Different fatty acids have different effects on human health, and saturated fat has had a bad reputation since the 1950s. It was around this time that the “diet-heart hypothesis” was first formulated and popularized (1). According to this hypothesis, consumption of saturated fat increases circulating cholesterol levels, which increases the risk of heart disease. To make matters worse for saturated fat’s public relations team, it’s not uncommon to find high levels of saturated fat in many foods that are colloquially labeled as “junk food.” It’s also true that these types of energy-dense, hyperpalatable foods tend to be associated with excess adiposity (2), and excess adiposity is independently associated with elevated heart disease risk (3). Taken together, these observations lead many to associate saturated fat with a vague cluster of deleterious outcomes related to body composition, blood lipids, and overall health.

Having said that, the carnivore diet started getting popular a handful of years ago. I use the term “popular” loosely – I don’t believe a large number of people actually adopted the diet, but it was certainly the talk of the town in fitness circles. The carnivore diet was more like a movement or a counterculture than a simple diet, and the community that formed around it became quite outspoken online. In response to the conventional (negative) messaging surrounding saturated fat, carnivore diet proponents popularized a number of rebuttals, counterpoints, and competing theories that framed saturated fats in a categorically positive light. Notably, many of these content creators lost considerable weight or became outright shredded on the diet, which caused them to claim that diets high in saturated fat actually make you leaner (and healthier) than alternative diets. Understandably, this fueled a great deal of confusion pertaining to the net impact of saturated fat consumption on body composition. In this article, I will address two recent studies that shed light on the topic.

Claim #1: Saturated fat has a lower thermic effect of feeding and smaller impact on satiety than all other types of fat.

There are credible reasons to believe that certain types of saturated fat might impact appetite responses or the thermic effect of feeding in a manner that promotes overconsumption of calories. Fortunately, a somewhat recent article by Nguo and colleagues (4) addressed both outcomes in a well-controlled study. The researchers used a crossover design, meaning that each participant completed all three experimental conditions. The sample included 13 overweight men between 18-40 years old and BMI values between 25-35 kg/m². All participants were generally healthy, and the average age and BMI were 23.8 ± 1.4 years and 29.3 ± 0.6 kg/m², respectively. Participants were excluded if they smoked, took medications known to alter body composition or metabolism, had obesity caused by a genetic or endocrinological disorder, were lactose intolerant, vegetarian, or were diagnosed with diabetes, high blood pressure, high cholesterol, or cardiovascular disease. Participant characteristics are presented in Table 1.

On three separate occasions separated by a washout period of at least one week, participants consumed three isoenergetic (~900kcal), high fat (~45% of energy) test meals. One of the meals was high in short and medium chain saturated fats, with cream, milk powder, and coconut cream used as fat sources. A second meal was high in long chain saturated fats, with palm olein and beef fat used as fat sources. The third meal was high in monounsaturated fats, with avocados, olive oil, and olives used as fat sources. Full meal details are provided in Table 2.

Each experimental visit took place after an overnight (12-hour) fast, with a standardized low-fat dinner (~467kcal; 15% fat) provided prior to the fast. Upon arrival at the laboratory, the researchers took baseline measurements including anthropometrics (height, weight, and body composition), resting metabolic rate, a blood sample, and questionnaires pertaining to hunger and fullness. After baseline measurements, participants ate their assigned meal. Measurements were repeated every hour for six hours after the ingestion of the test meal. The researchers were primarily interested in determining if different fatty acids (short/medium-chain saturated fat, long-chain saturated fat, and monounsaturated fat) have differential impacts on meal-induced thermogenesis and appetite responses.

As for the findings, Nguo and colleagues found no significant differences in meal-induced thermogenesis among the three experimental meals (p = .888). All three meals led to values ranging from 192.6 to 204.2 kJ over the six-hour period, which equates to roughly 50kcal. These results are presented in Figure 3. In addition, there were no statistically significant differences among conditions for postprandial hunger or fullness ratings (p = .181 and p = .383, respectively). These values are presented in Figure 4.

The presently reviewed study suggests that there are no meaningful differences in hunger or energy expenditure responses to meals with long-chain saturated fats, short- and medium-chain saturated fats, and monounsaturated fats. These findings are compatible with other research showing no meaningful differences between saturated and monounsaturated fat with regards to post-meal energy expenditure (5, 6, 7, 8, 9). Notably, this study provided large high-fat meals that were designed to produce exaggerated differences between meals. In other words, this study represents an “extreme” scenario meant to amplify any potential effects, rather than representing a real-world scenario meant to reflect a typical meal or diet. If significant differences had been found, we’d need to take them with a grain of salt – it’s one thing to demonstrate that an effect could exist, but a very different thing to demonstrate that an effect matters in real-world scenarios. The presently reviewed study spared us from wrestling with that conundrum, as all three test meals led to pretty similar appetite responses and post-meal energy expenditure. 

In summary, saturated fat does not appear to meaningfully alter a meal’s impact on energy expenditure or appetite responses. If you expected saturated fat to be inherently fattening (or, conversely, to facilitate leanness), these are two key outcomes that might be of interest. These null results are informative, but energy expenditure and appetite are merely proxies related to the outcome of interest. To know if saturated fat causes more (or less) fat gain than other fat sources, we should compare them head-to-head in an overfeeding study that actually induces weight gain. If you’re familiar with the literary device known as “foreshadowing,” you probably know where this article is going.

Claim #2: Saturated fat causes proportionally more fat gain and less fat-free mass gain than unsaturated fat

Back in 2014, Rosqvist and colleagues completed a double-blind, parallel-groups, randomized trial to compare the effects of saturated fat (palm oil) and polyunsaturated fat (sunflower oil) overfeeding on body composition (10). Overfeeding occurred for a period of seven weeks in a sample of 39 non-diabetic adults between the ages of 20–38 years with BMI values between 18-27 kg/m². For the duration of the 7-week trial, participants were randomly assigned to add muffins containing saturated fat (palm oil) or polyunsaturated fat (sunflower oil) to their diet, with diets continuously adjusted to achieve a 3% weight gain over the duration of the study. Both groups gained roughly 1.6kg of total weight, but the saturated fat group gained more fat volume (1.5 versus 0.97 L) compared to the polyunsaturated fat group. Conversely, the group assigned to muffins high in polyunsaturated fat gained almost triple the lean tissue volume than the saturated fat group (0.86 L vs 0.31 L). Of course, we shouldn’t get too carried away from a relatively small study reporting relatively small differences in body composition. Fortunately, the same exact lab group recently attempted to replicate this finding.

That brings us to the 2024 study by Rosqvist and colleagues (11). The main portion of the study took the same general form as their prior study: participants were randomly assigned to consume muffins high in polyunsaturated fatty acids (primarily linoleic acid from sunflower oil) or muffins high in saturated fatty acids (primarily palmitic acid from palm oil) daily for 8 weeks. This time around, they recruited a larger sample that included 61 free-living healthy males and females with ages between 20-55 years and BMI values between 25-32. In addition to running this replication trial, the researchers also used observational data from 13,849 participants in the UK Biobank study to explore the relationship between polyunsaturated fat intake, saturated fat intake, and lean tissue mass.

Just like last time, weight changes observed for the two groups consuming different fat sources were not significantly different from one another. This time around, both groups gained similar volumes of lean tissue when assessed by MRI (+0.54 L in the polyunsaturated fat group and +0.67 L in the saturated fat group). When cross referencing this finding with the observational UK Biobank data, the researchers modeled the impact of making 100-kcal changes in various forms: adding 100kcal of a given fat source to one’s typical diet, swapping 100kcal of polyunsaturated fat in the place of 100kcal of saturated fat, or adding 100kcal of a given fat source to one’s diet while reducing 100kcal from elsewhere in the diet. None of these scenarios led to statistically significant effects on lean tissue volume. Baseline characteristics and body composition changes after 8 weeks of overfeeding are presented in Table 3.

In summary, the 2018 study by Nguo and colleagues casts doubt on the idea that saturated fat promotes fat accretion by impacting acute energy expenditure or appetite after meals. The unsuccessful replication attempt reported by Rosqvist and colleagues in 2024 casts doubt on their prior finding that saturated fat overfeeding leads to significantly greater fat accretion than polyunsaturated fat overfeeding, and their inability to detect meaningful relationships between between polyunsaturated fat intake, saturated fat intake, and lean tissue mass among 13,849 participants in the UK Biobank study casts further doubt on the concept. Based on the data available, it seems safe to conclude that any differential effects of saturated and polyunsaturated fat on whole-body indices of body composition are negligible when these fats are delivered in a standardized food item or meal in well-controlled studies.

Saturated Fat Could Promote to Excess Adiposity Without Causing Excess Adiposity

That may sound like an oxymoron, but I’m ultimately referring to the distinction between correlation and causation. It’s entirely possible that someone might lose fat when they decide to restrict their saturated fat intake, or might gain fat when they add more saturated fat to their diet. There are several potential explanations for these observations that have nothing to do with saturated fat being inherently fattening due to some unique characteristic related to biochemistry, physiology, or metabolism. At the end of the day, we select foods rather than nutrients when we order at a restaurant or fill our cart at the market. The decision to restrict saturated fat could look like ordering a chicken-based entree instead of beef, or eating oatmeal instead of a donut or bagel with cream cheese for breakfast. In the context of food swaps, the decision to restrict saturated fat may result in a general shift toward lower-calorie options, ultimately nudging energy balance in a direction that favors leanness. 

It’s also worth noting that many popular foods high in saturated fat are extremely palatable. Some examples that come to mind are fried foods and a wide variety of desserts that contain a great deal of cream or butter. As discussed in a fantastic guest article back in Volume 3 of MASS, hyperpalatable foods light up our brain’s reward centers, encouraging overconsumption. By incorporating more hedonically rewarding foods that happen to be higher in saturated fat, you might find yourself consuming larger portions than you otherwise would,  had you opted for a less palatable option. As a result, it’s entirely possible that certain diets high in saturated fat may promote fat gain in certain individuals. However, there doesn’t appear to be a unique structural, biochemical, or metabolic characteristic of saturated fat that promotes substantially more fat gain than unsaturated fat.

Does This Mean Dietary Fat Sources Are Interchangeable?

Not quite. The exact balance of saturated, monounsaturated, and polyunsaturated fat probably won’t make a detectable difference in terms of your overall body composition. However, different fatty acids have differential effects on several outcomes related to cardiometabolic health. First and foremost, artificial trans fats are objectively terrible. These fats are produced by heating unsaturated fat in a process called “hydrogenation.” The resulting fats take on physical characteristics that make them highly suitable for producing shelf-stable prepackaged foods. Unfortunately there are obvious links between artificial trans fat intake and chronic disease risk (12), so just about every set of nutrition guidelines you’ll ever see recommend avoiding them as much as possible.

In comparison to unsaturated fats, saturated fats tend to be associated with increased risk of cardiovascular disease (13). This is largely attributed to the effects of saturated fat on multiple blood lipid parameters associated with heart disease, including total LDL cholesterol, small LDL particles, and apolipoprotein B (14). Blood lipids aside, overfeeding studies indicate that saturated fat has more deleterious effects on the liver than unsaturated fat. In a trial that induced similar amounts of weight gain via polyunsaturated fat overfeeding and saturated fat overfeeding, saturated fat led to a substantially larger amount of liver fat storage (15). This is an adverse outcome, as excess liver fat is associated with fatty liver disease, dyslipidemia, and insulin resistance. 

I wish I could say the complete picture is as simple as saturated versus unsaturated, but unfortunately that’s not the case. While the general patterns described in the previous paragraph tend to hold true, it’s important to recognize that the cardiometabolic effects of saturated fat can differ considerably from one saturated fatty acid to another. As explained by Shramko et al (13): “myristic and lauric acids increase all the cholesterol fractions (e.g., total cholesterol, LDL cholesterol, HDL cholesterol, LDL cholesterol/HDL cholesterol ratio, TG, apolipoprotein A-I, and apolipoprotein B) more than does palmitic acid, and palmitic acid increases all the cholesterol fractions more than does stearic acid.” Of course, the same is true for unsaturated fats; there are modest distinctions between the physiological impacts of monounsaturated versus polyunsaturated fats, and specific polyunsaturated fats (such as EPA and DHA, found in fish oil) have properties that are unique in comparison to other polyunsaturated fats. So, it is true to say that artificial trans fats are pretty terrible and that you’re typically improving your long-term health prospects when you use unsaturated fat substitutions to get your saturated fat down within the recommended range. However, it’s also true to say this heuristic oversimplifies the more nuanced relationships between individual fatty acids and cardiometabolic outcomes. From a practical perspective, I don’t know many people who care to dig deep into the details of the specific fatty acids they are consuming. As a result, most people are better off using the slightly oversimplified interpretation that lumps saturated fats into one category and unsaturated fats into another.

How Much Saturated Fat is Too Much?

It’s easy to give a good answer to this question, but difficult to give a perfect answer. In other words, we know plenty about saturated fat, but we are still far from knowing everything. As I’ve mentioned in previous MASS articles (one, two), the Healthy Diet Indicator criteria from 2015 and 2020 are useful guidelines for constructing a health-compatible diet. Both guidelines recommend restricting dietary saturated fat to <10% of total energy, which is generally in line with epidemiological and mechanistic evidence linking high saturated fat intakes to morbidity and mortality (13). It is not true that less saturated fat is always better – the evidence to date suggests that intakes <10% of total energy are optimal, intakes above 10% are less optimal, and intakes above 18% of total energy tend to be least optimal with regards to long-term health outcomes.

I personally dislike dietary recommendations that are quantified as a percentage of total energy. As someone who has worked with bodybuilders trying to get as big as possible and bodybuilders trying to get as lean as possible, I’m not unfamiliar with constructing diets that are in excess of 5,000 kcal/day or diets that are comfortably below 1,500 kcal/day. When dealing with diets across this broad range of energy intakes, the shortcomings of percentage-based guidelines are particularly glaring. So, if you’d like to convert these to a raw number to shoot for, I’d suggest assuming a daily caloric intake of 2,000 to 2,500 kcal/day. With this assumed range, an individual’s typical daily saturated fat intake would be limited at roughly 22 to 28 g/day. Alternatively, you could scale this to the typical “reference person,” who happens to weigh 70kg. This would lead to a scaled daily intake of roughly 0.3 to 0.4 g/kg/day. If you eat a pretty “typical” number of calories per day, then the 10% of energy heuristic should be adequate. If your caloric intake or body size is far from the population average, you might want to use absolute upper limits (22-28 g/day) or upper limits scaled to your body mass (0.3-0.4 g/kg/day) for saturated fat intake.

Next Steps

This literature would be strengthened by more longitudinal trials that randomly assign participants to a variety of dietary conditions featuring different fatty acid compositions. These studies are few and far between, and as we saw from the 2014 and 2024 studies by Rosqvist et al, it’s incredibly important to carry out replication attempts with sufficiently large sample sizes. Ideally, these studies would measure changes in appetite, energy intake, and body composition over a sufficiently long timeline (at least 8-12 weeks). These trials require a great deal of time, effort, and money, but they yield very valuable insights.

Application and Takeaways

Saturated fat doesn’t appear to dramatically alter postprandial appetite or energy expenditure, nor does it appear to be inherently fattening (in terms of whole-body indices of body composition). However, that doesn’t mean that all fatty acids are physiologically equivalent. Artificial trans fats should be avoided as much as possible due to substantial negative impacts on blood lipids. While there are some nuanced distinctions between different saturated fatty acids, saturated fats generally have less favorable effects on liver fat accumulation and blood lipids when compared to unsaturated fats. Swapping unsaturated fat in place of saturated fat won’t directly make you leaner, but dialing back on saturated fat might be advisable (as part of a multifaceted plan of action) if your biomarkers pertaining to liver function, insulin sensitivity, or heart disease risk are a little out of whack.

References

1. Keys A, Anderson JT, Fidanza F, Keys MH, Swahn B. Effects of diet on blood lipids in man, particularly cholesterol and lipoproteins. Clin Chem. 1955 Feb;1(1):34–52.
2. Linde JA, Utter J, Jeffery RW, Sherwood NE, Pronk NP, Boyle RG. Specific food intake, fat and fiber intake, and behavioral correlates of BMI among overweight and obese members of a managed care organization. Int J Behav Nutr Phys Act. 2006 Nov 26;3:42.
3. Powell-Wiley TM, Poirier P, Burke LE, Després JP, Gordon-Larsen P, Lavie CJ, et al. Obesity and Cardiovascular Disease: A Scientific Statement From the American Heart Association. Circulation. 2021 May 25;143(21):e984–1010.
4. Nguo K, Huggins CE, Truby H, Sinclair AJ, Clarke RE, Bonham MP. No effect of saturated fatty acid chain length on meal-induced thermogenesis in overweight men. Nutr Res. 2018 Mar 1;51:102–10.
5. Casas-Agustench P, López-Uriarte P, Bulló M, Ros E, Gómez-Flores A, Salas-Salvadó J. Acute effects of three high-fat meals with different fat saturations on energy expenditure, substrate oxidation and satiety. Clin Nutr. 2009 Feb 1;28(1):39–45.
6. Soares MJ, Cummings SJ, Mamo JCL, Kenrick M, Piers LS. The acute effects of olive oil v. cream on postprandial thermogenesis and substrate oxidation in postmenopausal women. Br J Nutr. 2004 Feb;91(2):245–52.
7. Piers LS, Walker KZ, Stoney RM, Soares MJ, O’Dea K. The influence of the type of dietary fat on postprandial fat oxidation rates: monounsaturated (olive oil) vs saturated fat (cream). Int J Obes. 2002 Jun;26(6):814–21.
8. Clevenger HC, Kozimor AL, Paton CM, Cooper JA. Acute effect of dietary fatty acid composition on postprandial metabolism in women. Exp Physiol. 2014;99(9):1182–90.
9. Clevenger HC, Stevenson JL, Cooper JA. Metabolic responses to dietary fatty acids in obese women. Physiol Behav. 2015 Feb;139:73–9.
10. Rosqvist F, Iggman D, Kullberg J, Cedernaes J, Johansson HE, Larsson A, et al. Overfeeding Polyunsaturated and Saturated Fat Causes Distinct Effects on Liver and Visceral Fat Accumulation in Humans. Diabetes. 2014 Jun 14;63(7):2356–68.
11. Rosqvist F, Cedernaes J, Martínez Mora A, Fridén M, Johansson HE, Iggman D, et al. Overfeeding polyunsaturated fat compared with saturated fat does not differentially influence lean tissue accumulation in individuals with overweight: a randomized controlled trial. Am J Clin Nutr. 2024 Apr 16:S0002-9165(24)00400-3.
12. Islam MA, Amin MN, Siddiqui SA, Hossain MP, Sultana F, Kabir MR. Trans fatty acids and lipid profile: A serious risk factor to cardiovascular disease, cancer and diabetes. Diabetes Metab Syndr. 2019;13(2):1643–7.
13. Shramko VS, Polonskaya YV, Kashtanova EV, Stakhneva EM, Ragino YI. The Short Overview on the Relevance of Fatty Acids for Human Cardiovascular Disorders. Biomolecules. 2020 Aug;10(8):1127.
14. Chiu S, Williams PT, Krauss RM. Effects of a very high saturated fat diet on LDL particles in adults with atherogenic dyslipidemia: A randomized controlled trial. PloS One. 2017;12(2):e0170664.
15. Rosqvist F, Kullberg J, Ståhlman M, Cedernaes J, Heurling K, Johansson HE, et al. Overeating Saturated Fat Promotes Fatty Liver and Ceramides Compared With Polyunsaturated Fat: A Randomized Trial. J Clin Endocrinol Metab. 2019 Aug 1;104(12):6207–19.