I love cranberries. I mean “looking at Thanksgiving turkey as a mere platform for cranberry sauce“ love cranberries. And don’t get me started on cranberry muffins and breads…
So, when I started to read that cranberries are a superfood that can prevent urinary tract infections, that they improve the immune system, reduce the risk of heart disease, benefit the digestive system, and even prevent cancer, I said “Count me in!”
But then the physician side of me comes out for a reality check: “Really, is that true?” So today my job is to check the scientific literature to see what it says about the benefits of cranberry. I’ve got my fingers crossed….
What Is Cranberry?
Cranberries are a group of evergreen dwarf shrubs or trailing vines in the sub-genus Oxycoccus of the genus Vaccinium. The American cranberry (Vaccinium macrocarpon) is a fruit native to North America. The berries are larger than the leaves of the plant; they are initially light green, turning red when ripe. It is edible, but with a tart taste that usually overwhelms its sweetness.
Cranberry is a perennial crop, grown commercially in man-made wetlands or bogs and harvested from mid-September to November. Massachusetts produces about half of the total U.S. crop but cranberries are also grown in four other states, namely New Jersey, Wisconsin, Washington and Oregon.
According to the Agricultural Marketing Resource Center, Americans consume nearly 400 million pounds of cranberries per year, 20 percent of them during Thanksgiving week. (1)
The U.S. per capita consumption of cranberries is 2.3 pounds, mostly in the form of juice or juice blends.
Historically, the health-promoting properties of cranberries have been based on folkloric remedies, which have existed for centuries. The health benefits of this fruit were recognized by Native Americans, who taught early New England sailors to eat the vitamin C-rich wild cranberries to prevent scurvy.
The crews of Yankee whaler and clipper ships routinely brought barrels of cranberries stored in water on board their ships. Other traditional uses included use for bladder, stomach, and liver disorders, as well as diabetes, wounds and other conditions.
The Composition of Raw Cranberries
In addition, cranberry has a complex and rich phytochemical composition, including flavan-3-ol monomers and dimers, proanthocyanidins, anthocyanins, hydroxybenzoic acids, hydroxycinnamic acids, terpenes and flavonols.
Proanthocyanidins, a class of plant-based flavonoids, are currently being studied for their antioxidant activities. A-type proanthocyanidins (PACs) may play an important role in preventing urinary tract infections (more about this later). Cranberry contains one of the highest levels of PACs among plant foods.
Anthocyanins are the specific chemical compounds that are responsible for the pigmentation of various fruits and vegetables. They are remarkably high in cranberry, contributing to its dark red color.
Although cranberry is used in beverages and food, it is rarely consumed fresh, due to its tart and astringent taste. It is mainly consumed as processed juice, sauce, and sweetened dry fruit. The multistep process used to make juice leads to a substantial loss of phytonutrients. Anthocyanins are the most affected, with losses of more than 50%. 
In addition, juice is often diluted or blended with other juices which can further change its content. In fact, one very popular brand of juice is not called “cranberry juice” but rather “cranberry juice cocktail.” If you look at the ingredients, there are 28 grams of added sugar (regular Coke® has 39 grams).
Cranberry is also available as fresh or frozen barriers and in the form of extracts, powders, capsules and tablets. Their composition of these other forms (you can find dozens at online retailers) are also subject to the way the cranberry is processed to make them. And they are very expensive.
Is There Any Research
As of March 2018, the biomedical literature (PubMed.gov) has 1461 articles about cranberry dating back 100 years to 1918. Most of these research studies involve experiments with laboratory animals (mostly rats or mice) or “test tubes” (in vitro studies). Of these articles, only 8.4% (123 studies) involve human subjects.
The U.S. government’s database of clinical research trials (ClinicalTrials.gov) describe 51 mostly completed clinical trials which studied the effects of cranberry on medical conditions including urinary tract infection (27 studies), insulin resistance, atherosclerosis, and diabetes.
One thing that really complicates research conclusions is that, until recently, no “standard reference material” (SRM) has been used uniformly throughout all studies. For example, some studies used cranberry juice and others have used powdered extracts. It is now recommended by experts that all studies should be done with freeze-dried whole cranberry powder (FWCP). This same problem exists for many, if not all, dietary supplements.
Another thing to be aware of is who is paying for the research to be done and how this might influence the study designs or conclusions. More and more, authors are required to disclose whether or not they’ve received money or other considerations from organizations such as the Cranberry Institute, the U.S. Cranberry Marketing Committee or the American Cranberry Growers Association.
If research is funded by such groups it doesn’t mean that the conclusions are biased or untrustworthy. It just means that readers should be aware of the connections.
Does It Prevent and Treat Urinary Tract Infections?
According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), urinary tract (bladder) infections (UTI) are common, especially among women. Research suggests that at least 40 to 60 percent of women develop a UTI during their lifetime, and most of these infections are bladder infections. One in 4 women is likely to have a repeat infection.
Worldwide, UTI affects more than 150 million people/year at a cost of more than $3.5 billion/year.
The treatment of choice for UTI is antibiotics. For those who have recurrent episodes of UTI, prophylactic antibiotics are frequently prescribed to try and prevent infections. Because long-term use of antibiotics can lead to antibiotic-resistant strains of urinary tract bacteria, the search for non-antibiotic strategies to prevent UTI is an important subject for public health.
Cranberry juice (and supplements) have been used for many years as a preventive measure for UTI. One theory was that cranberry worked for urinary tract infections by making the urine acidic which would inhibit the growth of bacteria. But researchers have disproven this theory. On the other hand, they did find that some components in cranberries (especially those A-type proanthocyanidins and other polyphenols) keep bacteria from adhering (sticking) to the cells that line the urinary tract where they can multiply .
Cranberry, however, does not seem to have the ability to release bacteria which are already stuck to these cells. This may explain why cranberry is possibly effective in preventing urinary tract infections but is ineffective in treating them.
Assessments of whether cranberry can prevent UTIs have shown mixed results. For example, in 2012 there were two meta-analyses (a statistical technique that combines data from multiple studies) on the use of cranberry to prevent UTIs. Wang et al  evaluated 13 clinical trials and concluded that “cranberry products were associated protective effects against UTIs, particularly for women with recurrent UTIs.
On the other hand, Jepson et al , doing a meta-analysis by the Cochrane Collaboration (a non-profit, non-governmental organization formed to organize medical research findings) looked at 10 clinical trials concluded that cranberry juice is less effective than previously stated and “cannot currently be recommended for the prevention of UTIs”. Interestingly, this study was an update of a report done in 2008 which had resulted in the opposite conclusion .
Confusing? Yes. How can scientists looking the same evidence come up with opposite conclusions? According to Liska, Kern and Maku  the answer may lie in which specific studies each group selected to include in their analysis.
The Jepson meta-analysis included more patients with recurrent UTIs and with complicated UTIs than the Wang group. Complicated UTIs refer to UTIs in patients with a structural or functional abnormality of the urinary tract (such as urinary obstruction, neurologic disease or immunosuppression). These differences can certainly skew the findings one way or another.
The most recent meta-analysis of the efficacy of cranberry in the prevention of UTI by Fu and colleagues  that analyzed 7 randomized controlled trials in healthy women at risk of UTI. Their analysis showed that cranberry reduced the risk of UTI by 26%.
They pointed out that the studies were relatively small, only two of which had over 300 participants. They suggested that cranberry “may be effective in preventing UTI recurrence in generally healthy women.” They also stated that larger high-quality studies are needed.
There is some evidence that cranberry can decrease the risk of recurrence of urinary tract infections, particularly in otherwise healthy women. There is no evidence that it can treat an infection already underway. Also, it may not be as effective in those who have had many recurrences or in those with complex UTIs.
Does It Decrease Inflammation?
We all know that oxygen is essential for life. Mitochondria in our cells use electrons taken from oxygen molecules to generate ATP (adenosine triphosphate) which is the molecule our bodies use for energy transfer. A byproduct of oxidation is the production of free radicals that are oxygen molecules with unpaired electrons.
These molecules are unstable (reactive) and can chemically interact with cell components such as DNA, protein or lipid, stealing their electrons to become stabilized. This can then trigger a chain of free radical reactions, which have the potential to cause considerable damage to cells.
Evidence is mounting that most of the degenerative diseases that afflict us have their origin in these deleterious free radical reactions. These diseases include atherosclerosis, cancer, inflammatory joint disease, asthma, diabetes, senile dementia, Alzheimer’s disease, and degenerative eye disease.
Fortunately, our bodies have their own defense systems to counteract a potential free-radical free-for-all. Our cells produce a complex system of antioxidants, such as glutathione, that can donate electrons to free radicals without becoming damaged or destabilized themselves. Our internal defense systems also include enzymes such as catalase, and superoxide dismutase. In addition, antioxidant molecules such as vitamin A, vitamin C and vitamin E come from our diets.
Cranberries are a rich source of several vitamins and minerals, especially vitamin C. Vitamin C, also known as ascorbic acid, is one of the predominant antioxidants in cranberries. There are 13.3 mg of Vitamin C in 100 g of raw cranberries, about 22% of the daily recommended value.
Unsweetened cranberry juice provides 23.5 milligrams of vitamin C (31 percent of the recommended daily intake for women of 75 milligrams and 26 percent of the recommended daily intake for men of 90 milligrams per day). Some cranberry juice “cocktails” have extra vitamin C (and sweeteners such as sugar) added.
Phytochemicals, such as benzoic and cinnamic acid derivatives and flavonols (particularly anthocyanins), also contribute to the potential antioxidant properties of cranberry.
Most studies on the antioxidant properties of cranberry have been done in vitro (in a test tube) or in vivo (on lab animals). Nardi et al  looked at the antioxidant properties of cranberry, blueberry and goji berry both in vitro and in and ex vivo.
- In vitro: Evaluation consisted on chromatographic assays of the cranberry extracts, and assays for the total flavonoid and phenolic content. Antioxidant capacity was assessed through radical scavenging activities testing (DPPH test) and a reducing power assay.
- In vivo: Mice were divided into 8 groups treated with each of the berry extracts at one of two doses, or a placebo or indomethacin for 10 days. The mice had paw edema (swelling) induced in one paw by injection of an irritating chemical.
- Ex vivo: The mice were euthanized, and their livers and paws were prepared as homogenates. These were then tested for antioxidant activity through lipid peroxidation, reduced glutathione, and catalase activity testing.
- All three berries have phenolic compounds and flavonoids. Goji berries had the highest amounts. Blueberry came in second (at about half of goji’s) and cranberry third at ⅙th to ¼ of goji’s phenol and flavonoid concentrations.
- Goji had the highest DPPH scavenging activity and was 4 times higher than cranberry (twice as high as blueberry). Results for reducing power were similar.
- All three berries showed reduction in the amount of inflammation found in the paws where swelling was induced.
- The antioxidant effect of cranberry is directly related to the content of quercetin and rutin (both flavonoid polyphenols) present in the extract.
- They cautioned that the study results are “valid for animals and that transfer of these results to humans must be done carefully.”
There are two small studies of the effect of cranberry on antioxidant capacity in humans. Pedersen et al.  looked at 9 healthy women volunteers who drank blueberry juice, cranberry juice or sugar solution (control). They repeated the study at weekly intervals until they each had been in each group once.
Blood and urine samples were obtained before and after beverage consumption. Drinking cranberry juice resulted in a significant increase in tests which measured antioxidant capacity. This corresponded to a 30% increase in vitamin C and a small but significant increase in total phenols in plasma. Consumption of blueberry juice had no such effects.
The authors concluded that the “increase in plasma antioxidant capacity following consumption of cranberry juice could mainly be accounted for by an increase in vitamin C rather than phenolics. This also accounted for the lack of an effect of the phenolic-rich but vitamin C-low blueberry juice.”
A study by Basu et al. looked at 36 women with the diagnosis of metabolic syndrome (high blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol levels). They were given 2 cups low calorie cranberry juice or placebo each day for 8 weeks. Blood was tested before the start and at the 8-week mark. In their study, there was a significant increase in plasma antioxidant capacity in patients receiving cranberry juice over placebo.
Cranberries, like many other berries, are a source of dietary antioxidants, both in the form of Vitamin C and from the phenolic and flavonoid compounds they contain. Other berries, such as goji berries and blueberries may have even higher levels of these compounds.
Does It Reduce Heart Disease Risk?
There are a variety of mechanisms by which cranberry could have a positive effect on the risk of cardiovascular disease (CVD). These include effects on CVD risk factors such as dyslipidemia (abnormal blood fats, such as cholesterol), diabetes, high blood pressure, inflammation, oxidative stress, endothelial dysfunction (abnormalities in the lining of blood vessels), arterial stiffness, and platelet function. Researchers have looked at the effect of cranberry on many of these factors and the results have been mixed.
Scientists looked at whether cranberry juice and cranberry anthocyanins can lower LDL and increase HDL. In vitro studies on cultured liver cells suggest that cranberry extract can increase the expression of LDL receptors, increasing the uptake of LDL-cholesterol and decreasing blood cholesterol levels . Qin et al’s study  on 120 patients with dyslipidemia suggests that increases in HDL-cholesterol may be due to the inhibition of cholesterol ester transfer protein (CETP) which raises HDL-cholesterol levels.
Studies on Syrian hamsters on a high fat diet , rats who had had their ovaries removed  and swine with high cholesterol all showed improvements in the lipid profiles after consumption of cranberry bioactives. In humans, a placebo-controlled trial by Zhu et al  150 participants with hypercholesterolemia (high blood cholesterol) received either 320mg twice a day of purified anthocyanin mixture or placebo for 24 weeks.
Patients who received anthocyanins had significantly lower LDL-cholesterol level and higher HDL-cholesterol levels at the end of the study. Other clinical trials on various populations [15, 19, 20] also showed favorable effects on cholesterol profiles.
Other studies on patients with CVD and healthy volunteers did not show significant effects on lipid profiles . Duthie et al looked at 20 healthy females (aged 18-40) who drank 750 ml/day of either cranberry juice or placebo for two weeks.
Although the plasma vitamin C levels of those receiving cranberry juice significantly increased, no anthocyanins (plasma) or catechins (plasma or urine) were detectable and plasma total phenols, total cholesterol, triglycerides, HDL and LDL were unchanged. Although Basu et al  showed increased oxidative capacity in their subjects taking cranberry juice (as above), they found no significant improvements in blood pressure, glucose and lipid profiles.
To date, cranberry has not been shown to lower blood pressure in humans.
Vascular endothelial cells are the innermost layer of the linings of blood vessels. They have many functions which maintain stability of the vascular system They play a role in controlling vasomotor tone, blood clotting, inflammation, platelet function, and the production of new blood vessels. Some of these functions depend on endothelium-derived nitrous oxide (NO). Dysfunction of the endothelium is one of many factors that contribute to cardiovascular disease.
A few in vitro studies using animal tissues have looked at the effect of cranberry bioactive compounds on endothelial function and NO bioavailability. Yung, et al  showed improved endothelium-dependent dilation and increased concentration of NO synthase (an enzyme which promotes the production of NO) in aortic tissue taken from rats treated with cranberry juice.
Clinical studies of cranberry on endothelial vasodilator function have mixed results. Zhu et al.  did a short-term crossover study on 12 patients with high cholesterol. Using an ultrasound test called flow-mediated dilation (FMD) to measure endothelial function, they measured the response of patients to oral anthocyanins given for a period of 12 weeks.
They found significant increases in FMD in the patients who received anthocyanins. However, Dohadwaia  found no increase in FMD in patients receiving cranberry juice for 4 weeks. Flammer  also observed no significant improvement in endothelial function in patients with risk factors and endothelial dysfunction given double doses of cranberry juice.
Arterial stiffness occurs because of biological aging and arteriosclerosis. Inflammation plays a major role in arteriosclerosis development, and consequently it is a major contributor in large arteries stiffening. Arterial stiffness is influenced by structural factors (including the relative amounts of elastin and collagen) and dynamic factors such as arterial tone and the balance of vasodilators and vasoconstrictors produced locally.
Pulse wave velocity (PWV) is the velocity at which the arterial pulse propagates through the circulatory system. PWV is used clinically as a measure of arterial stiffness.
Although Dohadwaia  did not find an increase in FMD in patients receiving cranberry juice for 4 weeks, he did demonstrate a significant decrease in central aortic stiffness (measured by PWV). Ruel et al found no effect of cranberry juice given for 4 weeks to obese men, however they used a different test (augmentation index) as a measure of arterial stiffness.
There are many mechanisms by which cranberry bioactive compounds could reduce the risk of cardiovascular disease. Although in vitro and in vivo studies have shown promise, clinical studies have had mixed results. Large scale, double-blind studies using standard reference material (SRM), such as freeze-dried whole cranberry powerd (FWCP), are needed.
Does it Improve Immune Function?
I’ve already talked about inflammation which is a function of the immune system. Acute inflammation (indicated by redness, swelling, heat and pain) is an important signal from our bodies telling us that something is wrong and we need to do something about it. However, chronic inflammation can be silent and is increasingly implicated in a number of human diseases.
The flavonoids in cranberries, especially the proanthocyanidins, have been the subject of research into the possible effects on immune function. These studies have been in a variety of diverse systems: Han et al  used rabbits who were subjected to infection-induced kidney damage.
Those treated with cranberry showed a reduction in inflammation when examined under a microscope. Hochmann et al  used a immune-compromised mouse lymphoma model which showed an increase in anti-lymphoma antibodies in mice who had an intraperitoneal injection of a bioactive material (“nondialyzable” material, NDM) obtained from cranberries.
Tipton et al  also used NDM to incubate epithelial and fibroblast cells from human gingiva (gums). They found that, in both cell types, there was a significant increase in interleukin-6 and -8 production. Interleukins are a group of cytokines (secreted proteins and signal molecules) that were first seen to be expressed by white blood cells (leukocytes). The function of the immune system depends in a large part on interleukins.
Most interleukins are synthesized by helper CD4 T lymphocytes, as well as through monocytes, macrophages, and endothelial cells. They promote the development and differentiation of T and B lymphocytes, and hematopoietic (blood cell-producing) cells. They suggest that cranberry components could be useful as a therapeutic agent to treat periodontitis (inflammation of the gums). Clinical studies would need to be done to support this.
Macrophages are a type of white blood cell that engulfs and digests cellular debris (a process called endocytosis). They also function as antigen-presenting cells, displaying antigen complexed with something called the major histocompatibility complexes (MHCs) on their surfaces so that they can be recognized by T-cells.
Carbello et al  looked at the interaction of cranberry proanthocyanidins (PACs) with cow protein and enzymes from hen egg whites, testing for the effect on microphage activation and antigen presentation. They found that PACs increased the rate of endocytosis and influenced antigen presentation in mice macrophages.
Nantz et al  looked at the ability of cranberry polyphenols to modify immunity, specifically γδ-T cell proliferation, after daily consumption of a cranberry beverage, and its effect on health outcomes related to cold and influenza symptoms.
The study was a randomized, double-blind, placebo-controlled, parallel intervention. Subjects (n= 45) drank a low calorie cranberry beverage or a placebo beverage daily for 10 wk. After the trial period, blood was drawn, and peripheral blood mononuclear cells were isolated and cultured for six days. Participants also kept a diary of any cold and/or influenza symptoms they may have exhibited.
They found that the proliferation index of γδ-T cells in culture was almost five times higher after 10 week of cranberry beverage consumption (p <0.001). In the cranberry beverage group, the incidence of illness was not reduced, however significantly fewer symptoms of illness were reported (p = 0.031).
Although bioactive components appear able to influence a number of cells and functions within the immune system, much more research is needed to draw any general conclusions about positive impacts of cranberry compounds on immune function.
Does it Help the Digestive Tract?
A quick primer on microbiomes:
The microbiome (old name: microbiota) is the collection of all the microorganisms living in association with the human body. In recent years, studies of our microbiome have exploded and one popular science book on the subject is Ed Young’s I Contain Multitudes. You can now even find microbiome cookbooks and diet plans.
Microbiome communities consist of a variety of microorganisms including bacteria, viruses and fungi. Because of their small size, microorganisms make up only about 1 to 3 percent of our body mass (that's 2 to 6 pounds of bacteria in a 200-pound adult) but bacterial cells outnumber human cells by a factor 10 to 1. The biggest population of microbes reside in our gastrointestinal tract – our “gut” -- mostly in the colon.
The microscopic organisms that make up our microbiome are generally not harmful to us. Indeed they are essential for maintaining health because they:
- produce some vitamins that our bodies do not have the ability to make
- help break down our food to extract nutrients we need to survive
- teach our immune systems how to recognize dangerous invaders
- produce helpful anti-inflammatory compounds that fight off other disease-causing microbes
An ever-growing number of studies have demonstrated that changes in the composition of our microbiomes correlate with numerous disease states such as inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS).
Now, back to cranberry:
There are two relatively recent papers that discuss the potential impact of cranberry on digestive tract health. The first, by Blumberg et al , is part of a summary of the Proceedings of the Cranberry Health Research Conference in 2015. They report that there is an increasing base of knowledge concerning the effects of cranberry bioactives on the gut and its microbiome.
Studies have shown that only small flavonoids are able to be readily absorbed by the intestinal tract. Longer chain flavonoids, such as proanthocyanidins or PACs, remain in the intestines until they reach the colon. It is here where they are digested by the gut microbiota and can be absorbed into the bloodstream. Additionally, PACs and their metabolites may exert health benefits directly on the GI tract through their antioxidant, anti-inflammatory, and antibacterial effects.
Biofilm is a group of microorganisms that stick together in a slimy scaffolding made up of complex carbohydrates, protein, and DNA. The production of biofilm is an early step in the development of an infection. Cranberry PACs have been shown to interfere with the aggregation of bacteria and decrease biofilm formation.
Lastly, the authors acknowledged that interpreting and comparing the results of previous studies are hampered by a lack of a standardized preparation of cranberry bioactives. To that end, the Cranberry Institute has created a standard reference material (SRM) to ensure consistency of cranberry products used in research on human health. It is made from a freeze-dried whole-cranberry powder (FWCP).
The second paper, published by Cires et al  in 2017 looks at the gastrointestinal tract as a key organ of the health-promoting effects of dietary PACs. In addition to cranberries, many other plants, and plant extracts such as Pycnogenol® derived from pine tree bark, are rich sources of PACs .
The beneficial effects of PCAs begin in the mouth where PCAs bind to proteins in saliva and inhibit bacterial adhesion in the gums. It continues in the stomach, where it may have a protective effect against helicobacter pylori (H. pylori), a bacterium that causes gastric ulcers. Other potential effects include protection from gastric inflammation, improved GI hormone secretion and stomach emptying, and even anti-vomiting (antiemetic) properties.
PACs can aid the digestion and absorption of nutrients in the small intestine. In the colon, where most of the gut microbiome lies, it becomes a nutrient (prebiotic) for some bacterial species, and can exert antioxidant, anti-inflammatory, and possibly anti-cancer effects. These potential benefits are summarized by this figure from the paper.
The following infographic is from the 2017 Frontiers in Nutrition article by Cires and associates .
Increasing evidence suggests that PACs are important prebiotics that should be part of a healthy diet. They can be obtained from many dietary sources, including cranberries. Evidence also suggests that cranberry PACs can exert antioxidant, anti-inflammatory, and possibly anti-cancer effects in the digestive system. Large scale clinical trials are necessary to confirm this.
Is Cranberry Safe?
Current National Institute of Diabetes and Digestive and Kidney Diseases from the U.S. government promote the concept of healthy eating patterns that include the consumption of nutrient-dense foods. These guidelines note that tart fruits and vegetables, namely cranberries and rhubarb, are often consumed with added sugar to counteract their tart flavors. I have previously noted that common cranberry juice “cocktails” contain added sugars in amounts comparable to soft drinks such as colas. (6)
Cranberry juice contains oxalates and have been associated with a higher risk of calcium oxalate kidney stones. Oxalates are naturally found in many foods, including fruits and vegetables, nuts and seeds, grains, legumes, and even chocolate and tea. Some examples of foods that contain high levels of oxalate include: peanuts, rhubarb, spinach, beets, chocolate and sweet potatoes. (7)
Large doses of cranberry may alter levels of warfarin, an anticoagulant (blood-thinner).
People who think they have a UTI should see a health care provider for a diagnosis and treatment. Don’t use cranberry products in place of antibiotic treatments for acute infections.
What about processed cranberry products such as powders and capsules? Online you can find many brands marketed as dietary supplements. Most are probably safe, when taken in recommended doses, but they are a very expensive way to get the benefits that can be obtained through eating fresh or frozen cranberries or drinking (lightly-sweetened) juice products made from them.
Cranberry has been enjoyed for many hundreds of years, primarily as a food, but also as a treatment for scurvy and the prevention of urinary tract infections. Although potential benefits have been suggested by in vitro and animal studies, well designed double blind clinical studies are few, and are plagued with inconsistent methods and lack of a standardized bioactive material with which to compare results.
Hopefully more research using the standard reference material (SRM), consisting of freeze-dried whole-cranberry powder (FWCP), will allow researchers to study cranberry health effects in a more consistent manner.
Cranberry might be a good first choice for those who wish to prevent urinary tract infections prior to using chronic antibiotic prophylaxis (discuss this with your health care professional first!).
As cranberry has high levels of vitamin C, antioxidants, and those potentially helpful PACs, along a very good safety profile, I have no reservations recommending it as a part of a healthy diet. Muffins anyone?