OVERWEIGHT men need to shed the pounds and improve their diet if they want to increase their chances of becoming a father, according to a major review of studies investigating infertility and nutrition.

A team of Portuguese scientists reviewed all the studies on sexual dysfunction and nutrition using Cochrane Library, Medline and ScienceDirect databases.

Several studies showed that a Mediterranean-style diet, which emphasises eating primarily plant-based foods, such as fruits and vegetables, whole grains, legumes and nuts, can improve fertility rates.

For women trying to become pregnant naturally, without using IVF for example, a Mediterranean diet was also seen as having a positive effect. Vitamins and nutrients, such as folic acid, vitamin B12 and omega-3 fatty acids, were also linked to positive effects on fertility.

A low carbohydrate diet, vegetable proteins and antioxidants were also found to improve fertility in many of the studies.

When it comes to dairy products, whole milk improves women fertility, but men benefit from skim milk, the scientists found.

Dietary supplements were largely seen as having little overall effect, but the most “promising” supplements according to the review are yohimbine – made from the bark of an African evergreen tree and commonly used to treat erectile dysfunction – vitamin B, L-arginine – a chemical building block called an amino acid and necessary for the body to make proteins – and vitamin D.

Trans fat and “unhealthy diets” – those “rich in red and processed meats, potatoes, sweets, and sweetened beverages” – were found to have negative effects in another study.

Sexual dysfunction and infertility are conditions with high prevalence in general population – and the situation has reached a crisis point.

Sperm counts among men in the west have more than halved in the past 40 years and are currently falling by an average of 1.4 per cent a year, according to scientists at the Hebrew University of Jerusalem.

After the results of that study were published last year some commentators went as far as to suggest humanity could soon become extinct.

The UK is now the third fattest nation in Europe – fuelled by its boozy lifestyles, the World Health Organisation warned in September. Research on 53 countries found Britain rising up the obesity league tables, and among the “world champions” for alcohol consumption.

The WHO said the trends were “alarming” and threatened to cut short the lives of generations growing up on a diet of junk food and alcohol. Almost 28 per cent of adults are now obese, while 63.7 per cent are classed as overweight or obese.

Data from the Human Fertilisation and Embryology Authority, which regulates infertility treatment in the UK, shows the number of infertile men being treated has doubled in just four years.

Experts believe rising obesity and diabetes are major factors in increasing rates of male infertility, because they damage overall fitness.

Earlier this year, the World Health Organisation described current knowledge of male infertility as “very low”, a relative ignorance that has since been acknowledged by the UK Medical Research Council, which issued a call for scientists to put forward projects in the field for funding.

The problem is also solely afflicting Western nations – not developing countries and people are desperate to find out why – most believe we face a major social and biological problem.

The latest study suggests adds to the growing body of evidence that obesity has a negative influence in male fertility, and weight loss improves male fertility.

Lots of studies of men have found that semen quality improves with healthy diets, while the opposite has been linked with diets high in saturated or trans fat. Alcohol and caffeine appeared to have little effect, good or bad.

Importantly, semen quality is not a perfect predictor of fertility, and most studies did not actually examine the impact of paternal diet on the rate of successful pregnancies, one research team pointed out.

The Cochrane scientists, who publish their findings in the journal Current Pharmaceutical Biotechnology, concluded that “nutritional factors may affect the sexual and reproductive health in both men and women”.

However, they said that further studies will be necessary to clarify this association, and simultaneously improve the approach and treatment of patients with sexual and/or reproductive problems.

Dr Hana Visnova, medical director of the world-leading IVF Cube in Prague, Czech Republic, said: “We know that more and more men are seeking fertility treatment than ever before. We also know that obesity levels are rising.

“Whether fertility levels and obesity are connected still warrants further study, but a number of studies have indicated links between the diet and sperm quality.

“As well as diet, we need to as well look at the impact of things such as stress and smoking.”