It can feel impossible to determine who's right when it comes to nutrition. One person says eat more plants and less fat, another says eat mostly fat and avoid carbohydrates. You're told food timing matters and eat your breakfast, then you're told to skip breakfast (and lunch while you're at it) and keep your food to just four or eight hours a day.
Beyond the buzzwords and diet rules, there is a common denominator that links them all: a reduction in caloric intake.
Popular fad diets like keto, plant-based, and intermittent fasting can all lead to weight loss because all of them result in you somehow reducing the number of calories you eat - and this is the only true driver of fat loss.
The ketogenic diet, often known as "keto," has garnered substantial attention in recent years. The keto diet is a high-fat, moderate-protein, and low-carbohydrate diet. The party line goes, by significantly reducing carbohydrate intake, the body enters a state of ketosis, where it primarily burns fat for energy. While this approach may seem like magic, the key reason behind weight loss on keto is the restriction of calories. Ketosis is a real thing, but the reason it results in fat loss is not because of this switch to "fat burning mode." Rather, your body needs to burn more energy than you eat, and there are a few ways that this diet can lead to your reduced calorie intake. One is that when carbohydrates are limited, people often eat fewer processed foods, which are often high in calories. Additionally, people find themselves feeling more full because they've often increased their intake of protein and low-calorie veggies, so they unwittingly reduce food intake. These are fine things for fat loss, but have nothing to do with metabolic magic or hormones. Keto can work, and I don't hate it. But I do think it's a very bad idea for athletes who need carbohydrates to fuel intense training. So keto is not for everyone and shouldn't be lauded as some kind of weight loss panacea. Like any diet, keto works via calories in, calories out - the one law to rule them all.
Flexitarian, vegan, reducitarian, plant-based - whatever you want to call it - eating more plants is gaining popularity. And in general, that's great - most people need to eat more plants. Complete elimination of animal products, however, is neither necessary nor inherently healthy for all people in all situations, nor does it magically create an environment for fat loss. Putting aside the ethical and environmental drivers for plant-based approaches, it's important to understand that from a fat loss perspective, plant-based diets work via the same mechanisms as all diets - a reduction in calories.
Going plant-based often results in an increase in fiber consumption, which adds bulk to meals and helps promote satiety. As a result, people tend to eat less food overall. Additionally, plant-based diets often emphasize the consumption of fruits, vegetables, legumes, and whole grains, which are generally lower in calories compared to processed foods and animal products. By adhering to these dietary principles, individuals consume fewer calories, leading to weight loss.
Intermittent fasting (IF) is colloquially used as a term to describe an eating pattern where for some period of time on some or all days, people avoid food and limit consumption to a controlled "eating window". There are several variations, such as the 16/8 method (fasting for 16 hours, eating within an 8-hour window), 5:2, and alternate-day fasting. Despite the differences in approaches, the primary mechanism behind weight loss in intermittent fasting is once again the reduction in calories.
By restricting the time window for eating, people naturally consume fewer meals and snacks throughout the day. It makes it hard to find time for mindless snacking and also promotes eating most food at meals. I generally find in practice that people construct better meals than snacks because they are more inclined to include veggies, protein and a balanced plate than they might when grazing. All these little tweaks push people into a calorie deficit and often help promote a more balanced diet. No metabolic magic here, just an incidental reduction in calories!
This is why all roads lead to Rome: any fad diet that worked, worked via a reduction in caloric intake. They just got there through different paths. These fad diets can sometimes be helpful for people because they can provide structure, promote healthier food choices, and improve adherence to calorie reduction. But they can also be incredibly restrictive, promote a black-and-white approach to food, and prevent people from learning how to create a long-term balanced approach to nutrition. I always recommend getting a nutrition coach instead and learning how to make food work better for you.