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An F may mean failure in school, but the letter earns high marks in your diet. The two biggest dietary Fs — fiber and fermented foods — are top priorities to help maintain healthy digestion, and they potentially offer much more. How can you fit these nutrients into meals? Can this help your overall health as well as gut health?

Fiber, fermented foods, and the gut microbiome

The gut microbiome is a composed of bacteria, viruses, fungi, and other microorganisms living in the colon (large intestine). What you eat, the air you breathe, where you live, and many other factors affect the makeup of the gut microbiome. Some experts think of it as a hidden organ because it has a role in many important functions of the body — for example, helping the immune system function optimally, reducing chronic inflammation, keeping intestinal cells healthy, and providing some essential micronutrients that may not be included in a regular diet.

Your gut communicates with your brain through pathways in the gut-brain axis. Changes in the gut microbiome have been linked with mood and mental health disorders, such as depression and anxiety. However, it's not yet clear that these changes directly cause these types of problems.

We do know that a healthy diet low in processed foods is key to a healthy gut microbiome. And increasing evidence suggests that fiber and fermented foods can play important parts here.

Fiber 101

Fiber's main job is to make digestion smoother by softening and adding bulk to stool, making it pass quickly through the intestines.

But fiber has other benefits for your microbiome and overall health. A high-fiber diet helps keep body weight under control and lowers LDL (bad) cholesterol levels. Research has found that eating enough fiber reduces the risk of heart disease, type 2 diabetes, and some cancers.

What to know about fiber

There are two types of fiber: insoluble (which helps you feel full and encourages regular bowel movements) and soluble (which helps lower cholesterol and blood sugar). However, recent research suggests people should focus on the total amount of fiber in their diet, rather than type of fiber.

If you're trying to add more foods with fiber to your diet, make sure you ease into new fiber-rich habits and drink plenty of water. Your digestive system must adapt slowly to avoid gas, bloating, diarrhea, and stomach cramps caused by eating too much too soon. Your body will gradually adjust to increasing fiber after a week or so.

How much fiber do you need?

The fiber formula is 14 grams for every 1,000 calories consumed. Your specific calorie intake can vary depending on your activity levels.

"But instead of tracking daily fiber, focus on adding more servings of fiber-rich foods to your diet," says Eric Rimm, professor of epidemiology and nutrition at Harvard's T.H. Chan School of Public Health.

Which foods are high in fiber?

Fruits, vegetables, legumes, nuts, seeds, and whole grains are all high in fiber. The Dietary Guidelines for Americans has a comprehensive list of fiber-rich foods and their calorie counts.

What about over-the-counter fiber supplements that come in capsules, powders that you mix with water, and chewable tablets? "If you have trouble eating enough fiber-rich foods, then these occasionally can be used, and there is no evidence they are harmful," says Rimm. "But they should not serve as your primary source of dietary fiber."

Fermented foods 101

Fermented foods contain both prebiotics — ingredients that create healthy changes in the microbiome — and beneficial live bacteria called probiotics. Both prebiotics and probiotics help maintain a healthy gut microbiome.

What to know about fermented foods

Besides helping with digestion and absorbing vital nutrients from food, a healthy gut supports your immune system to help fight infections and protect against inflammation. Some research suggests that certain probiotics help relieve symptoms of gut-related conditions like inflammatory bowel disease and irritable bowel syndrome, though not all experts agree with this.

Many foods that are fermented undergo lacto-fermentation, in which natural bacteria feed on the sugar and starch in the food, creating lactic acid. Not only does this process remove simple sugars, it creates various species of good bacteria, such as Lactobacillus or Bifidobacterium. (Keep in mind that some foods undergo steps that remove probiotics and other healthful microbes, as with beer or wine, or make them inactive, like baking and canning.)

The exact amounts and specific strains of bacteria in fermented foods vary depending on how they are made. In addition to probiotics, fermented foods may contain other valuable nutrients like enzymes, B vitamins, and omega-3 fatty acids.

How often should you eat fermented foods?

There is no recommended daily allowance for prebiotics or probiotics, so it is impossible to know precisely which fermented foods or quantities are best. The general guideline is to add more to your daily diet.

Which fermented foods should you choose?

Fermented foods have a range of tastes and textures because of the particular bacteria they produce during fermentation or that are added to foods. Yogurt is one of the most popular fermented foods (look for the words "live and active cultures" on the label). Still, many options are available if you are not a yogurt fan or want to expand your fermented choices. Kimchi, sauerkraut, kombucha, and pickles are a few examples.

As with fiber, probiotics are also marketed as over-the-counter supplements. However, like all dietary supplements, they do not require FDA approval, so there is no guarantee that the types of bacteria listed on a label can provide the promised benefits — or are even in the bottle. "Therefore, it is best to get your probiotics from fermented foods," says Rimm.

To learn more about the value of fiber, fermented foods, and a healthy gut microbiome, listen to this episode of the Food, We Need to Talk podcast, "Understanding the Microbiome."

About the Author

Matthew Solan, Executive Editor, Harvard Men's Health Watch

Matthew Solan is the executive editor of Harvard Men’s Health Watch. He previously served as executive editor for UCLA Health’s Healthy Years and as a contributor to Duke Medicine’s Health News and Weill Cornell Medical College’s … See Full Bio View all posts by Matthew Solan

About the Reviewer

Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing

Dr. Howard LeWine is a practicing internist at Brigham and Women’s Hospital in Boston, Chief Medical Editor at Harvard Health Publishing, and editor in chief of Harvard Men’s Health Watch. See Full Bio View all posts by Howard E. LeWine, MD

Mosquitoes are not just a buzzing summertime nuisance; they can carry serious diseases like West Nile virus, malaria, and Zika. In recent months, the US has seen an unusual spike in the mosquito-borne illness dengue fever, also known as breakbone fever.

What is dengue, and where is it occurring in the US? How does it spread? And what steps can you take to protect against this and other mosquito-borne illnesses?

What is dengue and what are its symptoms?

Dengue is a viral disease caused by any of four closely-related viruses: dengue virus 1, 2, 3, and 4. Not everyone infected with dengue will become sick: only about one in four will experience symptoms that range from minimal discomfort to serious problems. A blood test is used to confirm the diagnosis.

When symptoms do occur, they tend to feel like the flu. The onset of feeling unwell usually begins within two to three days of being bitten by an infected mosquito but can take up to six to nine days, says Dr. Edward Ryan, director of global infectious diseases at Harvard-affiliated Massachusetts General Hospital.

Seek medical advice if you have recently traveled to an area with a risk of dengue and have a fever (101° F or higher) combined with any of these symptoms:

• aches and pains (often headache or pain behind the eye or in muscles, joints, or bones)
• nausea and vomiting
• a rash (faint red blotches).

About one in 20 people who gets sick will develop severe dengue, which requires emergency care. Those at an increased risk for severe dengue include infants a year or younger, pregnant women, adults ages 65 years or older, and those with previous dengue infections.

Symptoms of severe dengue require immediate medical care. They include:

• feeling very weak and lightheaded from low blood pressure
• bleeding from the nose or gums
• stomach swelling
• vomiting
• extreme fatigue.

How is dengue treated?

Dengue is cause by a virus, and no specific medications are available to combat it. Milder dengue symptoms typically last two to seven days, according to the Centers for Disease Control and Prevention (CDC). The symptoms are managed supportively: plenty of fluids, over-the-counter pain medications, and rest.

Severe dengue usually requires treatment in a hospital with intravenous (IV) fluids. Patients are monitored until their fever breaks and symptoms begin to wane. This usually happens in about three to five days. Recovery at home can last many more days before a person’s strength returns.

“While there are reports of people dying from severe dengue fever, deaths from dengue cases in the US are rare,” says Dr. Ryan.

How does dengue spread?

The viruses that cause dengue do not directly spread from person to person. Aedes species mosquitoes can become infected with the dengue virus when they bite a person who has the virus. Infected mosquitoes then spread the virus to other people.

US visitors to high-risk countries may unknowingly bring the virus home with them, giving dengue infection a chance to spread further. For example, an infected person will have dengue virus circulating in their bloodstream for up to one week after being bitten. Let’s say the person returns to the US sooner than seven days, where they get bitten by a mosquito. That mosquito in the US now carries the virus and could bite someone else, thereby spreading the infection.

Where is dengue occurring in the US?

Most dengue transmission in the United States happens in areas where this illness is already common, such as Puerto Rico, American Samoa, and the US Virgin Islands.

At this writing, the CDC has reported nearly 3,000 dengue cases in the United States and US territories. States with the highest reported instances include Florida, New York, Massachusetts, and California. (Puerto Rico, which declared a public health emergency in March, had reported almost 1,500 cases by late June 2024.) While US cases are higher than in previous years, they are still low compared to the global incidence of dengue, which hit a record 9.7 million cases in North, Central, and South America during the first six months of 2024.

It’s not clear what has caused the sudden rise in dengue cases. Dr. Ryan says it could be due to several factors, such as higher seasonal travel, more people living closer together in urban settings, and the fact that there are more mosquitoes are carrying dengue.

Is there a dengue vaccine?

Currently, there is no widely available dengue vaccine for US travelers. One FDA-approved vaccine, Dengvaxia, protects children ages 9 to 16 from all four types of dengue. It is used only for children who have had a previous infection and who live in areas where dengue is common. However, the vaccine will be discontinued in September 2025, with the final doses expiring in September 2026.

What steps can you take to prevent dengue?

The best way to prevent dengue when visiting high-risk areas and after returning home is to protect yourself from mosquito bites. Here are some tips from the CDC:

• Use EPA-registered insect repellents with one of the following active ingredients: DEET; picaridin (known as KBR 3023 and icaridin outside the United States); IR3535; oil of lemon eucalyptus (OLE) or 2-undecanone (plant-derived ingredients); or para-menthane-diol (PMD).
• Wear loose-fitting, long-sleeved shirts and pants.
• Wear clothing and gear treated with permethrin (an insecticide that kills or repels mosquitoes).
• Place screens on windows and doors.
• When traveling, stay in places with air conditioning and screens. Use a bed net if air-conditioned or screened rooms are unavailable or if sleeping outdoors.
• To prevent mosquitoes from laying eggs in or near water around your home, empty and scrub, turn over, cover, or throw out outdoor items that hold water. This includes tires, buckets, toys, kid pools, birdbaths, flower pot saucers, or trash containers.

About the Author

Matthew Solan, Executive Editor, Harvard Men's Health Watch

Matthew Solan is the executive editor of Harvard Men’s Health Watch. He previously served as executive editor for UCLA Health’s Healthy Years and as a contributor to Duke Medicine’s Health News and Weill Cornell Medical College’s … See Full Bio View all posts by Matthew Solan

About the Reviewer

Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing

Dr. Howard LeWine is a practicing internist at Brigham and Women’s Hospital in Boston, Chief Medical Editor at Harvard Health Publishing, and editor in chief of Harvard Men’s Health Watch. See Full Bio View all posts by Howard E. LeWine, MD

Global average temperatures have soared in 2024, surpassing records set just last year. Extreme heat poses numerous health risks, some of which are visible on our skin. Understanding how heat affects your skin can help you take measures to prevent or ease heat rash, eczema, rosacea, the discoloration of melasma, and many other skin conditions sparked or worsened by high temperatures.

How does heat affect your skin?

Heat waves can provoke or worsen several skin conditions.

• Heat rash (miliaria) occurs due to obstruction of sweat ducts on the skin, which are responsible for helping the body maintain a normal temperature by releasing water to the surface of the skin. As these sweat glands get blocked with excessive sweat on an extremely hot day, they do not function properly and red itchy bumps develop.
• Grover disease, a rash of small, red, itchy bumps on the chest and back, can also be triggered or worsened by excessive heat and sweating. Often this clears up within a few weeks or months. Less often symptoms may persist for years, with flares during the summer months.
• Eczema is a chronic skin condition characterized by dry, itchy plaques that affects millions of people in the United States. Increased sweating during heat waves can lead to skin irritation and inflammation, aggravating eczema symptoms. Dehydration caused by fluid loss through sweating can make the skin more susceptible to eczema flare-ups.
• Rosacea, which causes facial redness, visible blood vessels, and acne-like bumps, is also heat-sensitive. Heat causes blood vessels to dilate as the body attempts to cool itself, resulting in visible redness and flushing. High temperatures, especially combined with sun exposure, can cause flare ups.
• Melasma. Hyperpigmentation disorders like melasma also worsen with heat. Melasma is characterized by irregular dark patches on the face and is often triggered by UV exposure from the sun. Heat can increase the activity of melanocytes, the cells that produce pigment, making these patches more pronounced. Additionally, the combined effects of heat and UV radiation accelerate collagen and elastin degradation, leading to premature aging and loss of skin elasticity.
• Skin cancer risk rises with increased exposure to UV radiation. Heat waves — now beginning earlier, ending later, and lasting longer — contribute to UV exposure, particularly among people who work outdoors. And preliminary research suggests prolonged exposure to high temperatures may further boost skin cancer risk. This could be of particular concern for firefighters, who face extended exposure to extreme heat.

Heat waves, air pollution, and skin

During heat waves, levels of environmental pollutants like ozone and particulate matter can rise. And reactions between these pollutants, heat, and UV radiation spawn secondary pollutants such as peroxyacetyl nitrates (PANs). Pollutants like these can irritate the skin and contribute to inflammation through oxidative stress and DNA damage, so inflammatory conditions such as eczema and rosacea may worsen.

Can heat affect medications?

High temperatures compromise the effectiveness of certain medications. For instance, EpiPens, which are crucial for managing life-threatening allergic reactions, can lose their potency when exposed to high temperatures. Check the instructions on all of your medicines to see which ones should be stored in a cool, dry place or refrigerated. If you’re not sure which medicines might be affected by heat, talk to your pharmacist or doctor.

Retinoids in skin care products, antibiotics taken for acne, and immunomodulators for autoimmune diseases can make skin more susceptible to sun damage. This can lead to severe sunburns or rashes known as photodermatoses. If you experience this, contact your health care team for advice.

Protect your skin when temperatures climb

Keep skin cool

• Wear light, breathable clothing. Choose natural fibers like cotton and linen to help regulate body temperature and prevent sweat-induced skin issues. Avoid synthetic fabrics, which can trap heat and moisture.
• Take cool baths or showers. Use cool or tepid water to bathe. Avoid hot showers, which can strip the skin of natural oils, leading to dryness and irritation.
• Find cool spaces. If your home is not air-conditioned, seek out cooler places and ways to cool off during heat waves.

Keep skin hydrated

• Drink ample water and eat water-rich foods. Foods like watermelon and cucumbers can provide additional hydration.
• Moisturize your skin. Apply light, non-comedogenic moisturizers immediately after bathing. Look for ingredients such as hyaluronic acid and glycerin, which are beneficial for enhancing skin hydration.

Limit exposure to sun and pollution

• Use protective clothing. Wear wide-brimmed hats, UV-blocking sunglasses, and long-sleeved shirts to shield the skin from harmful radiation.
• Wear sunscreen. Apply broad-spectrum mineral sunscreens containing zinc oxide, titanium dioxide, or iron oxide to protect against UV radiation and pollutants.
• Use topical antioxidants. Use products like vitamin C in your morning skin care routine to mitigate oxidative stress.
• Wash up. Cleansing face and body at the end of the day may help you limit skin exposure to pollutants while also rinsing off any sunscreen residue.

About the Authors

Kathyana P. Santiago Mangual, Contributor

Kathyana P. Santiago Mangual is a clinical research fellow in dermatology at Massachusetts General Hospital and Harvard Medical School. She is also a medical student at the University of California, Los Angeles, and will be graduating … See Full Bio View all posts by Kathyana P. Santiago Mangual

Arianne Shadi Kourosh, MD, MPH, Contributor

Arianne Shadi Kourosh, MD, MPH, is a board certified dermatologist and associate professor of dermatology at Harvard Medical School. She is a graduate of the Harvard T.H. Chan School of Public Health and the University of … See Full Bio View all posts by Arianne Shadi Kourosh, MD, MPH