Can you eat whatever you want and still lose weight?

Short answer

Yes, for body weight. Weight change is driven by how much energy you eat in total against how much you burn, not by which foods the energy comes from. Controlled feeding studies find the food mix matters far less than the amount. The hard part is the arithmetic: working out how much of everything else to eat around the foods you want, while keeping protein high enough to hold on to muscle.

What the evidence says

Energy balance decides body weight. Reviews of controlled studies conclude that weight change follows the gap between energy eaten and energy spent; diet composition has, at most, small effects at the same calories.¹
Cutting carbs or cutting fat, at the same calories, both work. In an NIH metabolic-ward study where every meal was measured, people lost body fat on both restricted diets; the difference between them was small (the reduced-fat diet lost slightly more).²
A snack-cake diet still lost weight. In 2010, Kansas State nutrition professor Mark Haub lost 27 lb in ten weeks eating mostly snack cakes and packaged sweets, by keeping his total below about 1,800 kcal a day. He did it to make a point, not as a recommendation.³
Loss slows as you get lighter. The old "3,500 kcal = 1 lb" rule overestimates long-term loss, because a smaller body burns less; targets need recalculating as weight changes.⁴
Protein protects muscle. For people who lift, gains in muscle mass level off at roughly 1.6 g of protein per kg of body weight per day, so a flexible diet should still reach that.⁵

What food choice still changes: fullness, micronutrients, and long-term health. "Eat what you want" works best as "include what you want", with the rest of the day built to cover protein and fibre.

How to do it

Set a calorie target a few hundred kcal under your maintenance for fat loss (above it to gain).
Set protein first, around 1.6 g per kg of body weight, then split the rest between carbs and fat.
Put the foods you want in the day first, at the amounts you want: the burger, the dessert, the drink.
Solve the rest: choose a few other foods and find the grams of each that close the gap on all four numbers at once. This is the step people give up on by hand; it is a small system of equations.

A real day: Big Mac and fries, on target

Target about 2,200 kcal: 165 g protein, 220 g carbs, 73 g fat. The Big Mac and medium fries are locked at one serving each; vopti's optimizer solved the grams of everything else.

Food	Amount	Protein	Carbs	Fat
Big Maclocked	1	25	45	34
Medium French Frieslocked	1	5	43	15
Chicken Breast	315g	98	0	11
White Rice (cooked)	43g	1	12	0
Greek Yogurt 0%	61g	6	2	0
Oats	178g	30	118	12
The day	2,196kcal	165	220	73

Which tools do this

Calorie trackers such as MyFitnessPal, MacroFactor and Cronometer record what you already ate. Meal planners such as Eat This Much choose meals for you. vopti works the other way round: you choose the foods, including fast food, lock any of them at a fixed amount, and it solves the exact grams of the rest to hit calories, protein, carbs and fat together.

Try vopti free →

Pick 2–4 foods and a calorie target. No sign-up for the first run.

Disclosure: vopti is made by VirtueNeuro LLC, which publishes this page.

Questions

Does sugar make you gain weight?

Not on its own. Weight gain comes from eating more energy than you burn, from any source. Sugar is easy to overeat and not very filling, which is why it is often part of the problem, but within a calorie target it does not cause fat gain by itself.

Will I lose muscle if I eat junk food while dieting?

Muscle is protected mainly by eating enough protein (around 1.6 g per kg of body weight per day for people who train) and by resistance training, not by avoiding particular foods.

How fast can I lose weight eating what I want?

The same as on any diet with the same calorie deficit. A deficit of a few hundred kcal a day is a common, sustainable starting point; loss slows over time as body weight falls.

Sources

Hall KD, Guo J. "Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition." Gastroenterology, 2017. https://doi.org/10.1053/j.gastro.2017.01.052
Hall KD et al. "Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity." Cell Metabolism, 2015. https://doi.org/10.1016/j.cmet.2015.07.021
CNN. "Twinkie diet helps nutrition professor lose 27 pounds." 8 November 2010. http://www.cnn.com/2010/HEALTH/11/08/twinkie.diet.professor/index.html
Hall KD et al. "Quantification of the effect of energy imbalance on bodyweight." The Lancet, 2011. https://doi.org/10.1016/S0140-6736(11)60812-X
Morton RW et al. "A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults." British Journal of Sports Medicine, 2017. https://doi.org/10.1136/bjsports-2017-097608

More answers: How to lose weight while still eating fast food · How much of each food should I eat to hit my macros?
Fit a chain into your macros: McDonald's · Burger King · Chipotle · Chick-fil-A