Fermented foods have become popular in recent years, partly due to their perceived health benefits.

For instance, there is some evidence eating or drinking fermented foods can improve blood glucose control in people with diabetes. They can lower blood lipid (fats) levels and blood pressure in people with diabetes or obesity. Fermented foods can also improve diarrhoea symptoms.

But can they help you lose weight, as a recent study suggests? Let’s look at the evidence.

Remind me, what are fermented foods?

Fermented foods are ones prepared when microbes (bacteria and/or yeast) ferment (or digest) food components to form new foods. Examples include yoghurt, cheese, kefir, kombucha, wine, beer, sauerkraut and kimchi.

As a result of fermentation, the food becomes acidic, extending its shelf life (food-spoilage microbes are less likely to grow under these conditions). This makes fermentation one of the earliest forms of food processing.

Fermentation also leads to new nutrients being made. Beneficial microbes (probiotics) digest nutrients and components in the food to produce new bioactive components (postbiotics). These postbiotics are thought to contribute to the health benefits of the fermented foods, alongside the health benefits of the bacteria themselves.

What does the evidence say?

A study published last week has provided some preliminary evidence eating kimchi – the popular Korean fermented food – is associated with a lower risk of obesity in some instances. But there were mixed results.

The South Korean study involved 115,726 men and women aged 40-69 who reported how much kimchi they’d eaten over the previous year. The study was funded by the World Institute of Kimchi, which specialises in researching the country’s national dish.

Eating one to three servings of any type of kimchi a day was associated with a lower risk of obesity in men.

Men who ate more than three serves a day of cabbage kimchi (baechu) were less likely to have obesity and abdominal obesity (excess fat deposits around their middle). And women who ate two to three serves a day of baechu were less likely to have obesity and abdominal obesity.

Eating more radish kimchi (kkakdugi) was associated with less abdominal obesity in both men and women.

However, people who ate five or more serves of any type of kimchi weighed more, had a larger waist sizes and were more likely to be obese.

The study had limitations. The authors acknowledged the questionnaire they used may make it difficult to say exactly how much kimchi people actually ate.

The study also relied on people to report past eating habits. This may make it hard for them to accurately recall what they ate.

This study design can also only tell us if something is linked (kimchi and obesity), not if one thing causes another (if kimchi causes weight loss). So it is important to look at experimental studies where researchers make changes to people’s diets then look at the results.

How about evidence from experimental trials?

There have been several experimental studies looking at how much weight people lose after eating various types of fermented foods. Other studies looked at markers or measures of appetite, but not weight loss.

One study showed the stomach of men who drank 1.4 litres of fermented milk during a meal took longer to empty (compared to those who drank the same quantity of whole milk). This is related to feeling fuller for longer, potentially having less appetite for more food.

Another study showed drinking 200 millilitres of kefir (a small glass) reduced participants’ appetite after the meal, but only when the meal contained quickly-digested foods likely to make blood glucose levels rise rapidly. This study did not measure changes in weight.

Another study looked at Indonesian young women with obesity. Eating tempeh (a fermented soybean product) led to changes in an appetite hormone. But this did not impact their appetite or whether they felt full. Weight was not measured in this study.

A study in South Korea asked people to eat about 70g a day of chungkookjang (fermented soybean). There were improvements in some measures of obesity, including percentage body fat, lean body mass, waist-to-hip ratio and waist circumference in women. However there were no changes in weight for men or women.

A systematic review of all studies that looked at the impact of fermented foods on satiety (feeling full) showed no effect.

What should I do?

The evidence so far is very weak to support or recommend fermented foods for weight loss. These experimental studies have been short in length, and many did not report weight changes.

To date, most of the studies have used different fermented foods, so it is difficult to generalise across them all.

Nevertheless, fermented foods are still useful as part of a healthy, varied and balanced diet, particularly if you enjoy them. They are rich in healthy bacteria, and nutrients.

Are there downsides?

Some fermented foods, such as kimchi and sauerkraut, have added salt. The latest kimchi study said the average amount of kimchi South Koreans eat provides about 490mg of salt a day. For an Australian, this would represent about 50% of the suggested dietary target for optimal health.

Eating too much salt increases your risk of high blood pressure, heart disease and stroke.

Evangeline Mantzioris, Program Director of Nutrition and Food Sciences, Accredited Practising Dietitian, University of South Australia

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Enzo Palombo, Swinburne University of Technology

Recent outbreaks of bird flu – in US dairy herds, poultry farms in Australia and elsewhere, and isolated cases in humans – have raised the issue of food safety.

So can the virus transfer from infected farm animals to contaminate milk, meat or eggs? How likely is this?

And what do we need to think about to minimise our risk when shopping for or preparing food?

How safe is milk?

Bird flu (or avian influenza) is a bird disease caused by specific types of influenza virus. But the virus can also infect cows. In the US, for instance, to date more than 80 dairy herds in at least nine states have been infected with the H5N1 version of the virus.

Investigations are under way to confirm how this happened. But we do know infected birds can shed the virus in their saliva, nasal secretions and faeces. So bird flu can potentially contaminate animal-derived food products during processing and manufacturing.

Indeed, fragments of bird flu genetic material (RNA) were found in cow’s milk from the dairy herds associated with infected US farmers.

However, the spread of bird flu among cattle, and possibly to humans, is likely to have been caused through contact with contaminated milking equipment, not the milk itself.

The test used to detect the virus in milk – which uses similar PCR technology to lab-based COVID tests – is also highly sensitive. This means it can detect very low levels of the bird flu RNA. But the test does not distinguish between live or inactivated virus, just that the RNA is present. So from this test alone, we cannot tell if the virus found in milk is infectious (and capable of infecting humans).

Does that mean milk is safe to drink and won’t transmit bird flu? Yes and no.

In Australia, where bird flu has not been reported in dairy cattle, the answer is yes. It is safe to drink milk and milk products made from Australian milk.

In the US, the answer depends on whether the milk is pasteurised. We know pasteurisation is a common and reliable method of destroying concerning microbes, including influenza virus. Like most viruses, influenza virus (including bird flu virus) is inactivated by heat.

Although there is little direct research on whether pasteurisation inactivates H5N1 in milk, we can extrapolate from what we know about heat inactivation of H5N1 in chicken and eggs.

So we can be confident there is no risk of bird flu transmission via pasteurised milk or milk products.

However, it’s another matter for unpasteurised or “raw” US milk or milk products. A recent study showed mice fed raw milk contaminated with bird flu developed signs of illness. So to be on the safe side, it would be advisable to avoid raw milk products.

How about chicken?

Bird flu has caused sporadic outbreaks in wild birds and domestic poultry worldwide, including in Australia. In recent weeks, there have been three reported outbreaks in Victorian poultry farms (two with H7N3 bird flu, one with H7N9). There has been one reported outbreak in Western Australia (H9N2).

The strains of bird flu identified in the Victorian and Western Australia outbreaks can cause human infection, although these are rare and typically result from close contact with infected live birds or contaminated environments.

Therefore, the chance of bird flu transmission in chicken meat is remote.

Nonetheless, it is timely to remind people to handle chicken meat with caution as many dangerous pathogens, such as Salmonella and Campylobacter, can be found on chicken carcasses.

Always handle chicken meat carefully when shopping, transporting it home and storing it in the kitchen. For instance, make sure no meat juices cross-contaminate other items, consider using a cool bag when transporting meat, and refrigerate or freeze the meat within two hours.

Avoid washing your chicken before cooking to prevent the spread of disease-causing microbes around the kitchen.

Finally, cook chicken thoroughly as viruses (including bird flu) cannot survive cooking temperatures.

Are eggs safe?

The recent Australian outbreaks have occurred in egg-laying or mixed poultry flocks, so concerns have been raised about bird flu transmission via contaminated chicken eggs.

Can flu viruses contaminate chicken eggs and potentially spread bird flu? It appears so. A report from 2007 said it was feasible for influenza viruses to enter through the eggshell. This is because influenza virus particles are smaller (100 nanometres) than the pores in eggshells (at least 200 nm).

So viruses could enter eggs and be protected from cleaning procedures designed to remove microbes from the egg surface.

Therefore, like the advice about milk and meat, cooking eggs is best.

The US Food and Drug Administration recommends cooking poultry, eggs and other animal products to the proper temperature and preventing cross-contamination between raw and cooked food.

In a nutshell

If you consume pasteurised milk products and thoroughly cook your chicken and eggs, there is nothing to worry about as bird flu is inactivated by heat.

The real fear is that the virus will evolve into highly pathogenic versions that can be transmitted from human to human.

That scenario is much more frightening than any potential spread though food.

Enzo Palombo, Professor of Microbiology, Swinburne University of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.