Based on the information it provided, I searched for the original article for this study. This article published in the Feb. 28 issue of Archives of Internal Medicine.  This study used 2005-2008 National Health and Nutrition Examination Survey (NHANES) data on 20-64 years old adults with greater than or equal to Body Mass Index (BMI) of 25.0. Since the data are snapshots without any temporal information, it is a cross-sectional study based on the definition from textbook.  The author aimed to find the influence of physician acknowledgment of patient' weight status on patient perceptions of overweight or obesity.
Here, the exposure would be whether the patients reported that their physician told them they were overweight, and the disease would be patient perceptions of overweight or obesity. They included 7790 people aged between 20-64 years from 2005-2008 NHANES data as a sample. Since they narrowed their sample within certain age range, it is a restriction sample. Because it is very hard to sample huge population, they weighted the data, so the 7790 people represent 174 million people.
They did have some confounding variables in their logistic regression model, such as age, sex, race, poverty to income ratio, marital status, education, whether the patient has a routine source of health care, and the number of physician visits in the last 12 months. But I think there might be other confounders, such as whether they have health insurance, and their occupations. I think those would also influence their perception of overweight or obesity. For example, uninsured people might not go to the doctor regularly, so the chance of being told their weight status by their physicians would be lower. Also, some job needs higher muscular-level work which might cause higher BMI (greater than 25), but people might not think they are overweight or obese, even they've been told so.
In the end of this articles, it points out its limitations: 1. no cause and effect association can be made; 2. no information about the extent of the physician intervention; 3. Using BMI standard to define overweight would be wrong to muscular people; 4. no information about the reason for people to see a doctor.
The authors concluded that patient-reported physician acknowledgment of patients' weight status is highly associated with patient perceptions of overweight and obesity based on the odds ratios for different groups. In this case, physicians need to tell more overweight and obese patients about their weight status so that it could help encourage them to change their behaviors to lose weight and lower their risk for many diseases.
I think this is a good article based on very solid foundation. The conclusion that author addressed is responsible as they know what limitations are. So they address the conclusion very carefully without suggesting any causality relationship. I think it could alarm physicians to pay more attention on telling overweight or obese patients their weight status, since most of patients would listen to their physicians and start to change their behaviors for weight loss.
1. Amanda Gardner. Doctors Can Influence patient to Lose Weight: Studies. 2011 HealthDay. http://health.msn.com/healthy-living/articlepage.aspx?cp-documentid=100270168
2. Robert E. Post, Arch G. Mainous III, et al. The Influence of Physician Acknowledgment of Patients' Weight Status on Patient Perceptions of Overweight and Obesity in the United States. http://archinte.ama-assn.org/cgi/content/full/171/4/316
3. Gordis L. Epidemiology. 4th ed. Elsevier. 2008. ISBN: 9781416040026