The Pros and Cons of Popular Diets

The Pros and Cons of Popular Diets

Rakesh Yadav brings us a comprehensive summary on the scientific consensus on some popular diets. In the comments on his original posts, he makes the following observations on Fats and Grains:

Fats: During recent years, there has been something of a backlash against low-fat eating recommendations because of the concomitant embrace of such recommendations and worsening of the obesity and diabetes epidemics. Data from the National Health and Nutrition Examination Survey, however, suggest that dietary fat intake did not appreciably decline; rather, total calorie intake went up, with the increase due to the adoption of many high-starch, high-sugar, low-fat foods. The decline in the percentage of calories from fat was more attributable to increased calorie intake than to decreased fat intake. The intent of low-fat guidance was, presumably, to encourage consumption of naturally low-fat foods—namely, plant foods direct from nature, rather than highly processed, fat-reduced foods (86). Adverse effects of low-fat eating may be associated with this misapplication of the original guidance rather than the intended guidance per se.

Grains: One of the more controversial aspects of plant-based eating is the role of grains. Evidence is fairly strong for a generous intake of dietary fiber over the life span, with whole grains representing an important source. Advocacy for the inclusion of whole grains in the diet is widespread, but concerns about the contributions of grains to obesity have been expressed quite vocally, in particular concerns about the rising prevalence of gluten sensitivity and concerns about genetic modifications of wheat. Data from National Health and Nutrition Examination Survey suggest that grain intake is related to positive nutrient profiles and improvement in chronic disease risk factors and is unrelated to obesity. The literature generally associates whole-grain intake with lower cancer risk, greater diet quality, and better control of body weight.

Originally shared by Rakesh Yadav

Recently a review article (http://goo.gl/wByL46; open access) was shared by Jim Carver .  The article had a somewhat provocative title “Can We Say What Diet Is Best for Health?” and I was sucked into the trap.

Such review articles are very important as they try to paint a comprehensive picture of the field of research and ideally provide an un-biased analysis of the different studies conducted in the past. In lack of such articles a non-expert can easily be lost in the myriad of studies, sometimes reporting conflicting results. Anyway, I read the abstract, and then the introduction and then the whole thing. I found it very interesting and hence I am sharing the main points (from my perspective). The summary below is mostly a cut-paste text from the original article with some paraphrasing done for the sake of coherence. This is of course in no way a comprehensive summary. If you are intrigued by some aspects, then you should definitely read the article! 

The authors basically talk about popular dietary choices in current culture. Using scientific studies they try to narrow down on health-promoting foods. They examine 7 diets which are described below.

[Warning: brace yourself, a long one is coming]

Summary starts here 

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Low carb diet 

Definition: total daily carbohydrate intake below 45% of total calories.

Interest in carbohydrate-restricted diets is long-standing, particularly in the context of diabetes management, and especially during the era before the advent of insulin therapy. It has become popular again in the context of weight control. Short to moderate intervention studies have shown efficacy in weight loss and favorably effects for quality of life. No long term intervention studies exist. Such studies cannot and do not, however, unbundle the effects of (a) carbohydrate restriction per se, on which the theory of the approach is predicated, and (b) calorie restriction, which is a virtually inevitable concomitant of choice restriction in general, and, perhaps especially, (c) restriction directed at carbohydrate, which constitutes the macronutrient class that provides the majority of calories for almost all omnivorous species. 

The basic principle for the diet is very general. For example the remaining calories can come from fat or protein, and, furthermore, these calories can be of plant of animal origin. The authors sum up by saying: The relevant literature remains equivocal, with most studies suggesting benefit from low-carbohydrate eating per se in comparison, generally, to either the typical Western diet or some version of a low-fat diet, with persistent concerns and uncertainty about longer-term effects on health outcomes.

Low-fat/vegetarian diet 

Definition: total daily fat intake below 20% of total calories.

Extensive body of literature exists. The diets of most primates are overwhelmingly plant based and low in total fat and are thought to be reflective of the earliest versions of the native human and prehuman diets, which evolved to include more meat in accord with hunting prowess. Intervention trials have long shown benefits from dietary fat restriction, ranging from weight loss to improvements in various biomarkers to reductions in cardiac events and mortality. Low-fat, plant-based eating has been associated with reductions in cancer and cardiometabolic disease. Uniquely, a very-low-fat diet has been shown to cause regression of coronary atherosclerosis (fat and plaque buildup on the walls of the arteries). High fiber intake in this (or others for that matter) diet might be crucial factor in their positive health effects. That said, there is no decisive evidence that low-fat eating is superior to diets higher in health-full fat in terms of health outcomes over the life span. When food choices are judicious in both contexts, the superiority of fat-restricted versus carbohydrate-restricted eating for weight loss and health is not reliably established.

Low-Glycemic diet

Definition: restricting/excluding foods with high-glycemic index

Clinical trial data are available and generally support efforts to reduce the glycemic load of the diet. Studies focused on this strategy have demonstrated benefits in the areas of weight loss, insulin metabolism, diabetes control, inflammation, and vascular function. Benefits have been seen in studies of both adults and children. Conversely, a high dietary glycemic load has been associated with adverse health effects. A recent meta-analysis concluded that high glycemic load and index are associated with increased risk of cardiovascular disease, especially for women. Most fruits are precluded by a preferential focus on the glycemic index as well. However, evidence that health benefits ensue from jettisoning fruits, or relatively high-glycemic-index vegetables, from the diet does not exist.

Often absent from discussions of low-glycemic diets is the consideration that, as with other dietary categories, there are various means to the same ends. McMillan-Price et al. studied alternative approaches to achieving a reduced glycemic load and demonstrated that a high-fiber, mostly plant-based approach offered metabolic advantages over a high-protein approach.

Mediterranean diet

Definition: based on the traditional dietary pattern that prevails in Mediterranean countries.

It has been associated with increased longevity, preserved cognition, and reduced risk of cardiovascular disease in particular, with some evidence for reduced cancer risk. However, longevity effects of diet, per se, are of course difficult if not impossible to unbundle from the effects of related lifestyle practices and cultural context. Adherence to a Mediterranean diet pattern is potentially associated with defense against neurodegenerative disease and preservation of cognitive function, reduced inflammation and defense against asthma, amelioration of insulin sensitivity, and relatively high scores of objectively measured overall diet quality. Studies have placed a particular emphasis on high intake of vegetables, fruits, nuts, olive oil, and legumes; moderate intake of alcohol; and limited consumption of meat. The contributions of cereal grains and fish are less apparent, perhaps because of lesser effects on health outcomes or less variation available for assessment.

Mixed, Balanced diet

Definition: it belongs to dietary patterns that include both plant and animal foods and conform to authoritative dietary guidelines, e.g. Dietary Recommendations of the World Health Organization

Such diets have figured prominently in the intervention trials of the National Institutes of Health (NIH). The Dietary Approaches to Stop Hypertension (DASH) diet is a salient example. Perhaps because of the ultimate accountability of the NIH to the tax-paying population at large, these federal diets have focused both on enhancements of nutrition and real-world applicability. Even so, efforts to translate the findings of intervention trials to community application have realized limited success. 

The DASH diet, as it has evolved, is a mostly plant-based diet inclusive of some animal products, with an emphasis on low-fat and nonfat dairy products. U.S. News & World Report has deemed DASH the most healthful diet in recent years. This designation, however, derives from the consensus opinion of a panel of expert judges rather than objective data. Data related to a direct comparison of DASH to other reasonable contenders for most healthful diet are lacking. There are some concerns about potential adverse effects of dairy intake that DASH-related literature tends to ignore. The Optimal Macronutrient Intake Trial for Heart Health has demonstrated short-term benefits for overall cardiovascular risk of several variations on the DASH diet theme – intake relatively high in carbohydrate, relatively high in protein, and relatively high in unsaturated fat – and suggested advantages to replacing some carbohydrate with either protein or fat. There are, however, no head-to-head comparisons of a DASH-style diet with other candidate dietary patterns to determine which produces the best long-term health effects.

Paleolithic diet

Definition: diet emulating pattern of our Stone Age ancestors with an emphasis on avoiding processed foods, and the intake of vegetables, fruits, nuts and seeds, eggs, and lean meats. In principle at least, dairy and grains are excluded entirely

Estimates of our Paleolithic dietary intake suggest that we are adapted to a high intake of plant foods and the nutrients they contain; a high intake of dietary fiber; and a fat intake of approximately 25% of total calories. One of the lesser challenges in reaching conclusions about the Paleolithic diet is variation in our ancestral dietary pattern and debate regarding its salient features. If Paleolithic eating is loosely interpreted to mean a diet based mostly on meat, no meaningful interpretation of health effects is possible. 

However even those emphasizing the role of hunting and meat suggest that some 50% of our Stone Age forebears’ calories came from gathered plant foods. Given the energy density of meat relative to most plants, even this translates to a diet that is, by bulk, mostly plants. Although superficially a departure from the other contending diets, a reasonable approximation of a true Paleolithic diet would in fact be relatively low in fat; low in the objectionable carbohydrate sources – namely, starches and added sugars; high in vegetables, fruits, nuts and seeds, and fiber; and low glycemic. An emphasis on lean meat remains distinctive, however, and may represent an advantage related particularly to satiety.

Vegan diet 

Definition: diets excluding all animal products – notably, dairy, eggs, and meats.

As with almost every other dietary approach, vegan eating can be done well or badly. Those committed to long-term veganism are typically well versed in the need to combine plant foods to achieve complete protein and in the role of select nutrient supplements. Those who adopt veganism for a short term, particularly adolescents seeking rapid weight loss, are not as reliably well informed. In general, vegan diets, when well constructed, are associated with health benefits. Intervention trials of short to moderate duration suggest benefits related to overall diet quality, inflammation, cardiac risk measures, cancer risk, anthropometry, and insulin sensitivity.

Intervention trials of vegan diets are limited to those willing to be assigned to such a diet for a span of weeks, months, or years. Given such constraints, data from intervention trials that are related to direct comparison of vegan diets with various other dietary patterns, that are defended from bias, and that examine long-term health effects are essentially nonexistent. This does not argue against vegan diets, but it does argue against overstating the basis for them in evidence related to human health outcomes.

Conclusions:

A common message can be drawn from the varies studies conducted on various food habits: the case that we should, indeed, eat true food, mostly plants, is all but incontrovertible. Perhaps fortuitously, this same dietary theme offers considerable advantages to other species, the environment around us, and even the ecology within us.

The clutter of competing claims likely obscures the established body of knowledge and forestalls progress, much like the proverbial trees and forest. We need less debate about what diet is good for health, and much more attention directed at how best to move our cultures/societies in the direction of the well-established theme of optimal eating, for we remain mired a long way from it.

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Summary ends here.

I hope this post motivates someone to read further and make responsible choices for themself. 

For ScienceSunday,  curated by Robby Bowles , Allison Sekuler , Rajini Rao , Chad Haney , Buddhini Samarasinghe , Aubrey Francisco , and Carissa Braun .

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