May 15, 2013
Dr. Darwin Labarthe is a Professor in Feinberg's Department of Preventive Medicine. Recognizing more than four decades of distinguished service, the American Heart Association gave Darwin Labarthe, MD, PhD, MPH, the 2012 Gold Heart Award, its highest honor. An AHA volunteer since 1971, Dr. Labarthe founded and for 25 years directed the U.S. Ten-Day Teaching Seminars on the Epidemiology and Prevention of Cardiovascular Disease. Mark Huffman sat down with Dr. Labarthe to learn more about his career and his time studying sodium intake and hypertension in China.
Mark: Can you tell me a little about the work you have been doing in China?
Darwin: My major activities in China were launched during my CDC career, from the Division for Heart Disease and Stroke Prevention, beginning in 2007. It had two main aspects; one was to bring together scientific and public health leadership in China in the area of hypertension, specifically to address the potential impact of sodium reduction in the dietary pattern of China, especially in the north. The other is an ongoing randomized community trial called the China Rural Health Initiative (CRHI), which was initiated under the Centers of Excellence Program supported by National Heart, Lung, and Blood Institute (NHLBI) and UnitedHealth with funding to the George Institute-Beijing. The trial involves 120 communities distributed in 5 provinces in northeast China where salt intake reaches 15+ grams per day with correspondingly high stroke mortality.
The trial began about 3 years ago with a factorial design in which, of the 120 communities, 60 communities are randomly assigned to sodium reduction, including availability of a reduced-sodium salt substitute. The salt substitute is a blend of sodium chloride and potassium chloride such that, with a constant level of salt use, sodium consumption decreases while potassium consumption increases, which may have synergistic effects in reduction of blood pressure.
Mark: Is the reduction in sodium by about a quarter or a third?
Darwin: The usual formulation of the potassium sodium salt substitute is about 65% sodium chloride and 35% potassium chloride. That level of potassium content is considered palatable without a metallic taste.
The other part of the factorial trial, again with 60 communities randomly assigned to each of the two conditions, is continuing conventional management of hypertension or training of village doctors on use of a simple blood pressure. I should emphasize that the CRHI is being implemented in rural settings in China, not in urban communities.
The initial phase of the trial has been completed and evaluation is ongoing, but a major expansion has been made possible by the Australian National Health and Medical Research Council, which allows doubling the sample size in number of communities and extension in time by 3-5 years, thereby creating the possibility of evaluating effectiveness of the intervention against stroke outcomes.
Mark: What is your role in the project? How do you help out?
Darwin: My role initially, during the period I was at CDC until mid-2011, was to oversee the engagement of US CDC in the project. US CDC was interested to assure full implementation of the sodium reduction/salt substitute intervention. We developed an inter-agency agreement with NHLBI through which CDC funding supplemented the funding from NHLBI and UnitedHealth to add substantially to the resources for implementing and evaluating the sodium related intervention.
My role then was as a member of the Scientific Committee, working with colleagues and counterparts in the George Institute-Beijing with respect to the trial itself. I mentioned earlier the broader CDC interest to bring together scientific and public health leadership in China on the topic of hypertension prevention and control, with a particular focus on sodium reduction. The Ministry of Health and specifically the Chronic Disease Bureau of the China CDC, as well as senior experts in this field in China were very directly and substantially involved. This had high-level visibility and participation from the corresponding leadership between China CDC and the US CDC. Tom Frieden, director of US CDC, visited there on multiple occasions through exchange visits between Beijing and Atlanta as well, reflecting US CDC interest in this activity in China.
One of the spinoffs was the development of what has been called the Shandong Project, which is to undertake as comprehensive as possible a strategy of intervention and evaluation of sodium intake and hypertension control in Shandong province, which is north of Beijing and is one of the major salt producing and consuming provinces of China. There was a high level of interest and collaboration between the national and provincial governments in carrying this out. That project is at an early stage of implementation now. I remain a member of the Scientific Committee for the CRHI. That basically is the story of the hypertension and sodium work in China in which I have been involved.
Mark: In the US we consume most of our sodium through processed foods. Is salt substitution a strategy you think either would be advocated or more widely implement in the US or would that you think would require testing in the US?
Darwin: I think there are a couple questions underlying your question. One is, to what extent does this sodium reduction strategy depend on the food sources of sodium in the population? The distinction your question points to is an important one, between a predominance of home prepared foods in rural China and commercially manufactured foods (or “food-like substances” some would say, rather than food; credit to Michael Pollan) in the United States and many western countries.
The question of the salt substitute China, in rural China, I think hinges in a sense on the expediency of initially substituting some potassium for some sodium in an environment where salt intake is expected to remain high. In principle, sodium reduction through reducing the use of salt altogether would be the longer-term, preferred strategy. On the other hand, among the scientific leadership in the field of hypertension in China, there is a strong current belief that a high intake of sodium is not the primary problem; rather, it is a low intake of potassium. As reflected in the symposium that I mentioned, which was broad-based, diverse, and representative of government, industry, health authorities, and non-governmental organizations, there were presentations about initiatives by the food industry in China to create niche salt products that combine sodium with potassium or other substitute minerals, often associated with marketing strategies to appeal to people’s beliefs about health.
The prevailing view influencing the China Rural Health Initiative has been that reducing sodium intake through the use of a salt substitute might be a more expedient means to reduce salt intake. This is at a time of food transition in China where food preparation remains very largely a household issue. The other side of that has two pieces, one that men in many of the northern rural areas in China do not work in the village of their residence. They work in the cities, and they may reside in the cities, spending substantially more of their time than they reside in their villages at home, presumably consuming foods that are available for purchase in the cities, which is believed to be increasingly “food products”. So, the supposition that the best strategy is salt substitute, rather than reducing sodium content of mass-produced foods is, perhaps in need of new and continuing monitoring. All indications are that marketing of processed foods is going to increase. So to rely on household salt use alone, to the neglect of what may be a tsunami of manufactured foods with very high salt content, would be a serious strategic error. So we have talk about surveillance not just of salt intake, blood pressure, and stroke, but also about surveillance of the food industry itself.
Mark: How do we measure salt in the US food supply?
Darwin: In the US there is a nutrient database that is maintained by the University of Minnesota, which more or less satisfactorily provides a current picture of the sodium content of various food products. FDA and USDA have the regulatory responsibility for monitoring the sodium content in foods, and CDC’s Division for Heart Disease and Stroke Prevention has been collaborating very closely in a tripartite activity with FDA and USDA to strengthen surveillance of sodium contents in the US food supply. This extends to developing a panel of so called “sentinel foods”, selected because of their high contribution proportionally to sodium intake in the US. These are not necessarily the saltiest foods, but the foods that contribute most to total salt intake because of frequency of consumption.
Mark: Foods like breads, chicken dinners, pizzas?
Darwin: Yes. This holds, in my view, high promise of substantially improving the quality of data on sodium intake in the US. As mentioned as a footnote, collaboration between the Division and the National Center for Health Statistics’ NHANES Division has also been very important to develop, demonstrate, and evaluate the feasibility of implementing 24-hr urine collections in NHANES. So, in that aspect the US is improving.
Back to the earlier question whether demonstration of effectiveness for stroke prevention through use of a salt substitute in rural northern Chinese villages would translate to advocacy for this approach in the US: I think it has to take account of the very different balance we talked about between household salt use and sources of sodium in the US food supply, which are overwhelmingly manufactured foods (77%). That does not take into account restaurant-prepared foods where the sodium content is even higher than in retail commercial food products, and the fact that the proportion of meals taken outside the home is increasing in the US. I think far the dominant strategy in the US to reduce overall sodium intake must be reduction of sodium itself in manufactured foods.
Mark: I use sodium chloride in my house; when I cook. I don’t use a lot. But I have it; I do not have a salt substitute. Should I have a salt substitute? Maybe I should be buying this?
Darwin: In my house, use of salt is very uncommon; very infrequent. I do a lot of the cooking and virtually never add salt to the foods that I prepare. I do not have a salt substitute in the house. It’s available at the stores. You can find it, but you would pay a premium for it, and it’s only a very small share of your total sodium intake.
Mark: How could trainees get involved?
Darwin: Things that comes to my mind that are relevant across a spectrum from public health to clinical domains are first, taking advantage of the surveillance data that will become available as 24-hour urine collections are implemented in NHANES.
Mark: So learning the methods and understanding sodium measurement methodology.
Darwin: Yes, I would say, including careful attention to the limitations of the data that have been available historically and recognizing the very high quality of some population studies, focusing on sodium intake and its relationship to blood pressure, such as INTERSALT, INTERMAP. We have published a recent American Heart Association Presidential Update reiterating the relationship between salt intake and blood pressure, despite some calls to the contrary, which have typically relied upon poorer quality or biased data.
Other things I would mention, an essential step for someone interested to work in the sodium arena, is to study the Institute of Medicine report on strategies on sodium reduction, which was published in 2010. That report provides a very thorough and balanced assessment of the role of sodium in manufactured food, the rationale for its persistence from the vantage point of the food industry, and the scientific basis for determination that a sodium intake of 1500 mg per day is already several times the nutritional requirement, a level that does not restrict food intake in such a way as to limit adequacy of other nutrients or total caloric intake.
Mark: In a country like India, the estimated salt intake is 9 grams per day to provide adequate iodine supplementation. One starts to ask, how would you increase iodine supplementation while others are calling for lower salt intake, unless salt were enriched with more iodine?
Darwin: You open the door on another, deep, and important conversation. I mentioned the IOM report 2010, but there is also the World Health Organization (WHO) report 2007, which is the product of a pair of meetings that address this very issue – the tension, I would call it, or conflict you could say, between two WHO policies, one being the target to reduce salt intake to less than 5 grams per day and the other to assure adequate iodine intake for prevention of the micronutrient deficiencies diseases attributed to insufficient iodine intake. It was pointed out in the first of those conferences that at the current level of iodine saturation of marketed salt, it was necessary to consume twice the amount of salt recommended under the salt reduction strategy from food other than manufactured foods in order to deliver the required amount of iodine.
Curiously, that first meeting was convened under the auspices of WHO’s Micronutrient Deficiency Disease Program, which vigorously supports iodine supplementation as its most important public health program. It was discussed, and a separate meeting was convened as a follow-up to further pursue that very question, that there is no technical limitation to the ability to increase the saturation of marketed sodium chloride to meet the iodine requirement while at the same time achieving the target level of sodium intake recommended by WHO. However, this will require conversion by the food industry to use iodized salt, which is to my knowledge altogether lacking in commercial food production. You need to produce iodized sodium in sufficient quantity and have its use adopted wholesale by the food industry. Now, you can ask whether that is an issue in the United States.
Mark: Is it?
Darwin: Here would be another topic for research. CDC’s Nutrition, Physical Activity, and Obesity Division in the Chronic Disease Center is a reference center under WHO’s Micronutrient Deficiency Disease Program, and there is some expertise there, on the issue of iodine intake population. And there are some concerns that reduction of salt intake in the US could more frequently risk iodine deficiency in some people (pregnant women in particular) for whom the goal iodine intake is set at a higher level to prevent fetal and childhood neurologic impairment. But, this is an area where an interested, young investigator could engage and for example, consider the extent to which if at all, there is a hazard to any part of the US population, through a reduction of the sodium iodide intake. It would be necessary to know what the actual intake of the sodium iodide is. It is not in manufactured food in the United States either. This is household use and to what extent would that be potentially modified by such recommendations as came from the IOM about reducing to 1,500 mg per day intake of sodium?
Mark: Very interesting.
Darwin: So topics for research in this area include surveillance, understanding the limitations of prior and current data for purposes of study of outcomes of sodium exposure, questions about iodine, and also questions about interventions. There are interventions being implemented in communities throughout the United States, in some instances through the CDC Community Transformation Grants as part of the Affordable Care Act. There is also a five-community study with direct, targeted support from the CDC to devise and implement sodium reduction strategies. A common one is the so-called procurement policy approach, in which purveyors of processed food in large quantities to large target populations have the opportunity to specify the sodium contents of the foods they purchase.
Mark: This would be like the military…
Darwin: The military, hospital systems, prisons, schools, other large organizational institutional agencies which, by proportionate market share, have the possibility to compel manufacturers to provide ingredients or food products which meet target standards for sodium reduction. So, in what kinds of issues could the young, interested investigator become involved? What are the policies? What are the issues in advocacy for those policies? What are the attitudes, beliefs, behaviors, practices of the public at large, the food purchaser, and the persons in decision-making positions with regard to these large food providers? What evidence is there that, once adopted, such policies have an effect on actual sodium intake? Is our food pattern changing? Do people compensate for reduced sodium in some foods by purchasing more of others that tend to replace that?
People who are the purchasers of food for their households, more often than not, do read the nutrient facts label on food products – not as often for sodium as for calories, but not a trivial proportion. People report great difficulty in actually limiting their salt intake, given the food that is in the market place. All of which adds up to suggest to me that there is far greater public readiness and desire for change in the food supply than industry certainly gives them credit for.