Learn Hardcore Spanish (Latin America): Tengo miedo de engordar si como pizza todos los días. - I am afraid of gaining weight if I eat pizza every day.

tener miedo de + infinitive
- Tengo miedo de engordar. – I’m afraid of getting fat.
tener miedo de que + clause
- Tengo miedo de que engorde. – I’m afraid that I’ll get fat.

You cannot normally drop the de in this structure, so:

✗ Tengo miedo engordar – incorrect
✓ Tengo miedo de engordar – correct

You may also hear tener miedo a + noun:

Tengo miedo a los perros. – I’m afraid of dogs.

But with an infinitive verb, de is the default and most natural choice.

Could I say tengo miedo de que voy a engordar or tengo miedo de que engorde instead of tengo miedo de engordar?

Yes, there are several correct ways to express this idea, but they’re not all equally natural or equivalent.

Tengo miedo de engordar.
- Structure: tener miedo de + infinitive
- Very natural and common.
- Subject is clearly yo (I): I’m afraid of getting fat (myself).
Tengo miedo de que engorde.
- Structure: tener miedo de que + subjunctive
- Also quite natural and common.
- Literally: I have fear that I (might) get fat.
- Engorde is subjunctive because this is about a possible/feared outcome.
Tengo miedo de que voy a engordar.
- Grammatically possible, but sounds less natural and a bit clumsy to many native speakers.
- The mix de que + voy a engordar (indicative future) is often avoided in careful speech.
- More idiomatic options would be:
  - Tengo miedo de que engorde.
  - Tengo miedo de que voy a engordar might be heard in informal speech, but it’s not the best model.

For a learner, the two best patterns to remember here are:

Tengo miedo de engordar.
Tengo miedo de que engorde.

Why is engordar not reflexive here? Could I say tengo miedo de engordarme or tengo miedo de que me engorde?

Engordar can be used:

intransitively: to get fat, to gain weight
transitively: to fatten (something/someone)
reflexively: engordarse – to make oneself fat / to get fat (with a reflexive pronoun)

In your sentence:

Tengo miedo de engordar already clearly means I’m afraid of getting fat myself, because the subject of engordar is the same as the subject of tengo (yo).

You can say:

Tengo miedo de engordarme. – also correct; it just makes the reference to myself more explicit.
Tengo miedo de que me engorde. – this sounds like someone/something else is going to fatten you up (e.g., Tengo miedo de que mi abuela me engorde – I’m afraid my grandmother will fatten me up). The me is now an object, not a reflexive subject.

So, in this sentence, engordar without a reflexive pronoun is completely normal and usually preferred. The reflexive version engordarme is possible, but not necessary.

Does engordar mean “to get fat” or just “to gain weight”? Is it rude?

Literally, engordar means to get fat / to put on fat, but in everyday speech it often just means to gain weight.

Nuance:

With people:
- He engordado un poco. – I’ve put on a bit of weight.
  Context and tone decide whether it feels neutral or a bit blunt.
With food:
- La pizza engorda. – Pizza makes you gain weight / Pizza is fattening.

More neutral or “softer” ways to say gain weight:

subir de peso – very common and neutral
aumentar de peso – a bit more formal
ganar peso – gain weight (also used literally)

So:

Tengo miedo de engordar. – I’m afraid of getting fat / gaining weight.
Perfectly natural. It doesn’t automatically sound rude; it’s self-directed.

Why is it si como (present) and not si coma or si comeré?

In Spanish si-clauses (if-clauses), for real, general, or likely situations, you normally use the present indicative, not the subjunctive or the future:

Si como pizza todos los días, engordo.
If I eat pizza every day, I get fat.
Si llueve, no salgo. – If it rains, I don’t go out.

So:

✓ si como pizza todos los días – correct for a general condition
✗ si coma pizza todos los días – incorrect in this context
✗ si comeré pizza todos los días – also incorrect

You can combine the si + present clause with different main clauses:

Si como pizza todos los días, voy a engordar.
Engordo si como pizza todos los días.
Tengo miedo de engordar si como pizza todos los días.

All of these follow the si + present indicative rule for real/general conditions.

Can I change the order of the parts? For example: Si como pizza todos los días, tengo miedo de engordar?

Yes. In Spanish you can freely move the si-clause before or after the main clause:

Tengo miedo de engordar si como pizza todos los días.
Si como pizza todos los días, tengo miedo de engordar.

Both are correct and natural. Differences:

If you start with si, you normally put a comma after that clause:
- Si como pizza todos los días, tengo miedo de engordar.
If the si-clause comes after, you usually skip the comma:
- Tengo miedo de engordar si como pizza todos los días.

Meaning stays the same.

Why is there no article before pizza? When would I say como pizza, como la pizza, or como una pizza?

In Spanish, whether you use an article with food depends on what you want to express:

No article: como pizza.
- General, unspecific: you eat pizza as a type of food.
- Como pizza todos los días. – I eat pizza every day. (habit, type of food)
Definite article: como la pizza.
- Refers to specific pizza already known in the context.
- Me como la pizza que sobró de ayer. – I’m eating the pizza that was left over from yesterday.
Indefinite article: como una pizza.
- Emphasizes one whole pizza (a unit/portion).
- Puedo comerme una pizza entera yo solo. – I can eat a whole pizza by myself.

In your sentence, we’re talking about pizza in general as something you eat habitually, so como pizza (no article) is the most natural choice.

Why is it todos los días and not todos días? Could I say cada día instead?

In Spanish, with todos + a countable noun like día, you normally need the definite article los:

todos los días – every day
todos los años – every year
todas las semanas – every week

So:

✓ todos los días – correct
✗ todos días – incorrect

You can say cada día instead:

como pizza cada día – I eat pizza each day / every day.

Both todos los días and cada día are correct, but:

todos los días is the more common everyday expression.
cada día can sound just slightly more formal or emphatic, depending on context.

Is miedo masculine or feminine? Does anything need to agree with it?

Miedo is a masculine noun:

el miedo – the fear

With tener miedo, nothing else in your sentence needs to agree with miedo, because:

tengo already agrees with the subject (yo),
miedo is a direct object noun that doesn’t control any adjectives in this sentence.

If you added adjectives, they would agree with miedo in gender and number:

Tengo un miedo terrible. – I have a terrible fear.
Ese miedo constante es agotador. – That constant fear is exhausting.

Other related words:

temor (also masculine): el temor – fear, worry
tener temor de / temer – to fear

Could I say me da miedo engordar instead of tengo miedo de engordar?

Yes. Both are natural, but they’re slightly different structures:

Tengo miedo de engordar.
- Literally: I have fear of getting fat.
- Structure: tener miedo de + infinitive.
Me da miedo engordar.
- Literally: Getting fat gives me fear.
- Structure: dar miedo + indirect object pronoun:
  - Me da miedo… – It scares me…
  - Te da miedo… – It scares you…
  - Le da miedo… – It scares him/her…

In practice:

Tengo miedo de engordar. – I’m afraid of getting fat.
Engordar me da miedo. / Me da miedo engordar. – Getting fat scares me.

Both are good everyday Spanish. Tengo miedo de engordar is a bit more neutral and is a great pattern to memorize.

Is this sentence specifically Latin American Spanish, or would it be different in Spain?

The sentence:

Tengo miedo de engordar si como pizza todos los días.

is perfectly natural in both Latin America and Spain. There’s nothing in it that is specifically regional.

Minor variation you might hear:

Some speakers (in both regions) also say tener miedo a in some contexts, especially with nouns:
- Tengo miedo a los perros. – I’m afraid of dogs.
But with an infinitive (engordar), tener miedo de + infinitivo is the standard and most natural everywhere.

So you can safely use this exact sentence anywhere in the Spanish-speaking world.