Skim Milk Gets a Move On

Abstract & Commentary

By Allan J. Wilke, MD, MA. Dr. Wilke is Chair, Department of Integrative Medicine, Ross University School of Medicine, Commonwealth of Dominica; he reports no financial relationship to this field of study. This article originally appeared in the November 15, 2010, issue of Internal Medicine Alert. At that time it was peer reviewed by Gerald Roberts, MD, Assistant Clinical Professor of Medicine, Albert Einstein College of Medicine, New York, NY. Dr. Roberts reports no financial relationship to this field of study.

Synopsis: Three 8-oz glasses of fat-free milk per day provided significant relief to patients with functional constipation.

Source: Aydin S, et al. Fat-free milk as a therapeutic approach for constipation and the effect on serum motilin and ghrelin levels. Nutrition 2010;26:981-985.

These investigators from turkey hypothesized that fat-free milk could improve constipation and that the hormones motilin and ghrelin are involved. Ghrelin, a "hunger" hormone produced in the stomach and pancreas, stimulates appetite. It has several other effects on the gastrointestinal (GI) tract, but for this study, its enhancement of motility was the effect of interest. Motilin is secreted in the small intestines and stimulates gastric motility and small intestinal peristalsis.

Thirty (30) constipated patients and 19 controls were recruited. All subjects underwent double-contrast barium enemas. Exclusion criteria included pre-existing GI pathology (cancer, lactose intolerance, ulcerative colitis, Crohn's disease, malabsorption syndrome, among others), and other disease or conditions associated with constipation (diabetes, thyroid disease, pregnancy, obesity, and tobacco use). The constipated subjects were classified by the Constipation Severity Instrument (CSI), a validated tool for accessing constipated patients,1 into three groups, mild, moderate, and severe, with 10 subjects in each group. The CSI for the mild group averaged 17.8, for the moderate group 20.2, and for the severe group 26.7. The controls were divided into two groups with CSIs of 10 and 9. All groups were evenly divided between men and women with average body mass indices around 26 kg/m2. The groups were well matched, except that the constipated patients ate fewer legumes, whole grains, and fresh fruits and vegetables. Milk consumption among the constipated patients was limited to 1-2 glasses of whole milk per week. None of them consumed fat-free milk.

Blood samples for electrolytes, lipids, ghrelin, and motilin were obtained at baseline and then after 3 days of fat-free milk consumption. All subjects were given a standard diet (45% carbohydrates, 35% fat, and 20% protein). The subjects in control group 1 (CG1) drank 400 mL of fat-free milk a day; those in control group 2 (CG2) drank the same amount of whole milk. The mild, moderate, and severe constipation cases received 400, 600, and 800 mL of fat-free milk, respectively, a day for 3 days.

CG1, which drank fat-free milk, saw an increase in ghrelin levels and a 3-point drop in CSI, while those subjects in CG2, who drank whole milk, saw a decrease in ghrelin and a 1-point drop in CSI. Motilin levels did not change significantly in CG1, but fell in CG2. Similar results were seen in the constipated groups, which all drank fat-free milk. In the mild group, the CSI fell 4 points, 12 points in the moderate group, and 17 points in the severe group. In fact, the post-fat-free milk CSIs in the moderate and severe groups were equal to the control groups. Ghrelin levels rose in all constipated groups. In contrast to CG1, however, motilin levels also rose.

Samples of the whole and fat-free milk were analyzed for chemical content. The only significant differences were the amount of iron (twice as much in whole milk than in fat-free milk) and the amount of ghrelin (more in fat-free milk than in whole milk). Motilin concentrations were equivalent.


This study was poorly written and confusing to follow. It was not a double-blinded, randomized, placebo- controlled study. It was not clear when the CSI was repeated, 3 or 30 days after baseline. The results need to be replicated in another environment with a larger and more ethnically diverse group of subjects and a study design that minimizes bias. Even with that, the results are interesting and raise several questions. Why is the concentration of iron higher in whole milk and does that matter for this study? Primary care physicians often receive anecdotal reports of constipation with iron supplementation, and research supports this observation in pregnancy.2 How much milk does one need to drink? For constipation, the magic number seems to be 600-800 mL (20-27 fluid oz) of fat-free milk a day. What would the effect of a similar volume of 1% or 2% milk be? These volumes of milk would cause symptoms in lactose-intolerant individuals. Would constipated, lactose-intolerant patients benefit from taking lactase before consuming fat-free milk or by drinking lactase-treated fat-free milk? What about fat-free fermented milk? How do we account for the increase in ghrelin and motilin serum values in subjects given fat-free milk? While it's safe to assume that the motilin increase represents endogenous production, since there is no significant difference between fat-free milk and whole milk in motilin concentration, the same cannot be said for ghrelin.

These subjects almost certainly had functional constipation, which is defined by the Rome III criteria as the presence of two or more of the following symptoms occurring for at least 12 weeks in the preceding 12 months: 1) straining during at least 25% of defecations; 2) lumpy or hard stool in at least 25% of defecations; 3) a sensation of incomplete evacuation in at least 25% of defecations; 4) a sensation of anorectal obstruction or blockage in at least 25% of defecations; 5) manual maneuvers to facilitate defecation used in at least 25% of defecations; and 6) fewer than three bowel movements in a week.3

Constipation has many causes. As a society, we aren't physically active, and we don't keep ourselves adequately hydrated. Our consumption of fat-free (skim) milk, whole grains, and fruit and veggies is low. Some constipation is iatrogenic from narcotics and medications with anticholinergic side effects. Sometimes constipation is a red flag for a more serious disease. When a patient presents with this complaint, we need to look at lifestyle issues and medication lists and search for GI pathology. Laboratory tests include thyroid-stimulating hormone (TSH), serum calcium, glucose, electrolytes, a complete blood count, and urinalysis.4 Advocating fat-free milk seems like a very reasonable approach, if the constipation is functional and the patient is not currently drinking it, especially since milk consumption has other benefits. For patients who complain that they don't like the taste of skim milk, suggest that they "wean" themselves off milk with higher fat concentrations gradually. Going from whole milk to 2% to 1% to skim, one bottle at a time, is doable.


1. Varma MG, et al. The constipation severity instrument: A validated measure. Dis Colon Rectum 2008;51: 162-172.

2. Melamed N, et al. Iron supplementation in pregnancy — does the preparation matter? Arch Gynecol Obstet 2007;276:601-604.

3. Longstreth GF, et al. Functional bowel disorders. Gastroenterology 2006;130:1480-1491.

4. Lembo A, Camilleri M. Chronic constipation. N Engl J Med 2003;349:1360-1368.