Patients allergic to soy, gluten or other foods found in the Ornish diet should not consume these foods.
Before beginning any new diet, a patient should consult with a qualified health professional.
The Ornish diet should not be used in place of a more proven traditional treatment for heart disease or prostate cancer unless a patient is directed to do so by a qualified health professional.
There have been no reported adverse effects while following the Ornish diet under the supervision of a physician.
Repeated large fluctuations in bodyweight have been associated with many adverse effects including increased mortality. Cyclical radical changes in bodyweight like one might experience by going back and forth between the Ornish diet and the average American diet might increase the risk of these adverse health outcomes. In addition, very low fat diets, while they decrease LDL, or bad cholesterol, have also been shown to decrease HDL or good cholesterol, as well as increase triglycerides (TGs) in the short run. While current available evidence with the Ornish diet does not support this theory, the long-term effects have not been well studied.
The Ornish diet is based on the belief that metabolism was altered when early humans could not depend on regular meals due to the scarcity of regularly available food. The body, to preserve energy, would store any extra energy as fat. People in modern societies generally have unlimited access to food but their bodies have not adapted to this new way of living.
Because the rate at which a person burns calories may decrease when a person consumes fewer calories, is it easy to hit a weight loss plateau soon after beginning a new, lower-calorie diet.
It is theorized that if individuals eat only when hungry on the Ornish diet, without portion control, that their metabolism will stay the same, or perhaps increase. The high-fiber content also slows down the absorption of food into the digestive system, so a person feels full longer with small portions than if eating calorie-restricted small portions. The complex carbohydrates may help blood sugar remain more stable.
The Ornish diet also encourages long, slow exercise that uses body fat as fuel. Moderate exercise done on a regular basis revs up the resting metabolism, while some have suggested that short periods of intense exercise decrease metabolism.
Meditation is used as a way of quieting the mind, increasing self-awareness and coping with stress. Various forms of meditation, including mindfulness, transcendental meditation, and "meditation-based stress reduction programs" have been studied for their effects on anxiety and stress. Although benefits have been reported, the available research has not been well designed or reported, and cannot be considered conclusive. Better studies are needed before a recommendation can be made.
A study conducted by Ornish for the American Cancer Society claimed that diet and exercise may play an important role in keeping prostate cancer at bay. The same lifestyle changes that help prevent heart disease also appear to keep prostate specific antigen (PSA) levels low, and may even have an impact on how quickly a tumor grows. Ornish and his colleagues studied 93 men with early stage prostate cancer who had decided to pursue watchful waiting rather than active treatment with surgery, radiation or chemotherapy. Men in one group were told to follow their normal lifestyle habits, while those in the other made some big changes. These men went on a strict vegan diet that contained only 10% of calories from fat, (most Americans get 30% or more calories from fat), supplemented with soy, fish oil, selenium and vitamins E and C. They also began regular exercise (30 minutes of walking 6 days a week), doing yoga or some other type of meditation and participating in support groups. After a year, the researchers measured the men's PSA levels. Men who had followed the diet and exercise plan saw their PSA levels decrease by 0.3 ng/mL (about 4%), while those who followed their usual routine saw their PSA levels increase by 0.4 ng/mL (about 6%). The study only measured PSA and other lab results; it did not look at survival or whether the disease had spread. Also, the follow up period was only a year, which may not be long enough to see all potential effects of the different approaches. Because there were so many aspects to the regimen, it is not possible to tell how much of an effect (if any) each individual component (vegan diet, supplements, exercise, stress reduction, support group participation) may have had on the PSA level. The study also could not determine whether making these lifestyle changes would actually prevent prostate cancer in the first place. Ornish, the developer of the diet, also conducted the study, which may be a source of potential bias. In the study, 3 men quit the plan because it they found it to be too difficult.
Another study compared the Atkins, Ornish, Weight Watchers and Zone diets for weight loss and heart disease risk reduction. The study set out to assess adherence rates and the effectiveness of the four popular diets for weight loss and cardiac risk factor reduction. A total of 160 participants were randomly assigned to either Atkins (carbohydrate restriction), Zone (macronutrient balance), Weight Watchers (calorie restriction) or Ornish (fat restriction) diet groups. The mean weight loss at one year was: 2.1 (4.8) kg for Atkins (53% participants completed), 3.2 (6.0) kg for Zone (65% completed), 3.0 (4.9) kg for Weight Watchers (65% completed) and 3.3 (7.3) kg for Ornish (50% completed). Greater effects were observed in study completers. Each diet significantly reduced the low-density lipoprotein/high-density lipoprotein (HDL) cholesterol ratio by approximately 10% with no significant effects on blood pressure or glucose at one year. Amount of weight loss was associated with self-reported dietary adherence level but not with diet type. For each diet, decreasing levels of total/HDL cholesterol, C-reactive protein and insulin were significantly associated with weight loss with no significant difference between diets. Each popular diet modestly reduced body weight and several cardiac risk factors at one year. Overall dietary adherence rates were low, although increased adherence was associated with greater weight loss and cardiac risk factor reductions for each diet group.
The Lifestyle Heart Trial studied the effects of the Ornish diet. On average, Ornish's patients lost 24 lbs in a year and had a 37% reduction in LDL cholesterol levels (HDL cholesterol levels were unchanged). Additionally, there was a 91% reduction in angina frequency (chest pain) and a significant degree of angiographically measured coronary stenosis regression. It was unclear to what degree other lifestyle modifications such as exercise and stress reduction, which are integral parts of the Ornish program, played in these results. Based upon these favorable findings, the National Institutes of Health is embarking upon a multimillion-dollar study of the Ornish diet vs. bypass surgery in patients with coronary disease.
Overall, there are mixed findings about the Ornish diet's efficacy compared with other diets (low carbohydrate, calorie restriction, etc). However, most results seem promising and the Ornish diet and lifestyle modification may be effective in reducing cardiac disease risks and aiding in weight loss.