纽约时报 | “兔子饥饿症”:高蛋白饮食对人体有何影响

We don't need nearly as much protein as we consume “兔子饥 […]

We don't need nearly as much protein as we consume

In the early 20th Century, Arctic explorer Vilhjalmur Stefansson spent a collective five years eating just meat. This meant that his diet consisted of around 80% fat and 20% protein. Twenty years later, he did the same as part of a year-long experiment at the New York City’s Bellevue Hospital in 1928.

20世纪初,北极探险家斯蒂芬森(Vilhjalmur Stefansson)曾连续五年内只吃肉食,这意味着他的饮食由约80%的脂肪和20%的蛋白质组成。二十年后的1928年,他在纽约市贝尔维尤医院再做了一次为期一年的只吃肉食的实验。

Stefansson wanted to disprove those who argued that humans cannot survive if they only eat meat. But unfortunately for him, in both settings he very quickly became ill when he was eating lean meats without any fat. He developed "protein poisoning”, nicknamed “rabbit starvation”. His symptoms disappeared after he lowered his protein intake and he raised his fat intake. In fact, after returning to New York City and to a typical US diet with more normal levels of protein, he reportedly found his health deteriorating and returned to a low-carb, high fat, and high protein diet until his death aged 83.


His early experiments are some of the few recorded cases of high protein intake having extreme adverse effects C but despite soaring sales of protein supplements, many of us are still unsure how much protein we need, how best to consume it, and if too much, or too little, is dangerous.


Despite obesity rates doubling over the past two decades, we’re becoming increasingly conscious of what we’re eating. In recent years many of us have swapped white bread for brown and wholemeal bread and full-fat milk for skimmed. Taking centre stage in our health kick is protein, with protein balls, bars and enhanced protein versions of staple products, from cereals to soup, dominating supermarket shelves. And with the global protein supplements market valued at $12.4bn (£9.2bn) in 2016, it’s clear we’re buying into the idea that we need as much protein as possible.


But some experts now argue that foods with inflated protein (and prices) are a waste of money.


Protein is essential for the body to grow and repair. Protein-rich food such as dairy, meat, eggs, fish and beans are broken down into amino acids in the stomach and absorbed in the small intestine, then the liver sorts out which amino acids the body needs. The rest is flushed out in our urine.


Adults who aren’t especially active are advised to eat roughly 0.75g of protein per day for each kilogram they weigh. On average, this is 55g for men and 45g for women C or two palm-sized portions of meat, fish, tofu, nuts or pulses.


Not getting enough protein can lead to hair loss, skin breakouts and weight loss as muscle mass decreases. But these side effects are very rare, and largely only occur in those with eating disorders.


Despite that, most of us have long associated protein with building muscle. This is accurate. Strength-based exercise causes a breakdown of protein in the muscle. For muscles to grow stronger, the proteins need to rebuild. A type of amino acid called leucine plays a particularly big part in triggering protein synthesis.


Some experts even argue that not consuming protein post-workout could cause the breakdown of muscle to be higher than the synthesis C meaning there’s no net gain in muscle mass. Supplement brands advise drinking protein shakes after a workout to help the growth and repair of muscle tissue, usually in the form of leucine-rich whey protein, a by-product of making cheese.

一些专家甚至认为,为增强肌肉而锻炼,但事后不补充蛋白质可能会导致肌肉流失得多,而蛋白质合成重建得少 - 这意味着肌肉不增反减。蛋白补充剂品牌建议在健身锻炼后喝蛋白质饮料奶昔(shakes),以帮助肌肉组织的生长和修复,这种饮料为富含亮氨酸的乳清蛋白(一种制作奶酪的副产品)。

Many consumers agree. Research company Mintel’s 2017 Report found that 27% of Brits use sport nutrition products such as protein bars and shakes. This figure rises to 39% for those who exercise more than once a week. But more than half of the individuals who use the products (63%) find it difficult to tell whether they’re having any effect.


Indeed, research on the muscle-building power of protein supplements is varied. A 2014 analysis of 36 papers found that protein supplements have no impact on lean mass and muscle strength during the first few weeks of resistance training in untrained individuals.


Over time and if the training becomes harder, supplements can promote muscle growth. However, the paper also concludes that these changes have not been proven over the long term. A 2012 review paper further says that protein “increases physical performance, training recovery and lean body mass”… but for the benefit to be optimal, it should be in combination with a fast-acting carbohydrate.


But even if athletes and gym goers may benefit from a post-workout protein boost, that doesn’t mean they should reach for the supplements and smoothies. Most people get more than their daily recommended allowance from food, says Kevin Tipton, a sport professor of the University of Stirling. “There’s no need for anyone to have supplements. They’re a convenient way to get protein, but there’s nothing in supplements you can’t get in food. Protein bars are really just candy bars with a bit of extra protein.”

但即使运动员和健身爱好者可能从运动后蛋白质补充中受益,这并不意味着他们应该食用蛋白补充剂和冰沙(smoothies,水果、碎冰和酸奶搅拌而成的健康饮料,也译为果昔)。斯特林大学体育教授蒂普顿(Kevin Tipton)说:“大多数人通过食物摄入的蛋白含量已高过每日所需,没有必要再服用补充剂。虽然补充剂是获得蛋白质的便捷方式,但任何补充剂内的要素都已经包含在食品内。蛋白棒实际上只是多一点额外蛋白质的糖块。”

Tipton adds that even among bodybuilders, products like whey protein aren’t as critical as they are hyped up to be. “There’s too much focus on which supplements to take, as opposed to getting in the gym and working harder. There are so many other variables, such as sleep, stress and diet,” he says.


Most experts agree with Tipton that protein is best consumed in food instead of supplements. But there are some exceptions, such as athletes who find it difficult to hit their daily protein targets, points out Graeme Close, professor of human physiology at Liverpool John Moores University. “I believe most need more than the recommended daily allowance, and there’s good evidence to support this,” he says. In this case, he says, a shake can be useful.

大多数专家都同意蒂普顿的观点,即蛋白质最好通过食物摄入,而不是补充剂。但也有一些例外,如有的运动员发现仅靠饮食很难达到每日蛋白质摄入目标量。利物浦约翰摩尔大学人体生理学教授克洛斯(Graeme Close)指出,“我相信大多数人身体对蛋白的需要应该比推荐的每日摄取量要高,而且有很好的证据支持这一点”,在这种情况下,饮一杯奶昔是有益的。

Another demographic who can benefit from extra protein? The elderly. That’s because as we age, we need more protein to retain muscle mass. But we also tend to eat less protein as we get older because our taste-buds begin to prefer sweet over savoury.


Emma Stevenson, professor of sport and exercise science at Newcastle University, is working with food companies to get more protein into snacks that the elderly  are known to regularly buy, such as biscuits. “We need to maintain our muscle mass as we age, because we become less active and frail,” she says.

纽卡斯尔大学体育与运动科学教授史蒂文森(Emma Stevenson)正在与食品公司合作,将更多的蛋白质加入老人常吃的零食中,例如饼干。她说:“随着年龄的增长,我们需要摄入蛋白质以保持肌肉质量,因为我们的活力下降,躯体渐趋孱弱。"

Close says the elderly should increase protein intake to around 1.2g per kg body weight.


Fortunately, it’s difficult to have too much protein. While we do have an upper limit of protein intake, it’s “virtually impossible” to reach, says Tipton. “There are concerns among some dieticians that a high protein diet can hurt the kidneys and bones, but evidence in otherwise healthy people is minimal. It is possible there could be a problem if someone with an underlying kidney [issue] eats high amounts of protein, but the odds of any adverse effects are very low.”


But while protein itself isn’t harmful, many protein supplements are high in carbohydrates called FODMAPs that trigger digestive symptoms like bloating, gas and stomach pain. Stevenson advises reading labels carefully on supplements, bars and balls. “Often, they’re very high in calories and contain huge amounts of carbs, often in form of sugar. You shouldn’t necessarily think that because it says it’s high protein that its healthy,” she says.


Weight loss


Protein has long been linked to weight loss, with low-carb, high-protein diets such as Paleo and Atkins promising to prolong the feeling of fullness. People fail to lose weight often because they feel hungry, and MRI studies have shown that a high-protein breakfast can help stop cravings later in the day.


There is sufficient evidence that protein is satiating, says Alex Johnstone of the University of Aberdeen. If you’re trying to lose weight, it’s therefore more important to have a high-protein breakfast, such as beans on toast or a dairy smoothie, rather than to have supplements.

阿伯丁大学的约翰斯通(Alex Johnstone)认为,有足够的证据表明蛋白质能带来饱腹感。如果你想减肥,你需要拥有高蛋白质的早餐,比如说烤面包上的豆子,或者牛奶冰沙,而不是摄入补充剂。

But she doesn’t advocate “Atkins-type” diets and has found that cutting out carbohydrates has adverse affects on gut health (and we now know that maintaining a healthy gut is crucial to many aspects of our health and well-being).


Instead, Johnstone recommends that overweight people eat a high-protein and moderate-carb diet, consisting of 30% protein, 40% carbs and 30% fat C compared to the average diet of around 15% protein, 55% carbs and 35% fat.


But, of course, upping protein intake alone won’t help you lose weight. Choosing lean meat such as chicken or fish is key. Studies also show that eating large amounts of animal protein is linked to weight gain and red meat in particular is linked to an increased risk of cancer as well as heart disease.


There are, though, healthy proteins which are not meat like mycoprotein, a plant-based protein derived from fungi. Mycoproteins like Quorn are high in fibre as well as protein.


Researchers now are looking into how this unique composition (of both protein and fibre) can affect satiety and insulin levels, which are linked to type two diabetes. One team compared a mycoprotein diet to a chicken diet and found that the insulin levels in those who ate quorn achieved the same sugar control, but needed less insulin to be produced by the pancreas.


The risk of consuming too much protein is small, but the bigger risk might just be falling for overpriced products offering us more protein than we need. “Some products labelled as high protein aren’t, and they’re quite expensive. Anyway, consuming more protein than need is wasteful in terms of money, and it’s paid down the toilet,” says Johnstone.


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