True, almost all regions of the world are currently experiencing an increase in the prevalence of NCDs – in part because, as deaths from acute infectious diseases and injuries decline, people live long enough to develop these diseases. But NCDs are increasing for many other demographic and epidemiological reasons as well – and understanding these has implications for health policy, and even for economic development.
In much of the world, populations are growing and aging simultaneously. Most NCDs increase in prevalence with age – a consequence of the cumulative exposure to risk factors (including unhealthy behaviors such as tobacco use and biological risk factors such as high blood pressure) over a lifetime. All else being equal, larger and older populations mean more people with NCDs.
This “population aging” effect is well understood. Far less well understood are the epidemiological forces that drive NCD dynamics. From an epidemiological perspective, NCD prevalence is determined by the difference between the rate at which previously healthy people become ill (incidence) and the rate at which ill people either recover or die (from any cause). If inflow exceeds outflow, prevalence rises.
Over the past several decades, standards of living, lifestyles, and biological risk factors have generally improved worldwide (obesity is an exception). So, contrary to popular belief, the incidence of most NCDs other than diabetes has actually been falling. Nevertheless, NCD prevalence has increased, because improvements in survival have outpaced reductions in incidence. Inflow and outflow have both fallen, but outflow has fallen further and faster.
Several factors underpin the recent dramatic gains in survival at older ages. People living with a chronic disease may die not only from that disease, but also from other causes – including other NCDs, acute infections, and injuries. In particular, more accessible and higher-quality health care has significantly improved survival rates for people living with NCDs, including diabetes.
Yet health care is not solely responsible for the observed improvement in survival rates. Improvements in lifestyle and related risk factors have contributed as well. A decline in the proportion of people who use tobacco, consume unhealthy diets, are physically inactive, and/or have elevated blood pressure and cholesterol does more than just prevent disease. Not only do fewer cases occur, but the NCD cases that do tend to be less severe, and to progress more slowly than was previously the case.
As a result, the increase in prevalence that has been seen in recent decades for many NCDs largely reflects an increase in the prevalence of early stages of the disease only. Increasing overall disease prevalence has hidden decreasing prevalence of late-stage or complicated disease. I have called this shift towards the milder end of the NCD spectrum the “severity effect.”
Most health problems linked to NCDs – problems such as chronic pain, disordered sleep, depression, disability, and premature death – are associated with late-stage or complicated disease, rather than with early-stage or uncomplicated disease. Whenever the “severity effect” outweighs the “prevalence effect,” the increasing overall prevalence of the NCD will be accompanied by a decreasing health impact, not an increasing disease “footprint,” as is widely assumed.
This is the paradox of NCDs: objective measures of poor health (severe symptoms, disability, premature death) are declining, even as the prevalence of these diseases is increasing. And, while this paradox is no excuse for complacency in responding to what the United Nations has rightly called a global NCD crisis, it does have practical implications for that response.
First, the primary concern should not be with reducing disease prevalence, but rather disease burden – the health impact as measured by disability and premature mortality. That means channeling resources according to burden rather than according to prevalence, particularly as co-morbidity (two or more diseases in the same patient) increases.
Second, we should concentrate less on improving health care and more on strengthening disease prevention, for example by driving down tobacco use, expanding opportunities for physical activity, and increasing the availability and affordability of a healthy diet. A greater focus on prevention can both reduce the incidence of NCDs and ensure that those cases that continue to occur will tend to be less severe and will progress more slowly, allowing scope for inexpensive but effective treatment in primary-care settings. Both mechanisms – lower incidence and lesser severity – will contribute to a smaller disease “footprint,” even as NCD prevalence continues to rise.
Martin Tobias is a public-health physician in Wellington,
New Zealand.
Martin Tobias: 非傳染病悖論
惠靈頓—艾爾伯特·愛因斯坦有一句名言：“越簡單越好，但不要太簡單。”但當下關於全球性爆發的非傳染性疾病（NCD，即心臟病、中風、糖尿病、癌症等慢性病）顯然沒有把這句話當回事。決策者將這一挑戰簡單化了，他們專注於NCD越來越普遍的發生——即罹患疾病人數的陡峭增加，但我認為，真正的問題並不在這裡。
誠然，幾乎在世界所有地區，都在經歷NCD的普遍發生，這部分是因為隨著急性傳染病和傷害致命人數的下降，人們的壽命得到了增加，使得越來越多的人患上了NCD。但CND的增加同樣也有人口和流行病因素在其中，弄清楚此中緣由將對衛生政策甚至經濟發展都帶來重大影響。
世界上有很多地區同時發生著人口的增加和老化。大部分NCD的出現伴隨著年齡的老化——這是一生中累積風險因素的結果（包括吸煙等不健康行為以及高血壓等生物性風險因素）。在其他條件相同的情況下，人口的增加和老化意味著更多的人患上NCD。
這一“人口老化”效應已為人所熟知。不為人所熟知的是NCD動態背后的流行性因素。從流行病學的觀點看，NCD的普遍性決定於先前健康的人群的發病率（事件）和病人被發現發病或死亡（不管什麼原因）頻率之差。
在過去幾十年中，生活水平、生活方式和生物性風險因素在全世界范圍內獲得普遍的改善（肥胖是一個例外）。因此，與流行的觀點相反，大部分NCD事件的發生（糖尿病除外）實際上是在下降的。盡管如此，NCD的普遍性在上升，因為生存率的改善要更甚於發病率的下降。流入和流出都在下降，但流出下降得更快更深。
老齡人口生存率的顯著上升有幾大原因。患有慢性病的人最終的死亡原因並不隻有這些疾病，還有其他因素——包括其他NCD、急性傳染病以及受傷。特別是，更便捷、更高質量的醫療已經顯著改善了患有NCD（包括糖尿病）的人群的生存率。
但對於我們所觀察到的生存率改善的事實來說，醫療並不是唯一因素。生活方式和相關風險因素的改善也起到了作用。吸煙、飲食不健康、缺乏體育鍛煉、高血壓和膽固醇人口比例的下降所起到的效果不僅僅局限於預防疾病。不但NCD的發病率下降了，而且即使發生了NCD，發病程度和勢頭也不像以前那樣嚴重和迅猛。
因此，最近幾十年來多種NCD普遍性的增加在很大程度上隻是早期疾病的普遍性增加了。總體NCD普遍性的增加隱藏了晚期或復雜病症普遍性的下降。我將這一NCD患病人群症狀的輕化趨勢稱為“嚴重度效應”。
與NCD有關的大部分健康問題——慢性病痛、睡眠不規律、壓抑、殘疾和過早死亡——大多與晚期或復雜病症，而不是早期和非復雜病症有關。隻要“嚴重度效應”壓過了“普遍性效應”，總體NCD的普遍性的增加就會伴隨著對健康影響的下降，而不是許多人認為的疾病“覆蓋面”的增加。
這就是NCD的悖論：不健康程度的客觀標准（嚴重症狀、殘疾、過早死亡）在下降，盡管這些疾病的普遍性在上升。這一悖論並不能作為對聯合國的“全球NCD危機”的正確說法的嗤笑，但對於如何應對具有實踐性影響。
首先，關注的焦點不應該是疾病普遍性的下降，而是疾病負擔——用致殘和致過早死亡的標准來衡量——的下降。這意味著應該根據負擔，而不是普遍性，來配置資源，比如（特別是）並發症（即兩種或以上病症在同一患者身上出現）的增加情況。
其次，我們不應當過多關注醫療的改善，而應當關注疾病預防，比如減少煙草的使用、增加體育鍛煉、增加健康飲食的可獲得性和成本。更關注預防既可以降低NCD的發生，也可以確保即使NCD發生，其症狀也會較輕，發展也會較慢，以便採取低成本、高效率的初級護理療法。這兩大機制——更低的發生率和更輕的嚴重度——將使疾病“覆蓋面”有所萎縮，盡管NCD的普遍性仍將繼續上升。
Martin
Tobias 是新西蘭惠靈頓的一位公共衛生醫師。