Fat and Fiction

Fat. Viewed over the years as both hero and villain, fat has been the subject of dietary debate since physiologist Ancel Keys presented a hypothesis linking saturated fats and heart disease at a 1955 meeting of the World Health Organization. 

Revised Dietary Guidelines for Americans issued late last year by the U.S. Department of Health and Human Services and the Department of Agriculture and updated Dietary Guidance to Improve Cardiovascular Health¹ published by the American Heart Association (AHA) in March 2026 have only added sizzle to the discussion. 

The facts around fat can seem bewildering—but Tufts experts are helping make sense of them. Diane McKay, assistant professor at the Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy, helps sort out six common assertions.

Fats (technically called lipids), along with proteins and carbohydrates, constitute one of three macronutrients required for our bodies’ most basic functions, like energy, growth and tissue repair. Fats yield more than twice as many calories per gram as carbs or protein, so fatty foods are favorites with human and non-human animals, who instinctively seek energy-rich foods. 

Ninety-five percent of the fats in our bodies and in our food are triglycerides, so-called because they’re composed of a molecule called glycerol attached to three fatty acids—basically chains of carbon atoms bonded to one another and to varying numbers of hydrogen atoms as well. The structure of the fatty acids determines if the fat is categorized as saturated, monounsaturated or polyunsaturated. 

If a fatty acid’s carbon atoms are fully bonded with hydrogen, it’s considered saturated. Saturated fatty acid chains are straight. As a result, they readily align to form rigid structures that, for example, contribute to healthy cell membranes. Saturated fats are typically solid at room temperature—think of butter or the marbling in a steak. 

If one or more carbon atoms in a chain are not fully bonded with hydrogen, that fatty acid is considered monounsaturated or polyunsaturated. Unsaturated fatty acid chains are kinked, making them more fluid than their saturated counterparts. Liquid oils at room temperature, unsaturated fats are found in fish and seafood and plants like nuts, avocados and seeds. 

Most fats actually contain a combination of different fatty acids, with one predominating, and good health requires a balance of fatty acids.

Not exactly, said McKay. Everyone’s body includes some saturated fat, and saturated fats play an important role in cell membrane structure and hormone production. However, our bodies can synthesize such fats so we don’t have to get them from food. The two essential fats that we must consume through diet because our bodies can’t produce them are the polyunsaturated fats alpha-linolenic acid and linoleic acid. 

But problems arise when we consume more saturated fat than our body needs. Excess saturated fat reduces the body’s ability to remove low density lipids (LDL) from the blood, which can lead to atherosclerosis (clogged arteries). In contrast, when substituted for saturated fatty acids in the diet, both monounsaturated and polyunsaturated fats have similar effects on lowering total and LDL cholesterol in our blood. 

And, although dietary fat doesn’t appear to initiate cancers, said McKay, it can promote development once cancer has started; prostate cancer, for example, appears to be correlated with dietary fat intake, particularly from animal-based foods. Some research also links excess saturated fat with increased inflammation, which can be an early step in cardiovascular disease processes.

At the same time, evidence is emerging that saturated fat from certain dairy and fermented foods—such as aged cheeses, yogurt, and kefir—may be neutral or even beneficial when it comes to health, possibly due to beneficial compounds created during fermentation and aging. 

The latest AHA dietary guidelines note that there is some evidence that certain saturated fats may reduce cardiovascular disease mortality, but concludes that evidence for benefits of saturated fats is still limited across the board. 

A good guideline is limiting saturated fats to 6 to 10% of your daily calories and total fats to 20 to 35%.

Yes. Although it’s difficult for children under a year old to digest protein in cow’s milk, the American Academy of Pediatrics recommends full-fat (whole) dairy for children between one and two years of age. “That’s the critical stage when kids need lots of energy for rapid growth and neurological development,” said McKay. 

In addition to calories, full-fat dairy products are a good source of nutrients that are particularly important during this growth period, including protein, calcium, phosphorus, potassium, riboflavin, and vitamins A, D, and B12. McKay noted that although there are a lot of high-fat foods available, not many are also a significant source of the nutrients essential to building healthy bones and supporting the development of other vital organs and systems. 

After age two, children can transition to reduced or non-fat dairy depending on their energy needs and family history of obesity or heart disease. While removal of fat from dairy decreases its fat-soluble vitamins, in the U.S. vitamins A and D are typically added back to milk during processing. The comparative benefits of full-fat, reduced-fat, low-fat and fat-free dairy for older children and adults continue to be studied—and debated. 

Yes, in moderation. While red meat is higher in saturated fat than poultry or fish, McKay pointed out that red meat is also a great source of readily bioavailable iron and zinc. “Nothing says you have to eliminate every single bit of saturated fat from your diet,” she said. But those who choose to eat red meat should limit portion size and frequency—no more than three portions per week (each 4 to 6 ounces when cooked)—and choose lean cuts of meat.  

Just because it’s natural doesn’t necessarily mean it’s good for you, said McKay. Poisonous mushrooms, for example, are natural but they’re not preferable to canned. There’s not yet a standardized definition of “ultra-processed.” Minimal processing such as canning, pasteurizing, freezing, or cooking can make food safer and fresh for longer, but consumers should moderate their intake of meats, including seafood, that have been smoked, cured, or salted, or that contain ingredients added to enhance flavor or appearance. When it comes to butter, McKay suggested substituting olive oil or another unsaturated vegetable oil for cooking and adding butter (if you choose) via a small pat either in the pan or on the food itself.

The 10 percent guideline for saturated fat is a well-established number, said McKay. It continues to be the recommendation of the most recent national dietary guidelines as well those of the AHA. Overall, fats should make up no more than 35 percent of our calories. More research is needed before revising these numbers.

It all depends on the animals and the plants, said McKay. The fatty acid profile of fish—containing less saturated fat than beef and rich in polyunsaturated omega-3 fatty acids—is healthier than the oils from tropical plants such as coconuts and palms, which are very high in saturated fats. Plus, fats aren’t the only factor in the healthfulness of a food. Case in point: along with healthy fats, plants usually provide more fiber than animal foods; examples include avocados, nuts, and whole olives.

McKay added that we’re moving into an age of precision and personalized nutrition that explores varied individual responses to different foods and nutrients. “We’ve already seen these variations in response to carbohydrate-rich foods and that may turn out to be true of fats as well,” she said. 

Meanwhile, moderation—neither overindulging in one nutrient or food group, nor trying to cut one out entirely—continues to be the foundation of a sensible approach to fats and overall dietary health.

Reference:

1) https://www.ahajournals.org/doi/10.1161/CIR.0000000000001435

(Newswise/HG)