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Which diet and lifestyle recommendations for men with prostate cancer are supported by science?

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Men diagnosed with prostate cancer often wish to improve their diet and lifestyle to help fight this disease. The internet is full of advice on foods, supplements and other lifestyle changes. But which advice can be trusted? This week’s research blog examines the scientific evidence around diet and lifestyle changes for men with prostate cancer.

Quality of evidence

Sellers of supplements and “super-foods” often claim there is evidence that these products fight cancer. But there are differing levels of evidence quality. If there is only low-quality evidence, this should be insufficient for making such claims. Claims made on the basis of low-quality evidence, more often than not, turn out to be wrong, once higher quality evidence becomes available.

There are three broad categories of evidence for making such claims:

  1. Laboratory experiments on cancer cell lines

Cancer cells can be grown in a laboratory. These cells are grown in a liquid packed with nutrients, under sterile conditions. The cells often need to be modified to make them grow well. They are grown in the absence of any other cells or parts of the human body.

A typical laboratory experiment involved treating these cells with foods or supplements, such as pomegranate juice, or a chemical called lycopene from tomatoes. These substances can sometimes reduce the growth of the cancer cells. Experiments can then be done to determine how the substance affected the cells. These experiments are very useful for determining how different chemical affect cells. But the cells used in laboratory experiments are usually very different from the tumours inside our bodies. It is very common to find that a substance that slows the growth of laboratory-grown cancer cells does not do the same for tumours growing in humans. Another problem is that substances that kill cells in the laboratory are often toxic, or have nasty side effects, when used at the same concentration by a human.

  1. Observational studies

These experiments are conducted in humans. An example would be asking people who had prostate cancer whether they ate a lot of tomatoes or not. A study like this may find that men with prostate cancer were less likely to eat a lot of tomatoes than men who do not have the disease. The conclusions from such a study would be that tomatoes are a potential protective factor for prostate cancer. But there are many problems with this approach.

Another type of observational study is called a cohort study. These studies identify groups of people who do or don’t choose a diet or lifestyle change. These people are followed-up over time to ask what outcomes they experience.

Unfortunately, the quality of evidence from observational studies is not strong enough to make firm conclusions. These studies can easily be wrong. The problems come from bias in the study. We know that people who live a healthy lifestyle - who are not obese, exercise and eat a lot of fruit and vegetables, will be less likely to get cancer. It is possible that a combination of good habits is saving these people from cancer. People with healthy habits are likely to eat more tomatoes, on average. So those people eating more tomatoes may have less risk of cancer, because they have many other healthy habits. Tomatoes may not be contributing at all. Therefore, advice to eat more tomatoes, without improving all the other healthy habits, won’t be useful.

  1. Randomised controlled trials

During a randomised controlled trial, participants are randomly assigned into groups. One group have the intervention, such as a vitamin supplement, or eating tomatoes each day. A control group continue their normally daily routine without this change.

Randomised controlled trials provide the highest quality evidence. The reason for this is that the design of these experiments eliminates the problem of bias. Once people are randomly assigned into groups, this minimises differences between the groups. So people eating lots of tomatoes in a randomised controlled trial shouldn’t have more healthy habits than those in the control group.

The problems with observational studies for prostate cancer

In many cases, we only have evidence from observational studies to guide us. Randomised controlled trials are expensive and often take a long time to complete. So when we don’t have these trials to guide us, observational studies provide the best available evidence.

Unfortunately, observational studies have been a problem for prostate cancer in the past. Many observational studies indicated that taking vitamin E supplements or selenium supplements could reduce the risk of prostate cancer diagnosis. But when this was tested in a large randomised controlled trial, the results were not the same. Taking selenium did not reduce the risk of prostate cancer. Much more concerning was vitamin E intake. People taking vitamin E supplements in the randomised controlled trial actually had a slightly higher risk of being diagnosed with prostate cancer. In this case the observational studies were dangerously misleading.

A review of evidence for diet and lifestyle changes

A new study has been published that reviewed the scientific evidence for diet and lifestyle considerations for men with prostate cancer. The team behind this study are clinicians and researchers from San Francisco in the US. They reviewed all the published randomised controlled trials on diet and lifestyle. In their study, they also considered good quality evidence from observational and laboratory studies.

This review article concentrated on the question of diet and lifestyle for men who have been diagnosed. They examined the evidence for localised prostate cancer as well as advanced prostate cancer. By reviewing these studies, the authors sought recommendations for diet and lifestyle changes that are backed up by good evidence. They looked for high-quality evidence about reducing cancer progression, saving lives, reducing side effects, reducing pain, reducing PSA levels, increasing quality-of-life, and reducing anxiety and depression. They also asked about the safety of these diet and lifestyle modifications, and whether they were feasible for men with prostate cancer.

It’s important to recognise that this review is centred on the quality of evidence. In many cases, there have not been trials of sufficient quality to be sure of any benefit. Many of the diet and lifestyle changes have not been properly tested and may actually work. This study can only rule out these interventions if good quality trials have shown that they don’t work.

The researchers divided the results into various categories, based on the quality of the evidence, not the amount of benefit:

Category 1: Well-established benefits3409543739?profile=RESIZE_710x

According to the review article, these interventions are supported by high-quality randomised controlled trials and other forms of good evidence:

Exercise   

There is high-quality evidence from trials showing that exercise benefits men with prostate cancer. This was particularly so for men with advanced prostate cancer. The exercise interventions tested were mostly programs supervised by exercise physiologists. In many trials, exercise improved physical functioning and strength. Exercise also improved body composition (such as proportion of fat to muscle), quality-of-life, sexual interest and side effects from hormone therapy.

More information about exercise for men with prostate cancer is available from this link.

Pelvic floor muscle training

Multiple high-quality trials have demonstrated the benefits of pelvic floor exercises prior to treatment for men with localised prostate cancer. Pelvic floor exercises are often recommended before and after surgery or radiotherapy. Randomised controlled trials have shown that these exercises reduce the symptoms of incontinence. Incontinence becomes less severe and less frequent.

For more information about pelvic floor exercises, ask your doctor, visit a physiotherapist or visit this link.

Category 2: Appear to have benefits

Multimodal (holistic) lifestyle programs

Numerous studies and trials have examined the benefits of multimodal lifestyle programs. These programs take a holistic approach, assuming that a combination of many healthy changes is most likely to be beneficial. Typically, these programs increase intake of plants and whole grains, and decrease intake of meat, dairy and refined carbohydrates. Many multimodal programs combine dietary changes with exercise. These programs are often aimed at reducing weight and increasing cardiovascular fitness, therefore improving metabolic health. This is a good idea, as we know that improving metabolic health is good for prostate health. Although the trials have had mixed results, the authors concluded that these programs “appear to have benefit” for men diagnosed with prostate cancer.

Category 3: Possible benefits, but the evidence is limited

When scientists say “the evidence is limited” they mean that there there is a possibility for benefit, but the evidence is too weak to make any firm conclusions. These interventions are supported by logic and some observation data, but high-quality evidence is lacking. In some cases, randomised trials have been performed, but they have been very small or there were problems with the trial design.

Guided imagery and progressive muscle relaxation

These are relaxation techniques that are somewhat similar to meditation. They are used by people with cancer in the hope of reducing stress. One study has examined the use of these techniques in men with advanced prostate cancer. The men in this study experienced improvements in pain, fatigue, nausea/vomiting, anxiety and depression. However, more evidence is necessary to make conclusions.

Pilates

Pilates is an exercise program that aims to improve strength and flexibility. One randomised controlled trial has tested the effect of Pilates on urinary continence after prostate surgery. Although this study indicated Pilates could make improvements, more research is necessary to make conclusions.

Lycopene-rich diet

Lycopene is a chemical present in tomatoes and some other foods. Laboratory experiments show that lycopene can slow the growth of prostate cancer cells. There are many observational studies showing that men who ate a lot of tomatoes are less likely to develop prostate cancer.

There are mixed results from randomised controlled trials testing lycopene and tomatoes for men with prostate cancer. Some trials indicate benefit, whereas others do not. One study of men with metastatic disease showed those taking lycopene had a reduced chance of dying. But all of the men in this study had an orchiectomy – surgery to remove their testicles. This is rare in Australia, so it’s difficult to determine how relevant the results are for Australian men with prostate cancer. Due to the conflicting data, more research is necessary before making conclusions about tomatoes and lycopene.

Tomatoes are part of a healthy diet. So adding some tomatoes to your diet is a good idea in general. It’s best to consult with your doctor or a dietitian before making major changes to your diet, or if you are having chemotherapy.

Green tea

Components of green tea called catechins have been shown to have some anti-cancer effects in laboratory experiments. But observational studies have had inconsistent results. One randomised controlled trial for men with localised prostate cancer showed some indication of reduced inflammation and PSA levels. But a lower PSA does not necessarily mean improvement in the cancer. A study of advanced prostate cancer showed that men taking 6g of green tea powder a day had an increase in PSA levels and side effects from the tea powder such as nausea and trouble sleeping. Due to these inconsistent results, the evidence is insufficient to recommend green tea for men with prostate cancer.

Qigong

Qigong is a “mind-body” activity that involves physical activity and meditation. One study of older men with prostate cancer tested whether qigong could improve symptoms of fatigue and distress. The study went for 12 weeks and showed some benefits for both fatigue and distress.

Massage

Massage therapy for men with metastatic prostate cancer was tested in a small trial. This trial found some improvements in quality-of-life. But these were not sustained over time. Larger and longer trials are needed to test the benefits of massage therapy properly.

Avoiding non-prescribed vitamins and minerals

Good quality observational studies and some randomised controlled trials have shown that taking non-prescribed vitamins and minerals does not help prevent the progression of cancer. But this does not include people with specific vitamin or mineral deficiencies. It’s still a good idea to take supplements to address these problems, as guided by a doctor or dietitian.

Category 4: Needs evaluation

According to the review article, there are dietary and lifestyle recommendations that have some evidence from observational and laboratory experiments only. But the evidence is insufficient on which to make any assessment of possible benefits. These interventions need to be tested in randomised controlled trials, before any conclusions or recommendations can be made. These are:

Acupuncture

Coffee

Cruciferous vegetables

Fish

Larrea tridentate

Mushrooms

Vegetable-derived fats

Avoidance of eggs, dairy, poultry with skin

Avoidance of red meat and saturated fat

Category 5: Well-established as not beneficial

The review article also listed diet and lifestyle changes that had been studied in high-quality trials, where there was good evidence of no benefit for men with prostate cancer. These are:

Hypnosis

One trial tested hypnosis with the aim of improving well-being. This study found no improvement in emotional distress, sleep difficulties, fatigue and quality-of-life after hypnosis.

Milk thistle (silibinin)

A milk thistle component called silibinin was tested in a trial by men after surgery for prostate cancer. There was no benefit in taking the milk thistle compound. Some men taking the silibinin suffered side effects such as diarrhoea.

Pomegranate

Despite many claims on the internet about the benefits of consuming pomegranates for prostate health, randomised controlled trials have shown no benefits. A trial for men with localised prostate cancer showed that pomegranate extract made no difference to PSA levels. Similarly, trials for men with advanced prostate cancer showed no change in PSA levels or pain after drinking 500ml of pomegranate juice each day.

Soy

Soy contains chemicals called phytoestrogens. Laboratory research has shown that phytoestrogens have some anti-cancer properties. One trial showed that men with localised prostate cancer taking soy grits had a reduced PSA level. However, four other trials testing similar soy intakes and did not see the same effect. For men with advanced prostate cancer, two long-term trials of soy consumption have found no difference in cancer progression or side effects from hormone therapy.

Omega-3 fatty acid supplementation2857799723?profile=RESIZE_710x

Fish oil and flaxseed supplements are rich sources of omega-3 fatty acids. These supplements are widely believed to be beneficial for the cardiovascular system. They are also believed to help prevent cancer, but this has not been supported by randomised controlled trials.

There have been numerous trials examining the benefits of omega-3 supplements for men with localised prostate cancer. Unfortunately these trial have found no improvement in markers of disease progression such as PSA levels.

One reason for this lack of benefit could be that the supplements are not as good at supplying these fatty acids as fresh fish. There is some observational data suggesting that swapping red meat for fish is beneficial. This should be followed up with proper trials to make firm conclusions.

What does this mean for men with prostate cancer?

The study authors recommend a healthy diet, pelvic floor exercises and an exercise program (both aerobic and resistance training). They also recommend looking after wellbeing, but make no mention of how this should be done.

We know that men who maintain a healthy diet are less likely to get advanced prostate cancer and are more likely to have good outcomes when they have prostate cancer. But what we don’t know, is exactly what parts of a healthy diet are responsible for these good outcomes. There are many proposals claiming benefits from increasing intake of specific foods, such as pomegranate or soy. It’s common that no benefit is seen once these are tested in high-quality trials.

A more holistic approach is eating a range of different healthy foods (fruit and vegetables) and avoiding unhealthy foods (saturated fats and processed meats). This is the reasoning behind multimodal approaches that combine a healthy diet and exercise. These multimodal approaches appear more promising than dietary changes involving intake of a large amount of one type of food.

PCFA recommends men diagnosed with prostate cancer maintain a healthy diet in accordance with the Australian Guide to Healthy Eating, consider a tailored exercise program supervised by an expert such as a physiotherapist, and perform pelvic floor exercises as recommended by clinicians.

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