Basically: It depends. If you eat in a caloric surplus, a low-fat diet won’t make you lose weight. You need a balance of omega-3 and omega-6 fatty acids. Saturated fat won’t necessarily give you a heart attack (but too much trans fat may).
Is fat bad? Eat fat, gain fat, right? For many decades, the traditional way to lose weight has been to subject oneself to a low-fat diet, yet current evidence suggests that, given the same caloric deficit and protein intake, low-fat and low-carb diets produce similar weight losses.
Moreover, while low-fat diets are not inherently unhealthy, shunning all fat from your diet can be dangerous, since your body needs to consume at least some omega-3 and omega-6 fatty acids.
Is saturated fat good or bad? Many believe saturated fat is the main driver of cardiovascular disease: no, just another myth.
At the end of the day, trans fat is the only kind of fat that has been shown to be categorically detrimental to health.[30] Naturally occurring trans fat and industrially produced trans fat seem to have a similar effect on blood lipids,[31] but you don’t need to worry about the minute amounts of trans fat naturally occurring in whole foods (notably dairy products).[32] The trans fat you need to shun is a byproduct of partially hydrogenated oils: this type of trans fat was once a common ingredient of processed foods — so common that trans fat consumption was linked to more than half a million coronary heart disease (CHD) deaths worldwide … just in 2010.
Industrially produced trans fat was banned in the US in 2015, and all products were supposed to be phased out by June 2018, but manufacturers received an extension until July 2019.[35] That means that a lot of products with this type of trans fat are still on the shelves today.
And you might not even know it by looking at food labels, because the FDA used to allow for a product to be labeled as having 0 grams of trans fat as long as a serving of the product had less than 0.5 grams. However, even today, the manufacturer usually gets to decide what a “serving” is, which means that, while a 5-gram serving (maybe a small treat the size of your thumbnail) may have officially 0 grams of trans fat, 100 grams of the product may have 8 grams (if 5 grams of the product contains in fact 0.4 grams of trans fat).
References:
- Gardner CD, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. (2018)
- Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids.. The National Academies Press.. (2005)
- World Health Organization & Brouwer.. Effect of Trans-Fatty Acid Intake on Blood Lipids and Lipoproteins: A Systematic Review and Meta-Regression Analysis.
- Gayet-Boyer C, et al. Is there a linear relationship between the dose of ruminant trans-fatty acids and cardiovascular risk markers in healthy subjects: results from a systematic review and meta-regression of randomised clinical trials. Br J Nutr. (2014).
- Wang Q, et al. Impact of Nonoptimal Intakes of Saturated, Polyunsaturated, and Trans Fat on Global Burdens of Coronary Heart Disease. J Am Heart Assoc. (2016).
- Allen BC, et al. Meta-regression analysis of the effect of trans fatty acids on low-density lipoprotein cholesterol. Food Chem Toxicol. (2016).
- Institute of Medicine. Final Determination Regarding Partially Hydrogenated Oils.. U.S. Food and Drug Administration, Center for Food Safety & Nutrition. (2016)