Among the most common health improvements reported on anti-inflammatory diets include weight loss, the reversal of type 2 diabetes and joint pain, and improvements in lipid profile (serum lipoprotein levels.) The claims of acid-alkaline diets, raw vegan diets, and detox/cleanse type diets to reverse cancer and other ailments are widely regarded to be quackery. However, they appear effective at promoting rapid weight loss due to the low calorie content.
I don't mean to imply that none of the diets listed to above can be beneficial under some circumstances. I also think that food quality, as well as healthy weight and calorie balance, is essential for good health. However, those who advocate these diets often make statements that are untrue or misleading. I seek to educate my readers on the pros and cons of this type of diet.
The benefits of these diets are primarily based on their low calorie content and subsequent weight-reducing effect:
1) Weight loss. Most fruits and vegetables are low in calories. Fresh fruit has between 200-400 calories per pound, while fresh non-starchy vegetables typically have less than 200 calories per pound. Other foods typically have greater than 500 calories per pound; some high-fat, energy-dense foods (like chocolate and potato chips) have more than 2000 calories per pound. Most of the diets listed recommend eliminating "junk" foods, which are high in calories, and replacing them with fresh fruits and vegetables. Due to the lower calorie content, people who eat the same serving size will eat fewer calories and lose weight.
One would wonder if eating a lighter meal causes one to eat more later, and to a certain extent this is true. However, the satiety of a food (effect of a certain number of calories in reducing hunger and food intake later on) is primarily determined by its protein, fiber, water, and fat content. Protein, fiber, and water make foods more satiating, while fat makes foods less satiating. Fruits and vegetables are low in fat and high in fiber and water. Thus, they are very satiating for the calorie content.
Epidemiological studies have shown that people who eat lower-energy-density diets tend to eat fewer calories and are less likely to be obese as people who eat higher-energy-density diets. (Note: lowering the energy density of your diet does not require any special diet books or planning. All you have to do is replace other foods with fresh fruits and vegetables.)
2) Type 2 diabetes. One of the diseases most tightly correlated with excess fat, type 2 diabetes also often resolves with significant weight loss. Bariatric surgery is effective in producing remisson of type 2 diabetes, probably because it leads to rapid, significant weight loss. Unfortunately, few people are able to lose the same kind of weight with diet and lifestyle change as with surgery. However, one would expect that a low-calorie diet that leads to significant weight loss could also lead to remission of type 2 diabetes. Even modest weight loss can lower blood glucose and delay the development of diabetes: according to this study, each kilogram of weight loss correlated with 16% lower rate of developing diabetes. (One kilogram is about 2 pounds.)
3) Osteoarthritis (joint pain). This is another illness strongly correlated with weight. It's easy to understand how excess body weight would stress the knees; overweight people have 4 to 5 times higher risk of developing knee osteoarthritis as normal-weight people. For unknown reasons, overweight people are also at higher risk for hand osteoarthritis than normal-weight people.
Given the strong effect of weight on osteoarthritis, it is recommended that osteoarthritis patients try to lose weight. It is also likely that weight loss is responsible for reduction in osteoarthritis, rather than elimination of certain foods or increased intake of polyphenols.
4) Dyslipidemia ("bad blood cholesterol numbers"). Being overweight is known to increase LDL ("bad cholesterol") and decrease HDL ("good cholesterol"), as well as increase the risk of heart disease. Weight loss is well known to improve these markers of cardiovascular health, and normal weight people have lower risk of heart disease than overweight people. (I put "cholesterol" in quotes because LDL and HDL are lipoproteins, not cholesterol. See this post for an explanation of terminology.)
Obviously, weight is not the only factor in dyslipidemia. Most of the diets I listed above limit animal fat, which would have the effect of reducing saturated fat and cholesterol intake. Reducing the amount of saturated fat and cholesterol in the diet is known to lower LDL "bad cholesterol." Also, increase in fiber intake and plant sterols has the potential to lower LDL levels.
5) Asthma. It is well known that asthma is exacerbated by obesity, and high body weight is a risk factor for developing asthma. The restrictions that severe asthma often place on the sufferer's physical activity and mobility can cause weight gain, which in turn worsens the underlying asthma in a vicious cycle. Weight loss improves asthma symptoms.
In other words, the most important and most highly touted benefits of these diets are due to the low calorie content. An isocaloric diet consisting of Twinkies, or meal-replacement shakes, would have the same benefit in the short term.* In the long term, the diet that one can stick to is the only one that will prove beneficial to health.
Improvement of other symptoms, especially qualitative symptoms, may be due to the placebo effect:
It has been demonstrated that placebos activate an "internal pharmacy" of opioids and endorphins, alleviating symptoms and making people feel better. Placebo diets presumably work in a similar manner as placebo pills. In other words, if you were told a certain diet would magically clear up your skin and make you sleep better, it would be likely to clear up your skin and make you sleep better. Then you would write a glowing review or testimonial, and reinforce the placebo effect for future individuals trying the diet. Be aware that not all reviews and testimonials are genuine, compounding the problem.
In the same manner, the nocebo effect is an important concern in the over-diagnosis of food sensitivities. (The nocebo effect is the opposite of the placebo effect.) If one is convinced that ingestion of a certain food will harm one's well-being and health, it might actually have that effect.
The advocates of these diets tend to ignore or downplay the health risks of these diets, including:
1) Binge eating. Dietary restriction of any kind and for any reason is known to increase the risk of binge eating. Many people develop binge eating disorder after dieting, and dieting is known to increase the risk of weight gain. Binge-restrict cycles, in which a person goes on a restrictive diet only to "cheat" and binge on "forbidden" food, are similar to bulimia except that they typically involve less severe purging behaviors. The Internet is full of stories of people who tried dieting only to develop severe eating disordered habits. Don't be one of them.
One strategy to prevent cravings and binge eating is to adopt a 80/20 strategy of eating. This means that 80% of the food you eat adheres to some healthy eating plan, and the other 20% allows you to indulge in moderate quantities of your favorite foods.
2) Underweight and malnutrition. This is typically only a problem with extreme diets; however, it is important to mention. It is possible to suffer severe complications of low weight, including hypothalamic amenorrhea and osteopenia, while being technically normal weight. The key factor in developing female athlete triad--a disease most common in young, female athletes who undereat and overtrain--is energy availability, although fat mass may also play a factor. Vitamin deficiencies become a problem on some extreme diets, especially vitamin B-12, which occurs only in animal products. Those who choose not to eat animal products must take B-12, or they will eventually develop a deficiency.
*Note: I am not recommending an unbalanced diet or meal replacement. These examples simply exist to illustrate my point about the calorie content of the diet.