What are the benefits and disadvantages of the popular diets (Zone, Atkins, Pritikin, Ornish and D'Adamo)? Since weight loss is always the result of a caloric or energy deficit, do these diets seem to work because they each are about eating fewer calories?

Most of the popular diets, in common, target either specific macronutrients (fat, carbohydrate, protein) or types of foods, and recommend various proportions of these. Other claimed health advantages, however, such as increasing longevity or regression of atherosclerosis or other disease states, have little to do with body weight.

The Zone Diet prescribes balances between carbohydrates, protein and fat that have to be eaten at each eating occasion (40% from carbohydrate, 30% from protein and 30% from fat). Most of these newer diets have macronutrient or food types prescriptions and not just calorie prescriptions. To keep on the Zone Diet requires an unreasonable amount of food preparation effort. As a result, the Zone dieter often needs to purchase Zone's food products, which are disproportionately expensive.

The Zone theory is based on a combination of combating insulin resistance and altering eicosonoid levels, the so-called "hormonal messengers" of the body that enable the dieter to achieve better control over energy intake and, thus, promote longevity. The term "Zone" is ambiguous but apparently refers to the maintenance of blood insulin levels at a low level in place of up and down swings that, supposedly, increase appetite. The diet recommends that adherents eat certain protein blocks, carbohydrate blocks and fat blocks at each meal and that meals occur every 4.5 hours (i.e., five times a day). A breakfast, typically, might contain egg white omelets, lots of lower glycemic carbohydrates, such as bulgur, or steel cut "whole-oat." Instead of juices, whole fruits or raw fruits are recommended. The Zone's saturated fat level is quite high (up to 20-25% of calories) as saturated fat compared with the traditional recommendation of 8% or less.

By contrast, the typical American diet has as much as 50-60% of energy from carbohydrate, and much less energy provided as protein (15%). No data exist that have prospectively shown that the Zone balance of macronutrients, or the alteration of the hormonal influences (such as insulin sensitivity or the mediators of inflammation), can result in preventing the aging process. Theoretically, the consumption of higher amounts of protein than is currently recommended to could result in increased bone loss and accelerated deterioration of renal function.

The Atkins Diet is, essentially, a high protein diet. It's low in carbohydrate and very high in fat. Atkins believes that this particular pattern of consumption will facilitate a decrease in circulating insulin and increased ketosis which will decrease appetite, decrease food consumption and, thus, result in weight loss. Consumption of the initial phase of the Atkins diet may, typically, lead to approximately 50-55% of calories from fat, 30-40% of protein and somewhere in the neighborhood of 5-15% from carbohydrate.

Meat, poultry, fish, dairy and oils are permitted. A typical breakfast consists of eggs, bacon and cream. The menus are monotonous. Most of the Atkins' recommended foods are located at the top of the Food Guide Pyramid, the "use sparingly" section, so, in effect, the Atkins diet is the classic food pyramid inverted. Theoretically, with the Atkins diet, there may be micronutrient deficiencies, since the diet is very low in fruits and vegetables. However, Atkins also recommends a number of supplements that he promotes to go along with the diet, which would, theoretically, prevent some of these deficiencies.

It is both difficult and impractical, given the amount of eating we do outside the home and the amount of preparation that it would require, to consume such a low carbohydrate diet. Also, the high degree of fat intake might increase risk for dyslipidemia, cardiovascular disease and certain types of cancers. The very low fiber intake, approximately 5 g/day (current recommendation: 20 - 30 g/day), might mean that constipation could be an issue with the Atkins diet. Furthermore, chronic conditions such as diverticulosis and possibly colon cancer could be exacerbated by low fiber intakes. Two new diets, "sugar buster" and "protein power" diets appear to be variations of the Atkins diet.

The Ornish and Pritikin diets have more in common with each other than with Zone or Atkins and, in fact, are their opposites. The Ornish diet emphasizes high levels of complex carbohydrates and low levels of fat, especially saturated fat. The Ornish diet originated as a way of combating atherosclerosis and even regressing coronary atherosclerosis. In the case of the Ornish diet, the fat content is approximately 10% of energy and the protein content is around 15% (which is the current average American intake), so the carbohydrate content is going to be quite high (~75% of calories). The authors recommend consumption of carbohydrates as whole grain products, fruits and vegetables. One might suppose that the reason why these diets are successful in inducing weight loss is because the energy density (or the calories per gram or kilogram of food ingested) is going to be quite low, given all the fruit, vegetable and whole grain intake, which includes a high fiber intake.

Dr. Ornish recognizes that diet is only one component in achieving cardiovascular health. His "Open Your Heart" program also advocates communication with others and spiritual connectedness. He conducts group therapy sessions and encourages physical activity as well as deep relaxation techniques. He also discourages alcohol intake, but he doesn't absolutely prohibit it.

People on the Ornish program are able to improve their cardiovascular risk factors and decrease their body weight. There are no studies documenting the insulin sensitivity or hyperinsulinemia on this particular diet, but level of insulin may not tell the whole story. Food choices are also important. Consumption of whole grains, fruits and vegetables, especially substances with soluble fiber, is going to blunt the postprandial insulin spike.

A new diet, known as the "Eat Right for Your Type" (meaning blood type), advocates "evolutionarily appropriate" diets for people with different blood types. It's author, Dr. Peter D'Adamo, a naturopathic physician, asserts that someone with an "O" blood type (as found in the oldest evolutionary animal) should not consume wheat or carbohydrate foods- but rather should consume a very high protein, high fat diet. From what we know, this is obviously not a healthy diet if eaten over a long time, and is very similar in pattern to the Atkins diet. The "A" type is the next blood type to evolve and this is someone who should eat a lot of carbohydrates. Then, there are the "B" and "AB" types who have been around the earth most recently, so they actually have a more "adaptive" intestinal capability. "B" and "AB" people can eat a varied diet, particularly dairy foods.

To follow the "Type" diet, you need to know your blood type and then figure out what foods you can eat. In terms of sensible nutrition goals, this diet is not useful and is more appropriately categorized as a "personality game."

In general, to lose weight, first identify the dietary and lifestyle issues that can be modified. For some, the issue is consumption of excess calories from foods that have no great nutritional value, such as sweets or certain high fat foods. A diet rich in fruits and vegetables and whole grains, in general, does promote weight loss, possibly in part by decreasing energy density or increasing volume. Trigger foods or food that stimulate consumption of more and more calories have to be controlled. Portion size is the key to meal planning. Severe restrictions, as seen in the popular diets, are not necessary. In the absence of physical activity, no weight loss program can be successful. Whatever you choose, remember to get enough micronutrients (vitamins and minerals), usually via a multivitamin or multimineral, in order to avoid deficiencies. Adequate hydration and dietary fiber are also the keys in any weight loss program. Everyone needs about 20-25 g of fiber/day.

Table 1: Comparison of Selected Popular Diets

Diet	Diet Composition*	Foods Specified	Diet Claim	Proposed Mechanism
Dr. Barry Sears' Zone Diet	40% CHO, 30%Pro, 30%fat. Calories are restricted.	Eating the "right food" leads to a metabolic state whereby the body has increased energy, decreased hunger.	Lower insulin levels, optimal eicosanoid levels are the "master switches" for decreased hunger.
Dr. Atkins' Weight Loss Diet	"Fortnight Diet": 36% Pro, 8%CHO, 53% Fat. Maintenance: 24% Pro, 40% Fat, 31% CHO. No calorie restriction.	Meats, cheese, eggs, poultry, fats and oils required. Grains, fruits and vegetables severely restricted.	Eating too many carbohydrates cause obesity. High protein, high fat leads to decreased hunger.	A ketogenic diet causes incomplete oxidation of fatty acids by the liver, resulting in ketone bodies in blood and urine.
Dr Dean Ornish's Reversing Heart Disease Program	Recommends whole grains, fruits and vegetables, and very little meat, dairy or fat.	Lowering fat lowers increased blood cholesterol, which forms plaque and tears the lining of coronary arteries.	Reduces plasma cholesterol, LDL cholesterol. Documented atherosclerotic regression.
Pritikin Diet	8-12% Fat, 12-15% Pro, 80% CHO. <100 mg cholesterol/day.	Recommends whole grains, fruits and vegetables, minimal fat and animal products.	Low fat diet increases energy.	Low fat diets reduce plasma cholesterol, LDL cholesterol. Low oxygenated blood is increased in a high fat diet, causing clogged arteries, capillaries, etc. Low fat diets will carry more oxygen to cells.
Food Guide Pyramid Diet	<30% Fat, 55-60% CHO, 10-15% Pro. Also defined as a diet that is adequate in vitamins, minerals and dietary fiber w/out excessive amounts of calories, fat, sat. fat, cholesterol, sodium, added sugars, alcohol.	Specifies servings of food groups: Grains, breads, etc:6-11 svgs. Fruits:2-4 svgs. Vegetables:3-5 svgs. Meat,eggs,nuts, dry beans, poultry, fish: 2-3 svgs. Dairy: 2 svgs Fats,sweets, alcohol: use sparingly & moderation	Diet was created to help healthy Americans follow the Dietary Guidelines for Americans: Variety Choose plenty of grains, fruits&veg. Keep diet low in sat. fat and cholesterol. Keep diet moderate in sugars, salt and sodium Moderate alcohol. Physical activity and maintain or improve your weight.	This diet was designed to address nutrient adequacy and nutrient excess in a healthy population. If followed over a period of time (ie, a week at least), this diet allows food choice flexibility and a practical balance in levels of calories, fat and sodium. Certain populations consuming lower calories (than 1600) may not meet the full RDA for certain nutrients.

*CHO=carbohydrate, Pro=protein, GI=glycemic index

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