glycemic: adj
表示"血糖的"，如：No significant differences were observed in glycemic measures.无显着差异，观察血糖的措施。
prostate: n
表示" 前列腺"，如：The prostate is a small gland in men.前列腺是男性体内的一个小腺体。
中文简要说明：
蛋白质的基本单元是胺基酸，有些胺基酸我们身体无法合成，必须靠食物摄取，被称为「必需胺基酸」。以往的营养学课本都会强调必需胺基酸的重要性，但是事事无绝对。一项针对小鼠的新研究发现，灭少一种必需胺基酸的摄取，竟然减缓老化影响，延长小鼠的寿命。
科学警报(ScienceAlert)报导，异白氨酸(Isoleucine)是我们用来在体内建构蛋白质的三种支链氨基酸之一。它就像基础元素，对我们的生存至关重要，但由于我们的细胞无法从头开始生产它，因此我们必须从鸡蛋、乳制品、大豆蛋白和肉类等来源获取。
但是再重要的东西，过量也是负担。一项统计研究，使用 2016~2017 年美国威斯康星州居民调查数据发现，BMI 较高的人，通常会消耗更多的氨基酸，而且代谢出现问题的居民，饮食中的异白氨酸都都偏高。
美国威斯康辛大学的营养学研究员勒明( Dudley Lamming)表示：我们一直在研究许多人摄入过多的营养成份，是否会造成负担。」
好消息是，只要改变饮食习惯，就能调整体质，即使年近中年候，饮食习惯的改变，仍然可以正向的改变健康状态。
研究人员以小鼠为实验，这些小鼠的基因很多样，排除基因单一化的问题。它们被喂食含有二十种常见氨基酸的饮食作为对照，有些组的小鼠所有氨基酸减少约三分之二的饮食，还有一些组的小鼠的异白氨基酸减少 。
研究开始时，这些小鼠大约六个月大，相当于人类的30岁。他们想吃多少就吃多少，但只能吃提供给它们的特定食物。
在三组小鼠当中，被刻意减少异白氨酸的那一组，它们的寿命和健康状况明显较好，身型状态好，血糖控制也较好。与异白氨酸未限制的小鼠相比，雄性小鼠的寿命延长了 33%，雌性小鼠的寿命延长了 7%。
在 26 项健康指标上，这些小鼠的表现都不错，包括肌肉力量、耐力、血糖水平、尾巴健康状态都是，而且罹癌的机率也较低。
奇怪的是，给予低异亮氨酸食物的小鼠，即使吃更了多的热量饮食，但体重不但没有增加，反而代谢的更多，继续保持更瘦的体重，尽管他们的活动量也没有什么不同。
研究人员认为，假如人体机能也类似，那么透过饮食或药物手段，限制人类体内的异白氨酸，有可能产生类似的抗衰老效果。当然，这在对进行人体实测之前，我们无法确定。
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