喝牛奶竟比喝水更补水，你真的懂怎么喝水吗？
大多数人都听说过，每天必须喝8杯水，以保持身体水分，但是令人意外的是，几乎没有数据支持这一建议。但是现在，一种新的补水指数为如何最有效地补水提供了以事实为依据的建议。
这一指数是根据英国威尔士班戈大学、英格兰罗浮堡（或译拉夫伯勒、拉夫堡）大学、苏格兰斯特灵大学在2015年12月第2届国际及第4届西班牙补水大会公布的一项随机研究开发出来的。该研究追踪了没有在运动的典型消费者喝下13种常见饮料后，身体里停留的时长。该研究摘要发表于西班牙肠外肠内营养学会官方期刊《医院营养》（Nutr Hosp. 2015 Dec 1;32 Suppl 2:10264），全文发表于美国营养学会官方期刊《美国临床营养杂志》（Am J Clin Nutr. 2016 Mar;103:717-23）。
补水指数是仿效著名的血糖指数设计的，测量身体对不同食物的碳水化合物的反应（血糖指数用于帮助个人控制葡萄糖胰岛素反应）。新的补水指数的指导原则是，有些液体比其他液体在身体里留存的时间更长，补水效果更好。毕竟，如果喝了一杯水，然后很快通过尿液排出一半水分，那么其实没有补充8盎司水，而是只补了4盎司。
该研究确定了13种常见饮料的补水指数。72名25岁左右的男性参与者喝下1升水作为基准。2小时后依然留存在参与者体内的水分含量（也就是没有通过尿液排出的水分）被指定为1.0。所有其他饮料以类似的方式进行评估，然后与水比较进行评分。得分高于1.0的饮料的体内水分留存率高于水；得分低于1.0的饮料的排泄率高于水。
结果表明，有4种饮料的补水指数比水高得多，分别是口服补水溶液（比如电解质液体）、脱脂牛奶、全脂牛奶和橙汁。前3种的补水指数在1.5左右，橙汁的指数略高于水，为1.1。口服补水溶液是为对抗严重脱水设计的，比如因为慢性腹泻导致的脱水。
为什么牛奶在补水方面如此有效？通常情况下，喝饮料意味着肾要通过制造更多尿液来去除多余的水分。不过，如果饮料中包含钠和钾等营养素和电解质，就像牛奶那样，那么胃的排空速度更慢，对肾的影响不那么剧烈。
也许令人意外的是，包含适量咖啡因和酒精或高水平糖分的饮料与水的补水指数没有差别。换而言之，咖啡和啤酒并不会让人脱水，尽管这与人们通常的看法相反。另外，普通汽水的补水效率和水一样。
咖啡因的确是利尿剂，不过大部分咖啡饮品中的咖啡因浓度达不到这个效果。人们口渴时，喝正常的茶、咖啡或可乐有助补充水分。非常浓的咖啡或酒精饮料，比如蒸馏酒，是例外。
在决定何时喝何种饮料时，补水指数可能被证明是有用的。比如，如果长时间开车，无法得到很多液体（也不能常去厕所），那么喝牛奶比喝水或冰咖啡更好，因为其补水指数较高。不过，不要忘记，牛奶的热量比水高，所以也不要喝得过多。
虽然严重脱水是剧烈运动、极端环境或患病情况下的罕见例外，但是多项研究表明，在高温天气下，高温和脱水可能导致死亡率升高。在热浪来袭时，死亡率猛烈升高，主要是因为人们没有摄入足够的水分，以弥补增加的水分流失。
这是非常聪明的研究，假定水是最理想的补水液体，从生物学角度讲是这样的，然后把其他液体与水进行比较。
在高温天气里，全天监测身体的水分状态很重要。因此，建议注意口渴状况，在需要时补水。另外，如果尿液颜色是深黄色的，就需要喝点清爽的饮料。
Nutr Hosp. 2015 Dec 1;32 Suppl 2:10264.
Development of a hydration index: a randomized trial to assess the potential of different beverages to affect hydration status.
Oliver S, Walsh N, Maughan RJ, Watson P, Cordery PA, Dolci A, Rodríguez Sanchez N, Galloway SD.
Bangor University, Bangor, Wales, United Kingdom; Loughborough University, United Kingdom; University of Stirling, United Kingdom.
PMID: 26615234
DOI: 10.3305/nh.2015.32.sup2.10264
Am J Clin Nutr. 2016 Mar;103(3):717-23.
A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index.
Maughan RJ, Watson P, Cordery PA, Walsh NP, Oliver SJ, Dolci A, Rodriguez-Sanchez N, Galloway SD.
Loughborough University, Loughborough, United Kingdom; Bangor University, Bangor, United Kingdom; University of Stirling, Stirling, United Kingdom.
BACKGROUND: The identification of beverages that promote longer-term fluid retention and maintenance of fluid balance is of real clinical and practical benefit in situations in which free access to fluids is limited or when frequent breaks for urination are not desirable. The postingestion diuretic response is likely to be influenced by several beverage characteristics, including the volume ingested, energy density, electrolyte content, and the presence of diuretic agents.
OBJECTIVE: This study investigated the effects of 13 different commonly consumed drinks on urine output and fluid balance when ingested in a euhydrated state, with a view to establishing a beverage hydration index (BHI), i.e., the volume of urine produced after drinking expressed relative to a standard treatment (still water) for each beverage.
DESIGN: Each subject (n = 72, euhydrated and fasted male subjects) ingested 1 L still water or 1 of 3 other commercially available beverages over a period of 30 min. Urine output was then collected for the subsequent 4 h. The BHI was corrected for the water content of drinks and was calculated as the amount of water retained at 2 h after ingestion relative to that observed after the ingestion of still water.
RESULTS: Total urine masses (mean ± SD) over 4 h were smaller than the still-water control (1337 ± 330 g) after an oral rehydration solution (ORS) (1038 ± 333 g, P < 0.001), full-fat milk (1052 ± 267 g, P < 0.001), and skimmed milk (1049 ± 334 g, P < 0.001). Cumulative urine output at 4 h after ingestion of cola, diet cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water, and a sports drink were not different from the response to water ingestion. The mean BHI at 2 h was 1.54 ± 0.74 for the ORS, 1.50 ± 0.58 for full-fat milk, and 1.58 ± 0.60 for skimmed milk.
CONCLUSIONS: BHI may be a useful measure to identify the short-term hydration potential of different beverages when ingested in a euhydrated state. This trial was registered at www.isrctn.com as ISRCTN13014105.
KEYWORDS: dehydration; electrolytes; euhydration; fluid balance; gastric emptying; intestinal absorption; macronutrients; rehydration; renal excretion; urine
PMID: 26702122
DOI: 10.3945/ajcn.115.114769
Milk and Other Surprising Ways to Stay Hydrated.
Most Americans have heard that they should drink eight glasses of water a day to stay hydrated, but there is surprisingly little data to support this advice.
But now, a new "beverage hydration index" provides evidence-based suggestions for how to most efficiently hydrate. The index was developed from a British study published in December that tracked how long 13 common beverages remain in the body after being consumed.
"In the last 25 years, we've done many studies on rehydration after exercise," said Ronald J. Maughan, a hydration expert from Loughborough University, and lead author of the study. "We thought it was time to look at hydration in typical consumers who aren't exercising."
The hydration index is modeled after the well-known glycemic index, which measures how the body responds to the carbohydrate content of different foods. (The glycemic index is used to help individuals keep their glucose-insulin response under control.) The guiding principle behind the new hydration index is that some fluids last longer in your body than others, providing more hydration. After all, if you drink a cup of water and then immediately excrete half that amount in your urine, you haven't added 8 ounces to your water supply, but only 4.
The British study determined the hydration index of 13 common beverages by having the participants, 72 males in their mid-20s, drink a liter of water as the standard beverage. The amount of water still remaining in subjects' bodies two hours later -- that is, not voided in urine -- was assigned a score of 1.0. All other beverages were evaluated in a similar manner, and then scored in comparison to water. A score higher than 1.0 indicated that more of the beverage remained in the body as compared to water, while a score lower than 1.0 indicated a higher excretion rate than water.
The results showed that four beverages -- oral rehydration solution, like Pedialyte; fat-free milk; whole milk and orange juice -- had a significantly higher hydration index than water. The first three had hydration index scores around 1.5, with orange juice doing slightly better than water at 1.1. Oral rehydration solutions are specifically formulated to combat serious dehydration such as that resulting from chronic diarrhea.
"It's a very clever, even brilliant study," said Lawrence Armstrong, a hydration expert at the University of Connecticut and immediate past president of the American College of Sports Medicine. "It assumes that water is the optimal rehydration fluid, which is biologically correct, and then compares other fluids to water."
Why is milk so efficient at rehydration? "Normally when you drink, it signals the kidneys to get rid of the extra water by producing more urine," Maughan said. "However, when beverages contain nutrients and electrolytes like sodium and potassium, as milk does, the stomach empties more slowly with a less dramatic effect on the kidneys."
Perhaps surprisingly, drinks containing moderate amounts of caffeine and alcohol or high levels of sugar had hydration indexes no different from water. In other words, coffee and beer are not dehydrating, despite common beliefs to the contrary, and regular soda can hydrate you just as well as water.
"It's true that caffeine is a diuretic, but not at the concentration found in most coffee drinks," Maughan said. "When we are thirsty, drinking normal tea, coffee or cola helps to rehydrate us. The exceptions are very strong coffee drinks or strong alcoholic drinks like distilled spirits."
The hydration index could prove useful when making decisions about what beverages to consume and when. For example, if you're going on a long drive and won't have access to fluids (or to bathrooms), you'd be smarter to drink milk with its high hydration index rather than water or iced coffee. But don't forget that milk has many more calories than water, so don't overdo it, either.
While severe dehydration is rare except in heavy exercise, extreme environments and disease, studies have shown that heat and dehydration can contribute to increased mortality rates during hot weather. "Mortality increases sharply during heat waves, mostly because people don't drink enough to compensate for their increased fluid losses," Maughan said.
Armstrong noted that in hot weather, it's important to monitor your hydration status throughout the day. He suggested paying attention to your thirst, and drinking when necessary. Also, if your urine color is a dark yellow, it's time for a refreshing drink.