Do you wonder how your body directs the smell of food into the nasal cavity and not in the other direction - into the lungs? The answer lies in a 3D-printed model of the human airway from nostril to trachea.

The model developed by a team of engineers from Pennsylvania State University shows that the shape of the airway preferentially transfers food chemicals to the nasal cavity and allows humans to enjoy the smell of good food.

And for a really good meal, taking time to slow down and breathe smoothly will deliver more smell and flavour, they said.

During quiet breathing, there is no valve that can control the direction of food volatile (chemicals) transport.

"However, something must be controlling the movement of these particles and keeping them out of the lungs," said Rui Ni, assistant professor of mechanical engineering.

In the past, physiologists looked at the nasal passages, but not at the pathway from the back of the mouth to the nose.

In the new study, the researchers used data from CT scans and the help of two radiologists to build a schematic of the human airway from the nostrils to the trachea, including the fine structure.

They then used the schematic to make a 3D model using a 3D printer.

Ni and colleagues then tested airflow into and out of the airway.

Chewed food particles end up in the back of the mouth in a sort of side cavity to the main airflow.

The team found that when air is inhaled through the nose, the air flow forms an air curtain to prevent volatile particles released from the back of the mouth from escaping into the lungs.

However, when air is exhaled, it sweeps into the area with abundant food volatiles moving them into the nasal cavity where they are sensed by olfactory cells.

Movement of the particles is also effected by the speed of breathing.

"Smooth, relatively slow breathing maximises delivery of the particles to the nose. Food smells and tastes better if you take your time," Ni said.

The results were detailed in the journal Proceedings of the National Academy of Sciences.

The simple way to boosting male fertility, a major problem across the world, could be gobbling a handful of walnuts, researchers say.

"The initial research we conducted regarding walnuts and male fertility found improvement in sperm parameters after adding walnuts to the diet and has set the stage for us to further explore the impacts of walnuts on male fertility and reproductive health," Professor Wendie Robbins, University of California, Los Angeles (UCLA) School of Nursing.

"New projects are underway that we hope to be able to share with you in the near future," she told PTI.

Infertility and subfertility affect a significant proportion of humanity and the burden in men is relatively unknown.

Infertility is defined as a "disease of the reproductive system" and results in disability.

Commenting on the study in 2012 by a team led by her, Robbins said that 75 grams of walnuts consumed daily improved sperm vitality, motility, and morphology (normal forms) in a group of healthy young men between 21-35 years of age.

These findings, published in the journal Biology of Reproduction, are of particular interest to the over 70 million couples worldwide who experience sub-fertility or infertility.

In fact, 30 to 50 per cent of these cases are attributed to the male partner, and in the US the prevalence of men seeking help for fertility is estimated at 3.3?4.7 million.

This research suggests that walnuts provide key nutrients that may be essential in male reproductive health.

According to Robbins, "the positive finding of walnuts on sperm may be a result of their unique nutrient profile."

Walnuts are the only nut that are an excellent source of alpha-linolenic acid (ALA) the plant-based omega-3 fatty acid.

The study also reported higher amounts of ALA provided by walnuts correlated with less frequent aneuploidy or abnormal cell sperm chromosome numbers which can result in genetic abnormalities such as Down syndrome, she said.

In addition to ALA, walnuts have high antioxidant content, along with numerous micronutrients that Robbins thinks may work together synergistically.

Food has been linked with human reproductive success but most of the emphasis has been on the maternal diet and very little focus has been given to the paternal diet, Carol Berg Sloan, Nutrition Consultant with the California Walnut Commission said.

She said the study by Robbins emphasises that a father's diet not only impacts fertility, but can also influence the health of the child and future generations.

The study by UCLA researchers included 117 healthy young men who routinely eat a Western-style diet. Approximately half consumed the 75 grammes of walnuts per day for 12 weeks, while the remaining half served as the control group.

After 12 weeks, compared to the control group, the walnut group experienced improvement in sperm vitality, motility, and morphology key components in male fertility, Robbins said.

Standing for at least one-quarter of the day is linked to to 32 percent reduced likelihood of obesity, new research has found.

While sedentary behaviour (such as watching TV and commuting time) has been linked to negative health effects, it is unclear whether more time spent standing has protective health benefits.

To investigate further, a research team led by Kerem Shuval from American Cancer Society examined reported standing habits in relation to objectively measured obesity and metabolic risk among more than 7,000 adults between 2010 and 2015.

Specifically, the association between standing time and obesity was determined through three measures: body mass index (BMI), body fat percentage, and waist circumference.

The association between standing and metabolic risk was assessed via metabolic syndrome, a clustering of risk factors that increase the risk for heart disease, stroke, and diabetes.

The study found that among men, standing a quarter of the time was linked to a 32 percent reduced likelihood of obesity (body fat percentage).

Standing half the time was associated with a 59 percent reduced likelihood of obesity. But standing more than three-quarters of the time was not associated with a lower risk of obesity.

In women, standing a quarter, half, and three quarters of the time was associated with 35 percent, 47 percent, and 57 percent respective reductions in the likelihood of abdominal obesity (waist circumference).

No relationship between standing and metabolic syndrome was found among women or men.

The study appeared in the journal Mayo Clinic Proceedings.

A diet missing soluble fibre promotes inflammation in the intestines and poor gut health, leading to weight gain, suggests new research.

Moreover, incorporating soluble fibre back into the diet can restore gut health, the research conducted in mice showed.

Foods rich in soluble fibre include oatmeal, nuts, beans, apples, and blueberries.

"If our observations were to prove applicable to humans, it would suggest that encouraging consumption of foods with high soluble fibre content may be a means to combat the epidemic of metabolic disease," the researchers said.

The research team at Georgia State University examined the effects of diets varying in amounts of soluble and insoluble fibres, protein and fat on the structure of the intestines, fat accumulation and weight gain in mice.

The researchers found that mice on a diet lacking soluble fibre gained weight and had more fat compared with mice on a diet including soluble fibre.

The intestines of mice on the soluble fibre-deficient diet were also shorter and had thinner walls. These structural changes were observed as soon as two days after starting the diet.

Introducing soluble fibre into the diet restored gut structure.

Supplementing with soluble fibre insulin restored the intestinal structure in mice on the soluble fibre-deficient diet.

Mice that received cellulose, an insoluble fibre, however, did not show improvements.

The data suggest a difference in health benefits between soluble and insoluble dietary fibres, the researchers stated.

The study was published in the journal American Journal of Physiology.